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Title: Linear Switching Voltage Regulator
Description: In most electronic systems, voltage regulation is required for various functions. Today’s complex electronic systems are requiring greater regulating performance, higher efficiency and lower parts count
Description: In most electronic systems, voltage regulation is required for various functions. Today’s complex electronic systems are requiring greater regulating performance, higher efficiency and lower parts count
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HB206/D
Rev
...
SCILLC reserves the right to make changes without further
notice to any products herein
...
“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different
applications and actual performance may vary over time
...
SCILLC does not convey any license under its patent rights nor the rights of others
...
Should Buyer purchase or use SCILLC
products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and
distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal
injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of
the part
...
PUBLICATION ORDERING INFORMATION
GLOBAL Literature Fulfillment:
Literature Distribution Center for ON Semiconductor
P
...
Box 5163, Denver, Colorado 80217 USA
Phone: 303-675-2175 or 800-344-3860 Toll Free USA/Canada
Fax: 303-675-2176 or 800-344-3867 Toll Free USA/Canada
Email: ONlit@hibbertco
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com
N
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HB206/D
Linear & Switching Voltage Regulator Handbook
HB206/D
Rev
...
All brand names and product names appearing in this document are registered trademarks or trademarks of their respective holders
...
SCILLC reserves the right to make changes
without further notice to any products herein
...
“Typical” parameters which may be provided in SCILLC data sheets and/or
specifications can and do vary in different applications and actual performance may vary over time
...
SCILLC does not convey any license under its patent rights nor the rights of others
...
Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold
SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable
attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim
alleges that SCILLC was negligent regarding the design or manufacture of the part
...
PUBLICATION ORDERING INFORMATION
Literature Fulfillment:
Literature Distribution Center for ON Semiconductor
P
...
Box 5163, Denver, Colorado 80217 USA
Phone: 303–675–2175 or 800–344–3860 Toll Free USA/Canada
Fax: 303–675–2176 or 800–344–3867 Toll Free USA/Canada
Email: ONlit@hibbertco
...
com
ON Semiconductor Website: http://onsemi
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2
Linear & Switching Voltage Regulator
Applications Information
In Brief
...
Today’s complex electronic systems are
requiring greater regulating performance, higher efficiency
and lower parts count
...
Available in a growing variety, ON Semiconductor offers a
wide range of regulator products from fixed and adjustable
voltage types to special–function and switching regulator
control ICs
...
Basic Linear regulator theory and switching
regulator topologies have been included along with practical
design examples
...
Basic Linear Regulator Theory
...
15
Linear Regulator Circuit Configuration
and Design Considerations
...
29
Linear Regulator Construction and Layout
...
59
Linear Regulator Circuit Troubleshooting
Check List
...
63
An Introduction to Switching
Power Supplies
...
74
Switching Regulator Component
Design Tips
...
88
Switching Regulator Design Examples
...
97
Heatsinking
...
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3
TABLE OF CONTENTS
Section 1
...
IC Voltage Regulator
...
The Error Amplifier
...
Page
6
6
6
10
13
Section 2
...
Selecting the Type of Regulator
...
Three–Terminal, Fixed Output Regulators
...
Selecting an IC Regulator
...
Linear Regulator Circuit Configuration and Design Considerations
...
Negative, Adjustable Regulator
...
Negative, Fixed Output Regulator
...
18
18
22
22
25
27
Section 4
...
Series Pass Element Configurations
...
Current Limiting Techniques
...
Foldback Current Limiting
...
29
29
30
31
31
34
36
Section 5
...
General Layout and Component Placement Considerations
...
Mounting Considerations for Power Semiconductors
...
Fastener and Hardware Characteristics
...
Appendix A: Thermal Resistance Concepts
...
37
37
37
39
43
47
55
56
57
Section 6
...
IC Regulator Selection
...
Determination of Component Values
...
Selection of Series Pass Element
...
Clamp Diode
...
59
59
59
59
60
60
61
61
61
Section 7
...
62
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Designing the Input Supply
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Surge Current
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Design Example
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An Introduction to Switching Power Supplies
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Basic Configurations
...
69
69
70
72
Section 10
...
FET and Bipolar Drive Considerations
...
Overvoltage Protection
...
Transformer Design
...
74
74
76
77
79
79
81
Section 11
...
Transistors
...
Rectifiers
...
Basic Switching Power Supply Configurations
...
Push–Pull and Bridge Converters
...
88
88
91
94
Section 13
...
96
Section 14
...
The Crowbar Technique
...
The Sense and Drive Circuit
...
MC34064 and MC34164 Series
...
Heatsinking
...
Selecting a Heatsink
...
Custom Heatsink Design
...
SOIC Miniature IC Plastic Package
...
SOP–8 and SOP–16L Packaged Devices
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106
106
107
107
109
112
112
113
113
114
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IC Voltage Regulator
The basic functional block diagram of an integrated circuit voltage regulator is shown in Figure 1–1
...
The output
voltage (VO), is equal to or a multiple of Vref
...
Therefore, the ideal voltage regulator could be
considered a voltage source with a constant output voltage
...
In this figure, the regulator is modeled as a voltage source with a positive output impedance (ZO)
...
Also, the regulator output voltage (VO) is affected by the voltage drop across ZO, caused by
the output current (IO)
...
B
...
1
...
It consists of a resistor and a zener
diode
...
In order to determine VZ, consider Figure
1–3b
...
Note that changes in the supply voltage give rise to changes in the zener current, thereby changing the
value of the reference voltage (VZ)
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6
Figure 1–1
...
Voltage Regulator Equivalent Circuit Model
VCC
ZO
IO
VO
V = f (VCC, Tj)
Figure 1–3
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7
2
...
The value of the zener current is largely independent of VCC and is given by:
IZ =
VBEQ1
RSC
(2)
where: VBEQ1 = base–emitter voltage of Q1
...
The reference voltage (about 7
...
This configuration has the additional benefit of better temperature stability than that of a simple
resistor–zener reference
...
The stability of zener diodes used in most integrated circuitry is about
+ 2
...
04%/°C (for a 6
...
If the junction temperature varies 100°C, the zener or
reference voltage would vary 4%
...
However, the circuit of Figure 1–4 does not have this drawback
...
2 mV/°C
temperature coefficient (TC) of the zener diode is offset by the negative 2
...
This results in a reference voltage with very stable temperature characteristics
...
Constant Current (Zener Reference)
VCC
R
Q2
Vref
VZ
VR1
IZ
Q1
VBEQ1
RSC
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Bandgap Reference
Although very stable, the circuit of Figure 1–4 does have a disadvantage in that it requires a supply
voltage of 9
...
Another type of stable reference which requires only a few volts to operate was
described by Widlar(1) and is shown in Figure 1–5
...
Figure 1–5
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9
C
...
Figure 1–6 shows a typical differential error amplifier in a voltage regulator
configuration
...
Note also that if AVOL is not
infinite, with constant output current (a non–varying output load), the output voltage can still be
“tweaked–in” by varying R1 and R2, even though VO will not exactly equal that given by Equation 11
...
Amplifier Input Offset Voltage Drift
The input transistors of integrated circuit amplifiers are usually not perfectly matched
...
At a given temperature, this effect can
be nulled out of the desired output voltage by adjusting Vref or 1/β
...
0 µV/°C to +15 µV/°C, causing a proportional change in the output voltage
...
2
...
In modern integrated circuit regulator amplifiers, the utilization of constant current sources gives
such large values of PSRR that this effect on VO can usually be neglected
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Typical Voltage Regulator Configuration
VCC
(+)
+VIO
Vref
IO
ZOL
υi
VO
AVOLυi
(-)
R2
R1
The definition of common mode voltage (VCM), illustrated by Figure 1–7a, is:
VCM +
where:
ƪV1 ) V2ƫ – ƪ (V)) ) (V–) ƫ
2
2
(12)
V1 = voltage on amplifier noninverting input
V2 = voltage on amplifier inverting input
V+ = positive supply voltage
V– = negative supply voltage
Figure 1–7
...
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11
VO
Figure 1–8
...
In fact, VCM does influence the amplifier output voltage
...
The latter figure is the same configuration as Figure 1–6, with amplifier input offset
voltage and output impedance deleted for clarity and common mode voltage effects added
...
This can be reduced
by designing the amplifier to have a high AVOL, a high CMRR, and by choosing the feedback ratio (β) to
be unity
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12
3
...
This can be accomplished by lowering ZOL, choosing an amplifier with high AVOL, and by selecting the
feedback ratio (β) to be unity
...
Given a change in output current (∆IO) the amplifier will see a
change of only ∆IO/hFEQ1 in its output current (IOȀ)
...
D
...
These are shown in Table
1–1 along with procedures which minimize their effects
...
Since a limit is soon approached in how much AVOL can be practically obtained in an
integrated circuit amplifier, selecting a feedback ratio (β) equal to unity is the only viable way of improving
total regulator performance, especially in reducing regulator output impedance
...
If the configuration of Figure 1–6 is used, the output
voltage cannot be adjusted to a value other than Vref
...
(2) Its greatest benefit is in reducing total
regulator output impedance
...
Emitter Follower Output
VCC
(+)
Vref
IO′
ZOL
Q1
IO
VO
(Ċ)
R2
R1
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Figure 1–10
...
J
...
1971, Vol
...
2–7
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SC–3, Number 4, Dec
...
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Selecting the Type of Regulator
There are five basic linear regulator types; positive, negative, fixed output, tracking and floating
regulators
...
1
...
However, depending on the system’s grounding requirements, each regulator type may
be used to regulate the “opposite” voltage
...
Note that the
ground reference for each (indicated by the heavy line) is continuous
...
If no other common supplies or system components operate off the input supply to the regulator, the
circuits of Figures 2–1c and 2–1d may be used to regulate positive voltages with a negative regulator and
vice versa
...
e
...
There are methods of utilizing positive regulators to obtain negative output voltages without sacrificing
ground bus continuity
...
An example of this technique is shown in Section 3
...
Three–Terminal, Fixed Output Regulators
These regulators offer the designer a simple, inexpensive way to obtain a source of regulated voltage
...
The advantages of these regulators are:
a) Easy to use
...
c) No circuit adjustments necessary
...
Their disadvantages are:
a) Output voltage cannot be precisely adjusted
...
b) Available only in certain output voltages and currents
...
(Methods for
obtaining greater output currents are shown in Section 3
...
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15
Figure 2–1
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16
3
...
These devices provide added flexibility with output voltage adjustable over a wide
range, from 1
...
A variety of current
ranges from 100 mA to 3
...
B
...
To provide
higher currents than are available from monolithic technologies, an IC regulator will often be used as a
driver to a boost transistor
...
Unfortunately, there is no exact step–by–step procedure that can be followed which will lead to the
ideal regulator and circuit configuration for a specific application
...
Because
of this, the following general design procedure is suggested:
1
...
At this point, do not be overly concerned with
the regulator capabilities in terms of output voltage, output current, SOA and special features
...
Next, select application circuits from Section 3 which meet the requirements for output current, output
voltage, special features, etc
...
From these designs a judgement can be made by the designer as to which regulator
circuit configuration combination best meets his or her requirements in terms of cost, size and complexity
...
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17
SECTION 3
LINEAR REGULATOR CIRCUIT CONFIGURATION
AND DESIGN CONSIDERATIONS
Once the IC regulators, which meet the designer’s performance requirements, have been selected,
the next step is to determine suitable circuit configurations
...
In this
section, several circuit configurations and design equations are given for the various regulator ICs
...
Organization is first by regulator
type and then by variants, such as current boost
...
A
...
Negative, Adjustable
C
...
Negative, Fixed
E
...
Special
1
...
Electronic Shutdown
G
...
If foldback limiting
is desired, see Section 4C for techniques and design equations
...
Positive, Adjustable Output IC Regulator Configurations
1
...
5 A, and 3
...
All of these devices utilize the same basic circuit
configuration as shown in Figure 3–1A
...
For
output voltages from ] 7
...
0 V to ] 7
...
2
...
The output currents which can be obtained with
this configurations are limited only by capabilities of the external pass element(s)
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18
Figure 3–1A
...
1µF
R2
CO
1
...
CO: improves transient response
...
R2
+ IAdj R2
Vout = 1
...
Figure 3–2A
...
1k
5
12k
7
0
...
1 µF
R2 =
Vref
VO
(R1 + R2) ≈
7
...
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01µF
RSC
10
6
+ 20V
12
Figure 3–3A
...
0 V ≤ VO ≤ Vref
Vin
12
11
+ 20V
RSC
10
2
7
VO
22Ω
+ 5
...
1k
5
R1
3
...
01µF
13k
R2
1
...
66V
; 10 kΩ < R1 + R2 < 100 kΩ
ISC
RSC ]
VO
R2 =
Vref
VO
(R1 + R2) ]
7
...
1 µF
Values shown are for a 5
...
To obtain greater output currents with the MC1723C the configurations shown in Figures 3–4A and
3–5A can be used
...
Figure 3–4A
...
3Ω
1/2W
10
VO
+ 15V
ISC = 0
...
1µF
4
5
R3
5
...
66 V
; 10 kΩ < R1 + R2 < 100 kΩ
ISC
Vref
VO
(R1 + R2) ≅
7
...
1 µF ; R3 ≅ R1 || R2
Selection of Q1 based on considerations of Section 4
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MC1723C PNP Boost Configuration
2N3791
or Equiv
Vin
+ 18V
R4
RSC
11
12
100
10
VO
0
...
75 A
3
MC1723C
R3
5
...
1µF
6
5
R1
4
13
10k
100pF
Cref
R2
7
13k
0
...
1 µF
ISC
Vref
7
...
0 mA
Selection of Q1 based on considerations of Section 4
...
3
...
0 V are desired, the configuration of
Figure 3–6A can be utilized to obtain increased operating efficiency
...
This method, however, usually necessitates that
separate short circuit protection be provided for the IC regulator and external pass element
...
Figure 3–6A
...
5V
Vin2
8
10
+ 10V
7
2
4
R1
Cref
3
R = 15Ω
IB
+5
...
0 A
MPS6512 or
Equivalent
4
1
...
0k
3
R2
MC1723C
0
...
0W
VO
R3
5
...
1µF
RSC ≅
0
...
66 V
; 10 kΩ
R2 =
VO
Vref
1
...
Selection of Q1 based on considerations of Section 4
...
0 V, 1
...
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1 µF
R3 ≅ R1 || R2
VO
(R1 + R2)
7
...
Negative, Adjustable Output IC Regulator Configurations
1
...
This is done by using an external pass element and a zener level shifter as shown in Figure 3–1B
...
In addition, it should be noted that this circuit will not operate with
a shorted output
...
MC1723C Negative Regulator Configuration
10
12
11
R1
12k
4
MC1723C
6
+
100pF
5
13
R2
10k
7
VO = -15 V
VZ = 14 V
Q1
Vin = -20 V to -23 V
10mF
2N3055
or Equiv
VO ≥ 10 V; 10 kΩ ≤ R1 + R2 ≤ 100 kΩ
R2 =
Vref
VO
(R1 + R2) ≅
7
...
0 V; VZ ≥ Vin - VO - VBE(Q1) + 6
...
Values shown are for a -15 V, 750 mA regulator using the MC1723CP with Q1
mounted on a 20°C/W heatsink at TA up to +70°C
...
C
...
Basic Regulator Configuration
The basic current configuration for the positive three–terminal regulators is shown in Figure 3–1C
...
0 A
...
Output Current Boosting
Figure 3–2C illustrates a method for obtaining greater output currents with the three–terminal positive
regulators
...
0 A
MC7800C in this configuration
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22
Figure 3–1C
...
33µF
2
IO
1
...
1 A
0
...
0 A
Device
MC78XX
MC78LXX
MC78MXX
MC78TXX
VO
CO
3
Cin: required if regulator is located more than a few (≈2 to 4) inches away
Cin: from input supply capacitor; for long input leads to regulator, up to 1
...
(Cin should be a high frequency type capacitor
...
XX: two digits of type number indicating nominal output voltage
...
Figure 3–2C
...
12Ω
5
...
0µF
3
2
ISCTOT
ISC(IC1)
VO
5
...
ăR:ăused to divert IC regulator bias current and determines at
ăăĂăwhat output current level Q1 begins conducting
...
6 V
; ISCTOT = ISC(Q1) + ISC(IC1)
ISC(Q1)
Selection of Q1 based on considerations of Section 4
...
0 V, 5
...
5°C/W heatsink and Q1 on a 1°C/W heatsink for TA up to 70°C
...
Obtaining an Adjustable Output Voltage
With the addition of an op amp, an adjustable output voltage supply can be obtained with the MC7805C
...
If lower output currents are required, then an MC78M05C (0
...
4
...
Figure 3–4C shows this configuration
...
0 mA (IQ, the regulator bias current) up to the available output
current of the regulator
...
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Adjustable Output Voltage Configuration
Using a Three–Terminal Positive Regulator
Vin
1
2
MC7805C
Figure 3–4C
...
33µF
VO′
3
3
R
VO
7
6
0
...
0k
IIB
2
0
...
0 V ≤ Vin ≤ 35 V
VO = 7
...
0 V
Vin ≥ 35 V
5
...
In these cases a preregulator circuit, as shown in Figure 3–5C, may be used
...
Preregulator for Input Voltages Above Vin(max)
2N6569
Vin
60V
R1
IC1
1
MC78XXC
Q1
750Ω
2
...
33µF
2
VO
3
IN5363
30V, 5
...
5
VCEO(Q1) ≤ Vin
) • hfe(Q1)
XX: two digits of type number indicating nominal output voltage
...
IC1 should be appropriately heatsinked for the package type used
...
High Output Voltage
If output voltages above 24 V are desired, the circuit configuration of Figure 3–6C may be used
...
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High Output Voltage Configuration for Three–Terminal Positive Regulators
IC1
2N6576
Vin
1
60V
R1
5
...
33µF
IN4751
30V,
1
...
0W
Z1
VO = VZ1 + 24; R1 = (
Vin - (VZ1 + VZ2)
1
...
0 A regulator
Q1 mounted on a 10°C/W heatsink
and IC1 mounted on a 2°C/W heatsink for TA up to +70°C
...
Negative, Fixed Output IC Regulator Configurations
1
...
Figure 3–1D
...
0 A
0
...
33µF
2
0
...
0 µF may be required
...
CO:ăimproves stability and transient response
...
See Section 15 for heatsinking
...
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25
Output Current Boosting
In order to obtain increased output current capability from the negative three–terminal regulators, the
current boost configuration of Figure 3–2D may be used
...
Figure 3–2D
...
56, 1W
Vin
Q1
0
...
56, 1W
RSC
ISCTOT
ISC(IC1)
IC1
MJE200
or Equiv
3
MC79XXC
R
5
...
0µF
2
1
...
See Section 2 for available voltages
...
O
VBE on(Q1)
IBias(IC1)
ISCTOT = ISC(Q1) + ISC(IC1)
RSC ≈
0
...
Values shown are for a -5
...
0 A regulator; using an MC7905CT on a 1
...
2
...
In order to obtain the greatest output voltage compliance range at a given input voltage,
the MC7905C or MC79L05C should be used in this configuration
...
Current Regulator Configuration for the Three–Terminal Negative Regulators
IO
Vin
3
1
...
0µF
IIB
Vin ≥ - 35 V for MC7905C
Vin ≥ - 30 V for MC79L05C
Vin ≤ VO + VO - 2
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26
VO
Constant Current
from Grounded
Load
F
...
These considerations
are given below
...
Regulator Voltages
For any circuit configuration, the worse–case voltages present on each pin of the IC regulator must be
within the maximum and/or minimum limits specified on the device data sheets
...
They include: a
...
Vin(max)
c
...
Vout(min)
e
...
5 V,
even instantaneously, or the regulator will not function properly, (see Figure 3–1B)
...
Regulator Power Dissipation, Junction Temperature and Safe Operating Area
The junction temperature, power dissipation output current or safe operating area limits of the IC
regulator must never be exceeded
...
Operation with a Load Common to a Voltage of Opposite Polarity
In many cases, a regulator powers a load which is not connected to ground but instead is connected
to a voltage source of opposite polarity (e
...
op amps, level shifting circuits, etc
...
This protects the regulator,
during startup and short circuit operation, from output polarity reversals
...
Output Polarity Reversal Protection
+ Vin
+ VO
Positive
Regulator
1N4001
or Equiv
Load
1N4001
or Equiv
- Vin
Negative
Regulator
- VO
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Reverse Bias Protection
Occasionally, there exists the possibility that the input voltage to the regulator can collapse faster than
the output voltage
...
If the output voltage is greater ≈ 7
...
To prevent this, a diode shunt can be
employed, as shown in Figure 3–2F
...
Diode D1 prevents CO from discharging through the regulator during an input short circuit
...
The
combination of diodes D1 and D2 prevents CAdj from discharging through the regulator during an input
short circuit
...
Reverse Bias Protection
Figure 3–3F
...
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28
SECTION 4
SERIES PASS ELEMENT CONSIDERATIONS
FOR LINEAR REGULATORS
Presently, most monolithic IC voltage regulators that are available have output current capabilities from
100 mA to 3
...
If greater current capability is required, or if the IC regulator does not possess sufficient
safe–operating–area (SOA), the addition of an external series pass element is necessary
...
For illustrative purposes, pass elements for only positive regulator types will
be discussed
...
A
...
This pass element could be a single transistor or multiple transistors arranged in
Darlington and/or paralleled configurations
...
Figure 4–1A
...
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(Regulators which possess an external sense
lead may also be used with this configuration
...
Figure 4–1B
...
The resistor (R) serves to route the IC regulator bias current (IBias) away
from the base of Q2
...
The value of R is low
enough to prevent Q2 from turning on when IBias flows through this resistor, and is given by:
0
VBEon(Q2 )
IBias
(4
...
Series Pass Element Specifications
Independent of which configuration is utilized, the transistor or transistors that compose the pass
element must have adequate ratings for IC(max), VCEO, hfe, power dissipation, and safe operating area
...
IC(max) — for the pass element of Figure 4–1A, IC(max) is given by:
IC(max)(Q2) ≥ IO(max) – IB(max)(Q2) = IO(max) –
≥ IO(max)
IC(max)(Q2)
hfe(Q2)
(4
...
2)
For the configuration of Figure 4–1B:
IC(max)(Q2) ≥ IO(max) + IB(max)(Q2)
≥ IO(max)
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3)
(4
...
VCEO — since VCE(Q2) is equal to Vin1(max) when the output is shorted or during start up:
VCEO(Q2) ≤ Vin1(max)
(4
...
hfe — the minimum DC current gain for Q2 in Figures 4–1A and 4–1B is given by:
hfe(min)(Q2) ≥
IC(max)(Q2)
@ VCE = (Vin1(min) – VO)
IB(max)(Q2)
(4
...
Maximum Power Dissipation PD(max), and Safe Operating Area (SOA)
For any transistor there are certain combinations of IC and VCE at which it may safely be operated
...
As an example, the safe–operating–area (SOA) curve for the well known 2N3055 NPN silicon power
transistor is shown in Figure 4–2
...
Notice that the power dissipation
and second breakdown ratings are given for a case temperature of +25°C and must be derated at higher
case temperatures
...
) These boundaries
must never be exceeded during operation, or destruction of the transistor(s) which constitute the pass
element may result
...
)
C
...
This locus of points is determined by the input voltage (Vin1, output voltage
(VO), output current (IO) and the type of output current limiting technique employed
...
All that is left to determine
is how the chosen current limit scheme affects required pass element SOA
...
1
...
The basic circuit configuration is shown in Figure 4–3A, and operates in the following manner
...
When the
output current has increased to the point that the voltage drop across RSC is equal to the base–emitter ON
voltage of Q3 (VBEon(Q3)), Q3 conducts
...
Base drive (IB(Q2)) of Q2 is therefore reduced and its collector–emitter
voltage increases, thereby reducing the output voltage below its regulated value, Vout
...
The value of ISC is given by:
ISC =
VBEon(Q3)
RSC
http://onsemi
...
7)
Figure 4–2
...
0
5
...
0
2
...
0
Second Breakdown @ TC = 25°C
0
...
5
0
...
2
VCEO
0
...
0
4
...
0
7
...
com
32
40
60
Figure 4–3
...
Of course, access to this point is necessary
...
(1)
The required safe–operating–area for Q2 can be obtained by plotting the VCE and IC of Q2 given by:
VCE(Q2) = Vin1 – VO – IORSC ] Vin1 – VO
IC(Q2) ] IO
(4
...
9)
where, VO = Vout for 0 ≤ IO ≤ ISC
(4
...
11)
The resulting plot is shown in Figure 4–4
...
Note that the greatest demand on the transistor’s SOA capability occurs when the
output of the regulator is short circuited and the pass element must support the full input voltage and short
circuit current simultaneously
...
Constant Current Limit SOA Requirements
(See Section 3 for Circuit Techniques)
IC(Q2) , COLLECTOR CURRENT LOG
IC(max)
Pass Element SOA
ISC
Vin1 - Vout
Vin1 VCEO
VCE(Q2), COLLECTOR-EMITTER VOLTAGE LOG
(1)
The three–terminal regulators have internal current limiting and therefore do not provide
access to this point
...
http://onsemi
...
Foldback Current Limiting
A disadvantage of the constant current limit technique is that in order to obtain sufficient SOA the pass
element must have a much greater collector current capability than is actually needed
...
This can be done
by using a “foldback” current limiting technique instead of constant current limiting
...
The circuit operates in a
manner similar to that of the constant current limiting circuit, in that output current control is obtained by
diverting base drive away from Q1 with Q3
...
Q3 is therefore
non–conducting and the output voltage remains constant
...
The output current at which this
occurs is IK, the “knee” current
...
Foldback Current Limiting
(a)
(b)
External Pass Element
VCE(Q2)
VA
Q2
Vin1
Ib(Q2)
Q1
IO
Vout
VO, OUTPUT VOLTAGE
IC(Q2)
VO
RSC
R1
IDrive
ISC
IO, OUTPUT CURRENT
Q3
IC Regulator
VBE(Q3)
R2
VR2
Figure 4–6
...
com
34
IK
The output voltage will now decrease
...
The output current will continue to “foldback” as the output voltage decreases, until an output
short circuit current level (ISC) is reached when the output voltage is zero
...
The values for R1, R2, and RSC (neglecting base
current of Q3) are given by:
RSC =
ǒ
Vout/ISC
Ǔ
I
Vout
– K
1 )
ISC
VBEon(Q3)
R2
V
= BEon(Q3)
ISC RSC
R1 + R2
and, R1 + R2 ≤
Vout
IDrive
(4
...
13)
(4
...
Note
that the pass element is required to operate with a collector current of only ISC during short circuit
conditions, not the full output current, IK
...
Although foldback current limiting
allows use of smaller pass element transistors for a given regulator output current than does constant
current limiting, it does have a few disadvantages
...
12), as the foldback ratio (IK/ISC) is increased, the required value of RSC
increases
...
In addition, it can be seen for
Equation (4
...
15)
For these reasons, foldback ratios greater than 2:1 or 3:1 are not usually practical for the lower output
voltage regulators
...
com
35
D
...
In these
cases it is necessary to parallel two or more transistors
...
In order to insure that the collector currents of the paralleled transistors are approximately equal, the
configuration of Figure 4–7 can be used
...
The collector–current mismatch can be determined by considering the
following, from Figure 4–7,
VBE1 + V1 = VBE2 + V2
(4
...
17)
where: VBE = VBE1 – VBE2 and, ∆V = V2 – V1
Assuming IE1 ] IC1 and IE2 ] OC2, the collector–current mismatch is given by,
V2
RE
IC2 – IC1
=
IC2
V1
RE
–
V2
RE
and, percent collector–current mismatch =
∆VBE
V2
=
V2 – V1
∆V
∆VBE
=
=
V2
V2
V2
(4
...
19)
× 100%
(4
...
20), the collector–current mismatch is dependent on ∆ VBE and V2
...
0 V to 0
...
RE is therefore given by:
RE =
0
...
0 V
0
...
0 V
0
...
0 V
=
=
IC1
IC2
IC/2
(4
...
Paralleling Pass Element Transistors
IC
IC1
IC2
IB
Q1
Q2
VBEI
IE1
V1
QN
VBE2
RE
IE2
V2
http://onsemi
...
In order to obtain excellent transient response performance, high
frequency transistors are used in modern integrated circuit voltage regulators
...
In this section, guidelines will be given on proper regulator layout and placement of circuit components
...
1
...
To insure stable closed–loop operation, all these
devices are frequency compensated, either internally or externally
...
Therefore,
it is important that the circuit lead lengths be short and the layout as tight as possible
...
Lead
lengths associated with the external pass element(s), if used, should also be minimized
...
If the
lead length from the input supply filter capacitor to the regulator input is more than a couple of inches, a
0
...
0 µF high frequency type capacitor (tantalum, ceramic, etc
...
2
...
The most common ground loop problem occurs when the return lead of the input supply filter
capacitor is improperly located, as shown in Figure 5–1
...
This is due to the high peaks of the filter capacitor ripple current
(Iripple) flowing through the lead resistance between the load and regulator
...
Since the regulator will only keep constant the voltage between its sense
lead and ground point, points “A” and “B” in Figure 5–1, this additional ripple voltage (Vlead), will appear at
the load
...
Remote Voltage Sensing — Closely related to the above ground loop problem is resistance in the current
carrying leads to the load
...
This is illustrated in Figure
5–3
...
From Figure 5–3 we can see that any lead resistance between these points and the load will
cause the load voltage (VL) to vary with varying load current, IL
...
http://onsemi
...
Filter Capacitor Ground Loop — WRONG!
A"
Regulator
Circuit
+
C
B"
to
XFMR
VL
Vout
RL
RLead
Iripple
VLead
Figure 5–2
...
The voltage
drops in the high current carrying leads now have no effect on the load voltage (V L)
...
The Ground and Sense leads should be formed into
a twisted pair lead to minimize their lead inductance and noise pickup
...
Effects of Resistance In Output Leads
Output
Vin
RLead
Regulator
Circuit
Sense
+
Gnd
VL
Load
IL
RLead
Figure 5–4
...
com
38
RL
IL
3
...
Except for lead–mounted parts used at low currents, a heat exchanger is required to prevent the junction
temperature from exceeding its rated limit, thereby running the risk of a high failure rate
...
(1)
Guidelines for designers of military power supplies impose a 110°C limit upon junction temperature
...
Most early life field failures of power semiconductors can be traced to faulty mounting procedures
...
With the widespread use of various plastic packaged
semiconductors, the prospect of mechanical damage is very significant
...
Figure 5–5 shows an example of doing nearly everything wrong
...
To use the
socket, the leads are bent — an operation which, if not properly done, can crack the package, break the
internal bonding wires, or crack the die
...
The force used to tighten the screw tends to pull the package into
the hole, possibly causing enough distortion to crack the die
...
If a rough heatsink surface and/or burrs around the hole were displayed in the
illustration, most but not all poor mounting practices would be covered
...
Extreme Case of Improperly Mounting
a Semiconductor (Distortion Exaggerated)
Plastic Body
Leads
Package Heatsink
Mica Washer
Equipment
Heatsink
Socket for
TO-213AA Package
Speed Nut
(Part of Socket)
Sheet Metal Screw
_________________________________
(1) MIL–HANDBOOK — 2178, SECTION 2
...
(2) Navy Power Supply Reliability — Design and Manufacturing Guidelines NAVMAT P4855–1,
Dec
...
, Philadelphia, PA 19120
...
Grafoil is a registered trademark of Union Carbide
Kapton is a registered trademark of E
...
Dupont
Rubber–Duc is a trademark of AAVID Engineering
Sil Pad is a trademark of Berquist
Sync–Nut is a trademark of ITW Shakeproof
Thermasil is a registered trademark and Thermafilm is a trademark of Thermalloy, Inc
...
http://onsemi
...
The isolation material is, to some extent, a thermal isolator as well, which raises junction operating
temperatures
...
Various regulating agencies also impose creepage distance specifications which further complicates
design
...
Proper mounting procedures usually necessitate orderly attention to the following:
1
...
Applying a thermal grease (if required)
3
...
Fastening the assembly
5
...
As newer packages are developed, it is probable that they will fit into the generic classes
discussed in this note
...
The following classes are defined:
Flange Mount
Tab Mount
Plastic Body Mount
Surface Mount
Appendix A contains a brief review of thermal resistance concepts
...
Mounting Surface Preparation
In general, the heatsink mounting surface should have a flatness and finish comparable to that of the
semiconductor package
...
In high power applications, a more detailed
examination of the surface is required
...
Surface Flatness
Surface flatness is determined by comparing the variance in height (∆h) of the test specimen to that
of a reference standard as indicated in Figure 5–6
...
The mounting surface flatness (i
...
Figure 5–6
...
com
40
∆h
Surface Finish
Surface finish is the average of the deviations both above and below the mean value of surface height
...
to 60 µin
...
A finer finish
is costly to achieve and does not significantly lower contact resistance
...
finish, showed that heatsink finishes between
16 µin
...
caused less than ±2
...
(3) Most commercially available cast or extruded
heatsinks will require spotfacing when used in high power applications
...
Mounting Holes
Mounting holes generally should only be large enough to allow clearance of the fastener
...
For these packages, a smaller screw
size must be used such that the hole for the bushing does not exceed the hole in the package
...
This “crater” in the heatsink around the mounting hole can
cause two problems
...
The first
effect may often be detected immediately by visual cracks in the package (if plastic), but usually an
unnatural stress is imposed, which results in an early–life failure
...
Although punched holes are seldom acceptable in the relatively thick material used for extruded
aluminum heatsinks, several manufacturers are capable of properly utilizing the capabilities inherent in
both fine–edge blanking or sheared–through holes when applied to sheet metal as commonly used for
stamped heatsinks
...
When mounting holes are drilled, a general practice with extruded aluminum, surface cleanup is
important
...
However, the edges must be broken to remove burrs which cause poor contact between device and
heatsink and may puncture isolation material
...
Anodizing results in significant electrical but negligible thermal insulation
...
Heatsinks are also available which have a nickel
plated copper insert under the semiconductor mounting area
...
Another treated aluminum finish is iridite, or chromate–acid dip, which offers low resistance because
of its thin surface, yet has good electrical properties because it resists oxidation
...
For economy, paint is sometimes used for sinks; removal of the paint where the semiconductor is
attached is usually required because of the paint’s high thermal resistance
...
Some manufacturers will provide anodized or painted surfaces
meeting specific insulation voltage requirements, usually up to 400 V
...
Immediately prior to assembly, it is a good practice to
polish the mounting area with No
...
________________________________
(3) Catalog #87–HS–9 (1987), page 8, Thermalloy, Inc
...
O
...
http://onsemi
...
Otherwise, the interface thermal resistance will be
unnecessarily high and quite dependent upon the surface finishes
...
They
have a resistivity of approximately 60°C/W/in whereas air has 1200°C/W/in
...
However, the grease causes a number of problems, as discussed in the following
section
...
These pads are conformal and therefore partially fill voids when
under pressure
...
Since some of
these compounds do not spread well, they should be evenly applied in a very thin layer using a spatula
or lintless brush, and wiped lightly to remove excess material
...
Some experimentation is necessary to
determine the correct quantity; too little will not fill all the voids, while too much may permit some compound
to remain between well mated metal surfaces where it will substantially increase the thermal resistance
of the joint
...
When the amount is
correct, a very small amount of grease should appear around the perimeter of each mating surface as the
assembly is slowly torqued to the recommended value
...
In production, assemblers should be trained to slowly
apply the specified torque even though an excessive amount of grease appears at the edges of mating
surfaces
...
To prevent accumulation of airborne particulate matter, excess compound should be wiped away using
a cloth moistened with acetone or alcohol
...
Data showing the effect of compounds on several package types under different mounting conditions
is shown in Table 5–1
...
The
joint compound also improves the breakdown rating of the insulator
...
Approximate Values for Interface Thermal Resistance Data
from Measurements Performed In ON Semiconductor Applications Engineering Laboratory
Dry interface values are subject to wide variation because of extreme dependence upon surface conditions
...
(See Appendix B for a discussion of Interface Thermal Resistance Measurements
...
5
0
...
3
0
...
2
1
...
4
1
...
See Figures 5–7 and 5–8 for additional data on TO–204AA and TO–220 packages
...
Screw not insulated (see Figure 5–12)
...
com
42
Conductive Pads
Because of the difficulty of assembly using grease and the evaporation problem, some equipment
manufacturers will not, or cannot, use grease
...
Data for a greased bare joint and
a joint using Grafoil, a dry graphite compound, is shown in the data of Figure 5–7
...
Another conductive pad available from AAVID is called KON–DUX
...
Highly compressible, it becomes formed to
the surface roughness of both the heatsink and semiconductor
...
Similar dry
conductive pads are available from other manufacturers
...
Insulation Considerations
Since most power semiconductors use are vertical device construction it is common to manufacture
power semiconductors with the output electrode (anode, collector or drain) electrically common to the
case; the problem of isolating this terminal from ground is a common one
...
Heatsink isolation is not always possible, however, because of EMI requirements, safety
reasons, instances where a chassis serves as a heatsink or where a heatsink is common to several
non–isolated packages
...
Newer packages, such as the ON Semiconductor Full Pak and EMS modules, contain the
electrical isolation material within, thereby saving the equipment manufacturer the burden of addressing
the
isolation problem
...
With many isolation materials reduction of interface thermal resistance of between 2 to
1 and 3 to 1 are typical when grease is used
...
Similar
materials to those shown are available from several manufacturers
...
Referring to Figure 5–7, one may conclude that when high power is handled, beryllium oxide is
unquestionably the best
...
(It should not be cut or abraided, as the dust
is highly toxic
...
It is a popular material for low power applications because of its low cost ability to withstand high
temperatures, and ease of handling in contrast to mica which chips and flakes easily
...
They cover a wide range of insulation
resistance, thermal resistance and ease of handling
...
Silicone rubber insulators have gained favor because they are somewhat conformal under pressure
...
When
first introduced, they suffered from cut–through after a few years in service
...
By comparing Figures 5–7(c)
and 5–7(d), it can be noted that Thermasil, a filled silicone rubber without grease, has about the same
interface thermal resistance as greased mica for the TO–220 package
...
com
43
Figure 5–7
...
0
1
...
4
(2)
(3)
(4)
(5)
1
...
0
0
...
6
0
...
2
(1) Thermalfilm,
...
05) thick
(2) Mica,
...
08) thick
(3) Mica,
...
05) thick
(4) Hard anodized,
...
51) thick
(5) Aluminum oxide,
...
57) thick
(6) Berylium oxide,
...
57) thick
(7) Bare joint Ċ no finish
(8) Grafoil,
...
13) thick*
*Grafoil is not an insulating material
(8)
0
0
...
8
0
...
6
0
...
4
(2) (3)
(5)
(4)
0
...
2
0
...
022 (
...
003 (
...
002 (
...
020 (
...
009 (
...
006 (
...
005 (
...
0
5
4
3
(1)
2
(2)
(3)
(4)
(7)
1
0
0
1
2
3
4
5
6
MOUNTING SCREW TORQUE (IN-LBS)
0
1
2
4
5 6
3
MOUNTING SCREW TORQUE (IN-LBS)
(c) TO–220
Without Thermal Grease
(d) TO–220
With Thermal Grease
Data Courtesy of Thermalloy
http://onsemi
...
Table 5–2 shows measured performance
of a number of these insulators under carefully controlled, nearly identical conditions
...
It is also clear that some of the insulators are
much more tolerant than others of out–of–flat surfaces
...
The Bergquist K–10 pad, for example, is described as having about 2/3 the interface
resistance of the Sil Pad 1000 which would place its performance close to the Chomerics 1671 pad
...
Published data from AAVID shows RθCS below 0
...
However, surface flatness and other details are not specified so a comparison
cannot be made with other data in this note
...
Thermal Resistance of Silicone Rubber Pads
Manufacturer
Product
RθCS @ 3 Mils*
Wakefield
Bergquist
Stockwell Rubber
Bergquist
Thermalloy
Shin–Etsu
Bergquist
Chomerics
Wakefield
Bergquist
Ablestik
Thermalloy
Chomerics
Delta Pad 173–7
Sil Pad K–4
1867
Sil Pad 400–9
Thermalsil II
TC–30AG
Sil Pad 400–7
1674
Delta Pad 174–9
Sil Pad 1000
Thermal Wafers
Thermalsil III
1671
0
...
752
0
...
735
0
...
664
0
...
592
0
...
529
0
...
440
0
...
5 Mils*
1
...
470
1
...
205
1
...
260
1
...
190
0
...
935
0
...
035
0
...
The thermal resistance of some silicone rubber insulators is sensitive to surface flatness when used
under a fairly rigid base package
...
Observe that the “worst case” encountered (7
...
In order for Thermasil
III to exceed the performance of greased mica, total surface flatness must be under 2 mils, a situation that
requires spot finishing
...
Effect of Total Surface Flatness on Interface
Resistance Using Silicon Rubber Insulators
INTERFACE THERMAL RESISTANCE (° C/W)
1
...
0
(1)
(2)
0
...
6
0
...
009 inches (0
...
(2) Thermalsil Il, 0
...
15 mm) thick
...
2
0
Data Courtesy of Thermalloy
0
0
...
004
0
...
008
TOTAL JOINT DEVIATION FROM FLAT OVER
TO-3 HEADER SURFACE AREA (INCHES)
http://onsemi
...
01
Silicon rubber insulators have a number of unusual characteristics
...
For example, in a study of the Cho–Therm 1688 pad
thermal interface impedance dropped from 0
...
70°C/W at the end of 1000 hours
...
74°C/W
...
With non–conformal
materials, a reduction in torque would have increased the interface thermal resistance
...
Table 5–3 shows data obtained from two sources
...
Appendix B
discusses the variables which need to be controlled
...
The conclusions to be drawn from all this data is that some types of silicon rubber pads, mounted dry,
will out perform the commonly used mica with grease
...
Table 5–3
...
033
0
...
233
—
0
...
267
0
...
009
0
...
329
0
...
433
0
...
533
0
...
400
0
...
440
0
...
Small particles of foreign
matter can puncture the insulation, rendering it useless or seriously lowering its dielectric strength
...
Dust and other foreign material can shorten creepage distances significantly, so having a clean
assembly area is important
...
Use of
thermal grease usually raises the withstand voltage of the insulation system but excess must be removed
to avoid collecting dust
...
Insulated Electrode Packages
Because of the nuisance of handling and installing the accessories needed for an insulated
semiconductor mounting, equipment manufacturers have longed for cost–effective insulated packages
since the 1950s
...
Although effective, the assembly is costly and requires manual
mounting and lead wire soldering to terminals on top of the case
...
These offerings presently
consist of multiple chips and integrated circuits as well as the more conventional single chip devices
...
The first has insulation between
the semiconductor chips and the mounting base; an exposed area of the mounting base is used to secure
the part
...
The Full Pak (Case 221C) illustrated in Figure 5–13, is an example of parts in the second category
...
com
46
Parts in the first category — those with an exposed metal flange or tab — are mounted the same as
their non–insulated counterparts
...
Fastener and Hardware Characteristics
Characteristics of fasteners, associated hardware, and the tools to secure them determine their
suitability for use in mounting the various packages
...
Compression Hardware
Normal split ring lock washers are not the best choice for mounting power semiconductors
...
A very useful piece of hardware is the conical, sometimes called a Belleville washer,
compression washer
...
When installing, the assembler applies
torque until the washer depresses to half its original height
...
) The washer
will absorb any cyclic expansion of the package, insulating washer or other materials caused by
temperature changes
...
They are used
with the large face contacting the packages
...
Called a Sync Nut, the patented device can be soldered to a PC board and the
semiconductor mounted with a 6–32 machine screw
...
Characteristics of the Conical Compression Washers Designed
for Use with Plastic Body Mounted Semiconductors
PRESSURE ON PACKAGE (LBĆF)
280
240
200
160
120
80
40
0
0
20
40
60
80
DEFLECTION OF WASHER DURING MOUNTING (%)
_________________________________
(4) ITW Shakeproof, St
...
http://onsemi
...
When only a few watts are being dissipated, the small
board–mounted or free–standing heat dissipators with an integral clip, offered by several manufacturers,
result in a low cost assembly
...
In order to provide proper pressure, the clip must be specially designed for a particular heatsink
thickness and semiconductor package
...
In addition to fast
assembly, the clip provides lower interface thermal resistance than other assembly methods when it is
designed for proper pressure to bear on the top of the plastic over the die
...
Machine Screws
Machine screws, conical washers, and nuts (or Sync Nut) can form a trouble–free fastener system for
all types of packages which have mounting holes
...
Torque ratings
apply when dry; therefore, care must be exercised when using thermal grease to prevent it from getting
on the threads as inconsistent torque readings result
...
Without a washer, cracking of the plastic case may occur
...
However, during the tapping
process with a standard screw, a volcano–like protrusion will develop in the metal being threaded; an
unacceptable surface that could increase the thermal resistance may result
...
If a self–tapping process
is desired, the screw type must be used which roll–forms machine screw threads
...
When a rugged metal
flange–mount package is being mounted directly to a heatsink, rivets can be used provided press–riveting
is used
...
Pop–riveting should never be used because
the high crimping force could cause deformation of most semiconductor packages
...
The hollow rivet, or eyelet, is preferred over solid rivets
...
Use of sharp blows could damage the
semiconductor die
...
However, user demand has led to the development of new packages
for this application
...
It is important that the semiconductor temperature not exceed the specified
maximum (usually 260°C) or the die bond to the case could be damaged
...
http://onsemi
...
(5)
Highly conductive types are available; a 10 mil layer has approximately 0
...
Different types are offered: high strength types for non–field–serviceable systems or low
strength types for field–serviceable systems
...
Plastic Hardware
Most plastic materials will flow, but differ widely in this characteristic
...
As previously discussed, loss of contact pressure will increase
interface thermal resistance
...
Details
pertaining to each type are discussed in following sections
...
To prevent galvanic action from occurring when devices are used on aluminum heatsinks in a corrosive
atmosphere, many devices are nickel or gold–plated
...
Another factor to be considered is that when a copper based part is rigidly mounted to an aluminum
heatsink, a bimetallic system results which will bend with temperature changes
...
If bending is excessive and the package is mounted by two
or more screws the semiconductor chip could be damaged
...
Mounting the component parallel to the heatsink fins to provide increased stiffness
...
Allowing the heatsink holes to be a bit oversized so that some slip between surfaces can
occur as temperature changes
...
Using a highly conductive thermal grease or mounting pad between the
heatsink and semiconductor to minimize the temperature gradient and allow for movement
...
The rugged base and distance between die and mounting hose combine to make it extremely
difficult to cause any warpage unless mounted on a surface which is badly bowed or unless one side is
tightened excessively before the other screw is started
...
After the screws are finger–tight the hardware should be
torqued to its final specification in at least two sequential steps
...
Machine screws (preferred), self–tapping screws, islets or rivets
may be used to secure the package using guidelines in the previous section, Fastener and Hardware
Characteristics
...
Moran, Heat Dissipation Through Thermalloy Conductive Adhesives, EMTAS ’83 Conference,
February 1–3, Phoenix, AZ; Society of Manufacturing Engineers, One SME Drive, P
...
Box 930, Dearborn, MI 48128
...
com
49
Figure 5–10
...
6 Sheet Metal Screw
Power
Transistor
Insulator
Insulating
Bushing
Heatsink
Socket
Tab Mount
The tab mount class is composed of a wide array of packages as illustrated in Figure 5–11
...
The rectangular washer shown in Figure
5–12a is used to minimize distortion of the mounting flange; excessive distortion could cause damage to
the semiconductor chip
...
140 inch (6–32 clearance)
...
250 inch
...
A maximum torque of 8 inch–pounds is suggested
when using a 6–32 screw
...
Such contact can result in damage to the plastic
body and internal device connections
...
TO–220 packages of other manufacturers may need a spacer or combination
spacer and isolation bushing to raise the screw head above the top surface of the plastic
...
com
50
The popular TO–220 package and others of similar construction lift off the mounting surface as
pressure is applied to one end
...
) To counter this tendency, at least one
hardware manufacturer offers a hard plastic cantilever beam which applies more even pressure on the
tab
...
Tab mount parts may also be
effectively mounted with clips as shown in Figure 5–14(c)
...
Interface thermal resistance with the
cantilever beam or clips can be lower than with screw mounting
...
Several Types of Tab Mounted Parts
CASE 221A
(TO–220AB)
CASE 314D
CASE 340
(TO–218)
Figure 5–12
...
4-40 Hardware is Used
...
Screw
a)ăis at Semiconductor Case Potential
...
Use Parts Listed Below
...
4-40 Pan or Hex Head Screw
6-32 Hex
Head Screw
Flat Washer
Insulating Bushing
(1) Rectangular Steel
Washer
Semiconductor
(Case 221, 221A)
Semiconductor
(Case 221,221A)
(2) Rectangular
Insulator
Heatsink
Rectangular
Insulator
Heatsink
(2) Bushing
(3) Flat Washer
Compression Washer
(4) Conical Washer
(1)ăUsed with thin chassis and/or large hole
...
(3)ăRequired when nylon bushing is used
...
Tripp Ave
...
http://onsemi
...
They
have been designed to feature minimum size with no compromise in thermal resistance
...
Because
the mounting force is applied to plastic, the mounting procedure differs from a standard TO–220 and is
similar to that of the Thermopad
...
With screws or eyelets, a conical washer should be used which applies the proper force to
the package over a fairly wide range of deflection and distributes the force over a fairly large surface area
...
Characteristics of a suitable conical washer is shown in Figure 5–9
...
As shown in Figure 5–14(c), one properly chosen clip, inserted into two slotted
holes in the heatsink, is all the hardware needed
...
This occurs because the clip
bears directly on top of the die and holds the package flat while the screw causes the package to lift up
somewhat under the die
...
) The interface should consist of a layer of thermal
grease or a highly conductive thermal pad
...
Both methods afford a major reduction in
hardware as compared to the conventional mounting method with a TO–220 package which is shown in
Figure 5–14(a)
...
Plastic Body Mounted Packages
CASE 221C
(Full Pak)
CASE 221D
(Full Pak)
CASE 340B
(Full Pak)
Figure 5–14
...
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52
(c) Clip Mounted
Full Pak
Surface Mount
Although many of the tab mount parts have been surface mounted, special small footprint packages
for mounting power semiconductors using surface mount assembly techniques have been developed
...
The thermal resistance values of the solder
interface is well under 1°C/W
...
Standard glass–epoxy 2 oz
...
As Figure 5–16 shows, thermal resistance assymtotes to about 20°C/W at 10 square
inches of board area, although a point of diminishing returns occurs at about 3 square inches
...
A dielectric coating designed for
low thermal resistance is overlayed with one or two ounce copper foil for the preparation of printed
conductor traces
...
(7) The substrate may be an effective heatsink
itself, or it can be attached to a conventional finned heatsink for improved performance
...
Figure 5–15
...
0
2
...
Copper
L
Minimum
Size Pad
60
1
...
8
50
40
1
...
4
PD(max) for TA = 50°C
Free Air
Mounted
Vertically
ÎÎÎÎ
ÎÎÎÎ
ÎÎÎÎ
ÎÎÎÎ
R θ JA, THERMAL RESISTANCE
100
0
...
0
10
15
20
25
30
PD, MAXIMUM POWER DISSIPATION (W)
Figure 5–16
...
http://onsemi
...
The leads of the various metal power
packages are not designed to support the packages; their cases must be firmly supported to avoid the
possibility of cracked seals around the leads
...
The leads should be as short as possible to increase vibration resistance and reduce thermal resistance
...
A plastic support for the TO–220
Package and other similar types is offered by heatsink accessory vendors
...
Manufacturers have provided sockets for many of the
packages available from ON Semiconductor
...
Sockets with Kelvin connections are necessary to obtain accurate voltage readings
across semiconductor terminals
...
Change in mechanical dimensions as a result of thermal cycling over
operating temperature extremes must be considered
...
Metal Packages
The pins of metal packaged devices using glass to metal seals are not designed to handle any
significant bending or stress
...
Wires may be attached using sockets, crimp
connectors or solder, provided the data sheet ratings are observed
...
Plastic Packages
The leads of the plastic packages are somewhat flexible and can be reshaped although this is not a
recommended procedure
...
Numerous lead and tab–forming options are available from ON Semiconductor on large
quantity orders
...
If, however, lead–bending is done by the user, several basic considerations should be observed
...
For forming small
quantities of units, a pair of pliers may be used to clamp the leads at the case, while bending with the fingers
or another pair of pliers
...
The following rules should be observed to avoid damage to the package
...
A leadbend radius greater than 1/32 inch for TO–220
...
No twisting of leads should be done at the case
...
No axial motion of the lead should be allowed with respect to the case
...
Force in this direction
greater than 4 pounds may result in permanent damage to the device
...
When wires are used for connections, care should be exercised to assure
that movement of the wire does not cause movement of the lead at the lead–to–plastic junctions
...
Wire wrapping of the leads is permissible, provided that the lead is restrained between the plastic case
and the point of the wrapping
...
http://onsemi
...
Alcohol and unchlorinated freon solvents are generally
satisfactory for use with plastic devices, since they do not damage the package
...
When using an ultrasonic cleaner for cleaning circuit boards, care should be taken with regard to
ultrasonic energy and time of application
...
Thermal System Evaluation
Assuming that a suitable method of mounting the semiconductor without incurring damage has been
achieved, it is important to ascertain whether the junction temperature is within bounds
...
In this case, use must be made of transient thermal resistance data
...
Other applications, notably switches driving highly reactive loads, may create severe current crowding
conditions which render the traditional concepts of thermal resistance or transient thermal impedance
invalid
...
Fortunately, in many applications, a calculation of the average junction temperature is sufficient
...
(See Appendix A
...
Average operating junction temperature can be computed from the following equation:
TJ = TC + RθJC × PD
where, TJ = junction temperature (°C),
TC = case temperature (°C),
RθJC = thermal resistance junction–to–case as specified on the data sheet (°C/W),
PD = power dissipated in the device (W)
...
Two commonly
used empirical methods are graphical integration and substitution
...
The pictures should be taken with the temperature
stabilized
...
Each pair of voltage and current values are multiplied together to give instantaneous values of power
...
The quotient is the average power dissipation
...
Substitution
This method is based upon substituting an easily measurable, smooth dc source for a complex
waveform
...
Case temperature is monitored
...
The dc supply is adjusted so that the
semiconductor case temperature remains approximately constant when the switch is thrown to each
position for about 10 seconds
...
It is generally necessary that a Kelvin connection be used for the device voltage measurement
...
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55
Appendix A
Thermal Resistance Concepts
The basic equation for heat transfer under steady–state conditions is generally written as:
q = hA∆T
(1)
where, q = rate of heat transfer or power dissipation (PD),
h = heat transfer coefficient,
A = area involved in heat transfer,
∆T = temperature difference between regions of heat transfer
...
From Equation 1, thermal resistance (Rθ) is
(2)
Rθ = ∆T/q = 1/hA
The coefficient (h) depends upon the heat transfer mechanism used and various factors involved in that
particular mechanism
...
Note that
T could be thought of as a voltage thermal resistance corresponds to electrical resistance (R); and, power
(q) is analogous to current (I)
...
Figure A–1
...
The thermal resistance junction–to–ambient is the sum of the individual components
...
The value for the interface thermal resistance (RθCS) may be significant compared to the other thermal
resistance terms
...
http://onsemi
...
The thermal resistance of
the semiconductor is also variable; it is a function of biasing and temperature
...
In some
applications such as in RF power amplifiers and short–pulse applications, current density is not uniform
and localized heating in the semiconductor chip will be the controlling factor in determining power
handling ability
...
All that’s apparently
needed is a thermocouple on the semiconductor case, a thermocouple on the heatsink, and a means of
applying and measuring dc power
...
The fastening method may also be a factor
...
Consequently, values for interface thermal resistance
presented by different manufacturers are not in good agreement
...
When fastening the test package in place with screws, thermal conduction may take place through the
screws, for example, from the flange ear on a TO–204AA package directly to the heatsink
...
MIL–I–49456 allows screws to be used in tests for interface thermal resistance
probably because it can be argued that this is “application oriented”
...
The ON Semiconductor fixture uses an insulated clamp arrangement to
secure the package which also does not provide a conduction path
...
These two methods often yield different values for
interface thermal resistance
...
To avoid this, the package should be covered
with insulating foam
...
Another significant cause for measurement discrepancies is the placement of the thermocouple to
measure the semiconductor case temperature
...
The
mounting pressure at one end causes the other end — where the die is located — to lift off the mounting
surface slightly
...
Use of
a spreader bar under the screw lessens the lifting, but some is inevitable with a package of this structure
...
JEDEC TO–220 Package Mounted to Heatsink Showing Various
Thermocouple Locations and Lifting Caused by Pressure at One End
Die
E
...
A
...
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57
Three thermocouple locations are shown
...
The
thermocouple is held in place by a spring which forces the thermocouple into intimate contact with the
bottom of the semi’s case
...
The thermocouple is swaged in place
...
The thermocouple is soldered into position
...
Consider the situation depicted in
Figure B–1
...
Consequently, the temperature at the JEDEC
location is hotter than at the Thermalloy location and the ON Semiconductor location is even hotter
...
Thus the choice of reference point for the case
temperature is quite important
...
If a mica washer with grease is installed between the semiconductor package and
the heatsink, tightening the screw will not bow the package; instead, the mica will be deformed
...
In this case, a small temperature
drop will exist across the vertical dimension of the package mounting base so that the thermocouple at the
EIA location will be the hottest
...
The ON
Semiconductor location will be coolest
...
It is of significance because
power ratings are supposed to be based on this reference point
...
The ON Semiconductor location is chosen to obtain the highest temperature of the case at a point
where, hopefully, the case is making contact to the heatsink
...
However, this location is not easily accessible to the user
...
However, it
also blemishes the case and may yield results differing up to 1°C/W for a TO–220 package mounted to
a heatsink without thermal grease and no insulator
...
When compared to the specified junction–to–case values of some of the higher power semiconductors
becoming available, however, the difference becomes significant and it is important that the semiconductor
manufacturer and equipment manufacturer use the same reference point
...
The washer is flat to within
1
...
, and has an imbedded thermocouple near its center
...
It is also easy to use but has not become widely accepted
...
If the junction–to–case
values remain relatively constant as insulators are changed, torque varied, etc
...
http://onsemi
...
Regulator Performance Requirements:
Output Voltage, VO = +10 V ± 0
...
0 A, current limited
Load Regulation, ≤ 0
...
1%
Output ripple, ≤ 2
...
1
...
This regulator must be current boosted to obtain the required
1
...
A rough cost estimate shows that an MC1723C series pass element combination is
the most economical approach
...
Circuit Configuration
In Section 3, an appropriate circuit configuration is found
...
3
...
a) Cref is chosen to be 0
...
b) R1 + R2 is chosen to be ≈ 10 k
...
0 V
(R1 + R2) = 0
...
0 k
VO
d) Since Vref can vary by as much as ± 5% for the MC1723C, R2 should be made variable by at least
that much, so that VO can be set to the required value of +10 V ± 0
...
R2 is therefore chosen to
consist of a 62 k resistor and a 2
...
e) R1 = 10 k – R2 = 10 k – 7
...
0 k
f) RSC ≈
0
...
6 V
= 0
...
56 Ω, 1
...
=
ISC
1
...
2 k
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...
Determination of Input Voltage (Vin)
There are two basic constraints on the input voltage: 1) the device limits for minimum and maximum
Vin and, 2) the minimum input–output voltage differential
...
5 V ≤ Vin ≤ 40 V and (Vin – VO) ≥ 3
...
0 V, where φ = VBEon ≈ 0
...
Since the base–emitter junction drops of Q1 and RSC have been added to the circuit, they must be
added to the minimum value of (Vin – VO)
...
0 V = 10 + 1
...
2 V
This condition also satisfies the requirement for a minimum Vin of 9
...
In order to simplify the design of the input supply (see Section 8), Vin is chosen to be 16 V average with
a 3
...
This assures that the input voltage is always above
the required minimum value of 14
...
Now, the output ripple can be determined
...
With an input ripple of 3
...
0 mVpp, which meets the regulator output ripple requirements
...
Selection of the Series Pass Element (Q1)
The transistor type chosen for Q1 must have the following characteristics (see Section 4):
a) VCEO ≥ Vin(max)
b) IC(max) ≥ ISC
ISC
@ VCE = Vin – VO – φ, where φ = VBEon [ 0
...
0 A
hfe ≥ 25 @ VCE = 5
...
0 A
PD(max) ≥ 16 W
θJC = 1
...
0 A @ 16 V
A 2N3055 transistor is chosen as a suitable device for Q1 using the selection guide of Section 4 and the
transistor data sheets (available from the device manufacturer)
...
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60
6
...
1 from Section 15)
(Equation 6
...
2)
From the 2N3055 data sheet, TJ = 200°C and θJC = 1
...
The transistor will be mounted with thermal
grease directly to the heatsink
...
1°C/W from Table 15–1
...
2:
θSA =
200°C – 70°C
– (1
...
1) °C/W
16 V × 1
...
6°C/W
A commercial heatsink is now chosen from Table 15–2 or one custom designed using the methods given
in Section 15
...
2°C/W, was used
...
Clamp Diode
Since the regulator can power a load which is also connected to a negative supply, a 1N4001 diode
is connected to the output for protection
...
Figure 6–1
...
0 A Design Example
2N3055 on
Thermalloy #6003
2
MC1723CL
6
VBE
IO
11
R3
2
...
0 A
RSC
Q1
Vin = +16 V
0
...
0W
3
θJA = 150°C/W
3
...
1µF
Cref
13
100pF
7
R2
2
...
2k
8
...
http://onsemi
...
If problems do occur,
the trouble can be traced to a design error in 99
...
As a troubleshooting aid to the designer,
the following guide is presented
...
However, the checklist provided will help the designer pinpoint the problem in the
majority of cases
...
If, after carefully rechecking the circuit, the designer is not successful in resolving the problem, seek
assistance from the factory by contacting the nearest ON Semiconductor Sales office
...
2
...
4
...
Emitter–base resistor in “PNP” type boost configuration too large
2
...
0 mA ‘‘minimum’’ load
...
Improper circuit configuration
5
3
5
5
4
3
Loss of regulation at heavy loads
1
...
3
...
5
...
1
...
Input Voltage Transient Vin(max), VCEO
Pass element fails during short
circuit
...
Insufficient pass element ratings SOA, IC(max)
2
...
1
...
Inadequate heatsinking
2
1
...
IC current or SOA capability exceeded as load (capacitor)
2, 3
4
3
5
15
15
2, 4, 5
is charged up
...
1
...
Out polarity reversal
2
...
(Usually seen with op amps, current sources, etc
...
Input supply filter capacitor ground loop
5
http://onsemi
...
The type of rectifier circuit used can be either a half–wave, full–wave, or full–wave bridge type,
as shown in Figure 8–1
...
The full–wave bridge is usually used in all other
high–current applications
...
Rectification Schemes
(a) Half–Wave
(b) Full–Wave or Full–Wave Center Tap
(c) Full–Wave Bridge
In this section, specification of the filter capacitor, rectifier and transformer ratings will be discussed
...
A detailed description of this type of filter can be found
in the reference listed at the end of this section
...
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63
1
...
The curves shown in Figures 8–2 through 8–5 give all the
required design information for half–wave and full–wave rectifier circuits
...
However, the rectifier forward drop often assumes more significance than the dynamic
resistance in low–voltage supply applications, as the dynamic resistance can generally be neglected when
compared with the sum of the transformer secondary–winding resistance plus the reflected
primary–winding resistance
...
0 V, which clearly cannot be ignored in supplies of 12 V or less
...
Relation of Applied Alternating Peak Voltage to Direct Output Voltage
in Half–Wave Capacitor–Input Circuits
100
0
...
5
1
2
RS
90
VM
C
VC
RL
4
80
% VC(DC) / VM
60
20
25
30
35
40
50
60
70
80
90
100
50
40
30
20
10
0
0
...
0
10
ωCRL
ω = 2 π f, f = Line Frequency
C in Farads
RL in Ohms
________________________________
(1)From
O
...
Schade, Proc
...
31, p
...
http://onsemi
...
5
15
70
Figure 8–3
...
05
0
...
5
1
100
VM
VM
90
RS
RS
Full-Wave
C VC
RL
2
4
Full-Wave
Bridge
6
80
8
12
...
1
1
...
02
0
...
1
0
...
5
1
...
0
5
...
0
2
...
0
5
...
0
10
20
30
nωCRL
50
70
100
200
300
1000
40
I F(Peak) IF(AV) (PER DIODE)
% RS /nR L
10
0
...
05
0
...
2
0
...
0
7
2
...
0
5
10
30
3
100
1
...
0
3
...
0
7
...
com
65
70
100
200
300
500
C in Farads
RL in Ohms
RS = RMS Equivalent Source Resistance
700 1000
% RS /nR L
F = IF(R ) I F(AV) (PER DIODE)
S
Figure 8–4
...
Root–Mean–Square Ripple Voltage for Capacitor–Input Circuits
100
70
50
Circuit
A
r f , RIPPLE FACTOR (%)
30
20
Half-Wave
Parameter
RS/RL (%)
A
0
...
0
10
30
0
...
0
10
30
Full-Wave
10
7
...
0
A
A
3
...
0
1
...
7
0
...
3
0
...
1
1
...
0
3
...
0
7
...
Figure 8–3 shows that a circuit
must operate with ωCRL ≥ 10 in order to hold the voltage reduction to less than 10% and ωCRL ≥ 40 to obtain
less than 2
...
However, it will also be seen that these voltage reduction figures require RS/RL,
where RS is now the total series resistance, to be about 0
...
These ratios
can be satisfied by many diodes; however, they may not be able to tolerate the turn–on surge current
generated when the input–filter capacitor is discharged and the transformer primary is energized at the
peak of the input waveform
...
In order to
control this turn–on surge, additional resistance must often be provided in series with each rectifier
...
If small voltage
reduction, that is good voltage regulation, is required, a much larger diode is necessary than that
demanded by the average current rating
...
The maximum instantaneous surge current is approximately VM/RS
and the capacitor charges with a time constant τ ≈ RS C1
...
3 ms
...
http://onsemi
...
Design Procedure
A) From the regulator circuit design (see Section 6), we know:
VC(DC) = the required full load average dc output voltage of the capacitor input filter
VRipple(pp) = the maximum no load peak–to–peak ripple voltage
Vm = the maximum no load output voltage
IO = the full–load filter output current
f = the input ac line frequency
B) From Figure 8–5, we can determine a range of minimum capacitor values to obtain sufficient ripple
attenuation
...
1)
2 √ 2 VC(DC)
A range for ωCRL can now be found from Figure 8–5
...
If the range of ωCRL values initially determined from Figure 8–5 is above ]10, RS/RL can be
found from Figures 8–2 and 8–3 using the lowest ωCRL value
...
D) Once ωCRL is found, the value of the filter capacitor (C) can be determined from:
ωCRL
C=
VC(DC)
2πf
IO
ǒ
Ǔ
(8
...
Average current per diode;
IF(avg) = IO for half–wave rectification
(8
...
RMS and Peak repetitive rectifier current ratings can be determined from Figure 8–4
...
The rectifier PIV rating is 2 Vm for the half–wave and full–wave circuits, Vm for the full
wave bridge circuit
...
4
...
4)
where, ESR = minimum equivalent series resistance of filter capacitor from its data sheet
...
Secondary leg RMS voltage, VS = {Vm + (n) 1
...
Total resistance of secondary and any external resistors to be equal to RS found from
Figures 8–2, 8–3, and 8–4 (see Part C)
...
Secondary RMS current; half–wave = Irms
full–wave = Irms
full–wave bridge = √ 2 Irms
where, Irms = rms rectifier current (from part E
...
2)
...
Transformer VA rating; half–wave = VS Irms
full–wave = 2 VS Irms
full–wave bridge = VS Irms (√ 2 )
where, Irms = rms rectifier current (from part E
...
2) and,
where, VS = secondary leg RMS voltage
...
com
67
(8
...
6)
(8
...
Design Example
A) Find the values for the filter capacitor, transformer rectifier ratings, given
Full–Wave Bridge Rectification;
VC(DC) = 16 V
VRipple(pp) = 3
...
0 A
f = 60 Hz
B) Using Equation (8
...
6%
2 √ 2 (16)
rf =
from Figure 8
...
64 = 64%
25
RS/RL = 20% or RS = 0
...
2
ǒVC(DC)Ǔ = 0
...
2 Ω
D) From Equation (8
...
IF(avg) = IO/2 = 0
...
3)
2
...
0 A from Figure 8–4
3
...
2 × IF(AVG) = 2
...
PIV = VM = 25 V (use 50 V for safety margin)
5
...
2 = 7
...
4), neglecting capacitor ESR
...
VS = {VM + n(1
...
5)}
2
...
2 Ω
3
...
4 A, (from Equation (8
...
4
...
7), the transformer should have a 27 VA rating
...
The designer should take these factors into account
when designing his input supply
...
________________________________
REFERENCES
1
...
H
...
IRE, Vol
...
2
...
http://onsemi
...
Its performance and size advantages meet the needs of today’s
modern and compact electronic equipment and the increasing variety of components directed at these
applications makes new designs even more practical
...
Also included are valuable design tips on both the major passive and active
components needed for a successful design
...
Comparison with Linear Regulators
The primary advantages of a switching power supply are efficiency, size,and weight
...
However switchers are being accepted in the industry, particularly
where size and efficiency are of prime importance
...
In the past the switcher’s advantage versus the linear regulator was in the high power arena where
passive components such as transformers and filters were small compared to the linear regulator at the
same power level
...
In recent years, Switchers have been significantly cost reduced because
designers have been able to simplify the control circuits with new, cost effective integrated circuits and have
found even lower cost alternatives in the passive component area
...
Switcher
efficiencies run from 70% to 80% but occasionally fall to (60% to 65%) when linear post regulators are used
for the auxiliary outputs
...
Most linears
operate with typical efficiencies of only 30%
...
Other
characteristics such as static regulation specs are comparable, while ripple and load transient response
are usually worse
...
Very often a 500 mV switching spike
at the output may be attenuated considerably at the load itself due to the series inductance of the
connecting cables and the additional filter capacitors found in many logic circuits
...
Hold–up time is greater for switchers because it is easier to store energy
in high voltage capacitors (200 V to 400 V) than in the lower voltage (20 V to 50 V) filter capacitors common
to linear power supplies
...
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...
20 kHz Switcher versus Linear Performance
Parameter
Switcher
Linear
Efficiency
75%
30%
Size
2
...
5 W/in3
Weight
40 W/lb
10 W/lb
Line and Load Regulation
0
...
1%
Output Ripple Vpp
50 mV
5
...
0 ms
20 µs
Hold–Up Time
20 ms to 30 ms
1
...
0 ms
Basic Configurations
A switching power supply is a relatively complex circuit as is shown by the four basic building blocks
of Figure 9–1
...
It is here
that the work of chopping the rectified line at a high frequency (20 kHz to 200 kHz) is done
...
The remaining blocks support this basic function
...
The task of regulating the output voltage is left to the control circuit which closes the loop from the output
to the inverter
...
Basically, the on–time of the square wave drive to the
inverter is controlled by the output voltage
...
The inverter configurations used in today’s switchers actually evolved from the buck and boost circuits
shown in Figures 9–2a and 9–2b
...
The forward
converter family which includes the push–pull and half bridge circuits evolved from the buck regulator
(Figure 9–2a)
...
The buck circuit interrupts the line and provides a variable pulse width square wave to a simple averaging
LC filter
...
This is satisfactory for most analysis work and
only the transformer turns ratio will have to be adjusted slightly to compensate for IR drops, diode drops,
and transistor saturation voltages
...
However, the flyback regulators which evolved from this
configuration delivers only the energy stored in the choke to the load
...
Here, when the switch is opened, only the stored
inductive energy is delivered to the load
...
Analysis of the boost regulator begins by dealing with the choke as an energy storage element which
delivers a fixed amount of power to the load: PO = 1/2 L IfO where, I = the peak choke current; fO = the
operating frequency; and, L = the inductance
...
It also makes a good battery charger
...
PORL = I
VO =
2
In this case, the choke current is proportional to the on–time or duty cycle of the switch and regulation for
fixed loads simply involves varying the duty cycle as before
...
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...
Functional Block Diagram — Switching Power Supply
50-60 Hz
20 to 25 kHz
DC
DC
High Frequency
Inverter
Output
Line
Input Rectifier
and Filter
Output Rectifier
and Filter
Control Circuit
PWM
Note: Input to output isolation is not shown
...
Nonisolated DC–DC Converters
+ Vin
+ Vout
+ Vin
Vout ≤ Vin
+ Vout
Vout > Vin
(a) Buck (Step–Down)
(b) Boost (Step–Up)
- Vout
+ Vin
(c) Buck–Boost Regulator
which Resembles the Flyback Regulator
(Step–Up or Down)
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...
They lead to what is termed a “load dump” problem
...
Practical solutions to this
problem include limiting the minimum load and using the right amount of filter capacitance to give the
regulator time to respond to this change
...
The latter are responding to the growth in microprocessor based equipment
as well as computer peripherals
...
Also, a trend
to higher switching frequencies to reduce size and cost even further has begun
...
At this time there are a lot of safety and noise specifications
...
It seems probable, however, that system
engineers or power supply designers will be able to add the necessary line filters and EMI shields without
evoking a significant cost penalty in the design
...
Switching power supply
components have actually evolved from components used in similar applications
...
The ultimate effect of this evolution will be to further simplify, cost reduce and increase the reliability
of these designs
...
SMPS Specifications
Specification
Area
UL 478, VDE 0730, VDE 0806
Safety
VDE 0871, VDE 0875
MIL–STD–217D
MIL–STD–461A
DOD–STD–1399
FCC Class A & B
CSA C22
...
As requirements for 2
...
0 V supplies emerge for use by fine geometry VLSI chips, the
only way to maintain decent conversion efficiency is to develop lower forward drop rectifiers
...
0 V and 5
...
At this time, ON
Semiconductor offers low VF Schottky and area efficient TMOS III FETs for this task and is considering a
variety of additional technology options
...
3 V to 0
...
The indirect approach involves using FETs or bipolar
transistors and slightly more complex circuitry like that shown in Figure 9–3
...
2 V and, in addition, the bipolar will have high EBOs (30 V) and high gain (100) with a recovery
time of 100 ns
...
The series resonant power supply topology seems to offer the possibility of
working in the 1
...
If components like the relatively exotic power transformer can be cost
reduced, then it will be possible for this topology to become dominant in the market
...
In a design now being studied in ON Semiconductor’s advanced products laboratory,
standard FETs, Schottkys and ultrafast rectifiers all appear to work very well at 1
...
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...
Synchronous Rectifiers
for 3
...
Synchronous Rectifier
Requirements
G
Output
Voltage
Rectifier Characteristics
Primarily to reduce VF
also to reduce trr
VR
5
...
5 V–1
...
3 V–0
...
0 V
S
S
VF
20 V–40 V
VO
G
Note: The FET must be operated below VF of the
diode in order to gain the trr advantage
...
SRPS Features
Feature
Description
High Frequency
Today’s line operated designs use sine waves in the 500 kHz to 1
...
Small Size
The ferrite transformer and polypropylene coupling capacitor are smaller than those
found in lower frequency square wave designs
...
Efficient
Because switching losses are reduced, efficiency is high (typically 80%)
...
Special Control Circuit
PDM (density) rather than PWM (width) control is used and requires a control IC
with a programmable VCO
...
Figure 9–4
...
0MHz
FET
Inverter
Series
Resonant
Tank and
Transformer
Schottky
Rectifiers
Output
Filter
PDM
Control Circuit
60Hz
1
...
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20kHz
Output
SECTION 10
SWITCHING REGULATOR TOPOLOGIES
FET and Bipolar Drive Considerations
There are probably as many base drive circuits for bipolars as there are designers
...
Many of today’s
common drive circuits are shown in Figure 10–1
...
FET drive circuits are another alternative
...
This transformer coupled circuit will produce forward and reverse voltages applied to the FET gate
which vary with the duty cycle as shown
...
And yet, minimum gate drive
levels of 10 V are still available with duty cycles up to 50%
...
Finally, one point that
is not obvious when looking at the circuit is that FETs can be directly coupled to many ICs with only
100 mA of sink and source capability and still switch efficiently at 20 kHz
...
0 A to 2
...
A simple example will serve to illustrate this point and also
show that the Miller effect, produced by CDG, is the predominant speed limitation when switching high
voltages (see Figure 10–2B)
...
0 V to 3
...
0 V to 8
...
Gate waveforms will show a porch at a point just above the threshold voltage which varies in duration
depending on the amount of drive current available and this determines both the rise and fall times for the
drain current
...
Typical Bipolar Base Drive Circuits
12V
MC34060
(a) Fixed Drive, Turn–Off
Energy Stored in Transformer
15Ω
9
MPS
U01
8
1
...
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Figure 10–2A
...
FET Drive Current Requirements
10 V
8
...
0 V
0V
VGS
100pF
CDG
500pF
Drive
Circuit
CGS
1
...
0 A
2
...
Miller Current for 30 ns:
ąąąIM = CDG dv/dt
= 100 pF ×
300 V
= 1
...
0 V
= 0
...
IM = CDGdv/dt and,
2
...
And IG is usually the lesser amount of current required to charge
the gate–to–source capacitance through the linear region (2
...
0 V)
...
IM = 100 pF × 300 V/30 ns = 1
...
IG = 500 pF × 6
...
1 A
This example shows the direct proportion of drive current capability to speed and also illustrates that for
most devices, CDG will have the greatest effect on switching speed and that CGS is important only in
estimating turn–on and turn–off delays
...
Today’s 400 V
FETs compete with bipolar transistors in many switching applications
...
The performance or
efficiency tradeoffs are analyzed using Figure 10–3, where typical power losses for switching transistors
versus frequency are shown
...
These curves are asymptotes of the actual device performance, but are useful in establishing the
“breakpoint” of various devices, which is the point where saturation and switching losses are equal
...
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TOTAL TRANSISTOR POWER LOSS (W)
PT = Pon + P swt
Figure 10–3
...
0 A (TJ = 100°C)
100
30
10
Bipolar
tf = 100 ns
FET
tf = 20 ns
3
...
0
1
...
0
10
Control Circuits
Over the years, a variety of control ICs for SMPS have been introduced
...
The basic regulating function is performed in the pulse
width modulator (PWM) section
...
The result is a variable duty cycle pulse train which, with suitable buffer or interface circuits, can
be used to drive the power switching transistor
...
Additionally, most ICs provide an error
amplifier and reference section shown as a means to process, compare and amplify the feedback signal
...
However, most of today’s current generation ICs have evolved with
the following capabilities or features:
• Programmable (to 500 kHz) Fixed Frequency Oscillator
• Linear PWM Section with Duty Cycle from 0% to 100%
• On Board Error Amplifiers
• On Board Reference Regulator
• Adjustable Deadtime
• Under Voltage (low VCC) Inhibit
• Good Output Drive (100 mA to 200 mA)
• Option of Single or Dual Channel Output
• Uncommitted Output Collector and Emitter or Totem Pole Drive Configuration
• Soft–Start
• Digital Current Limiting
• Oscillator Sync Capability
It is primarily the cost differences in these parts that determine whether all or only part of these features
will be incorporated
...
In addition to the control circuits listed in Table 10–2, ON Semiconductor also has two dc converter
control chips, the µA78S40 and the MC34063A
...
0 A switching
transistor and operate by dropping pulses from a fixed frequency, fixed duty cycle oscillator depending on
load demand
...
These ICs, like ON Semiconductor’s
MC34060A and MC34063A components, are used to run the low–power flyback type configurations and
are usually part of a three chip rather than a single chip system
...
When it is necessary to drive two or more power transistors, drive transformers are a practical interface
element and are driven by the conventional dual channel ICs
...
In this situation, an
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...
And
the error amplifier in this case is nothing more than a programmable zener like ON Semiconductor’s TL431
...
For
linear supplies, the pass transistor can fail shorted, allowing high line transformer voltage to the load
...
Table 10–1
...
Control Circuits
Overvoltage
Protection
(OVP)
Standard
TL431
Over/Undervoltage
Protection
(O/UVP)
MC3425
MC34161
High
Performance
MC3423
TL431A
Undervoltage
Sense MPU/MCU
Reset
MC34064–5
MC34164–3
MC34164–5
The list of available circuits is shown in Table 10–2 and a typical 0 V application is shown in Figure 10–4
...
The SCR will discharge C2 and either blow the fuse or cause the power supply to shut down
...
Crowbar Circuit
Power
Supply
C2
MC3423
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...
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...
They have very large
capacitive input filters with high inrush surge currents
...
Surge current limiting can be accomplished by adding RS and an SCR short after charging C1, as
shown in Figure 10–5, or by phase controlling the line voltage with a Triac
...
Surge Current Limiting for a Switching Power Supply
RS
AC
Line
Rectifier
Bridge
G
C1
RS
AC
Line
Rectifier
Bridge
G
C1
Transformer Design
With respect to transformer design, many of today’s designers would say don’t try it
...
It takes quite a bit of time
to develop a feel for this craft and be able to use both experience and intuition to find solutions to second
and third order problems
...
However, there is a considerable design challenge in this area and a great deal of satisfaction
can be obtained by mastering it
...
A
partial list of companies is shown in Table 10–3
...
E cores expose the windings to air so that heat is not trapped inside
and make it easier to bring out connections for several windings
...
The classic approach is to consult manufacturers charts like the
one shown in Figure 10–8 and then to pick a core with the required power handling ability
...
EC
cores offer a performance advantage (better coupling) but standard E cores cost less and are also used
in these applications
...
Size the wire for 500 circular mils (CM) per amp and then find a
core that has the required window area for this design
...
This
involves checking the turns per inch of the wire against the bobbin length
...
If the primary hangs over an extra half layer, try reducing the turns or the
wire size
...
e
...
This technique ultimately results
in the use of foil for the higher current (20 A) low voltage windings
...
Finally, once a mechanical fit has been obtained, it is time for the circuit tests
...
The inductance and saturating current level of the primary are inherent to the
design, and should be checked in the circuit or other suitable test fixture
...
The pulse
generator is run at a low enough duty cycle to allow the core to reset
...
Inductance is found using L = E/(di/dt)
...
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79
Figure 10–6
...
For these applications the core should be chosen large enough so that the resulting LI product
insures that IM at operating voltages is less than Isat
...
The gap will normally be quite large, Lg > > L m/u,
where, Lg
= gap length
Lm = magnetic path length, and
u
= permeability
...
Gaps of 5 mils to 20 mils are common
...
Table 10–3
...
Indiana General
Stackpole
TDK
Pulse Engineering
Coilcraft
Location
Code
Sauggerties, NY
Keasby, NJ
St
...
Here the designer must recheck three areas:
1
...
Correct amount of secondary voltage
3
...
However, if problems are minimal, or none exist, it is possible to stay with the same core or even
consider using the next smaller size
...
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80
Figure 10–7
...
The good news is that most have
been characterized, improved, and cost reduced for this application
...
e
...
If voltage is increased
beyond this point, the capacitor will begin to act like a zener and be thermally destroyed from high leakage
currents if the rating is exceeded for enough time
...
Here extra voltage capability may be needed to make up for the imbalances
caused by different values of capacitance and leakage current
...
The RMS current rating is also an
important consideration for input capacitors and is an example of improvements offered by today’s
manufacturers
...
Large capacitors that were
not needed for performance were used just to reduce this heating
...
A partial list of manufacturers
that supply both high voltage input and the lower voltage output capacitors for switchers is shown in
Table 10–4
...
Table 10–4
...
S
...
It turns out that for most
capacitors even in the so–called “low ESR” series, the output ripple depends more on this resistance than
on the capacitor value itself
...
This parameter is responsible for the relatively high
switching spikes that appear at the output
...
These LC noise or spike filters are made using small powdered iron toroids (1/2″ to 1″ OD) with
distributed windings to minimize interwinding capacitance
...
1 µF ceramic or a 10 µF to 50 µF tantalum or both
...
Calculations for the size of this component should take into account the minimum load so
that the choke will not run “dry” as stated earlier
...
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Figure 10–8
...
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...
For the final choice performance and cost
tradeoffs among devices from the same or several manufacturers have to be weighed
...
ON Semiconductor’s first line of devices for switchers were trademarked “Switchmode” transistors and
introduced in the early 70’s with data sheets that provided all the information that a designer would need
including reverse bias safe operating area (RBSOA) and performance at elevated temperature (100°C)
...
Finally, high voltage (1
...
The Switchmode II series
is an advanced version of Switchmode I that features faster switching
...
5 kV blocking capacity
...
A similar argument applies to ON Semiconductor TMOS Power FETs
...
This latter point is the one most often made to show that systems savings are again
quite possible even though the initial device cost is higher
...
ON Semiconductor High Voltage Switching Transistor Technologies
Approximate
Switching
Frequency
Typical
Device
Typical Fall
Time
SWITCHMODE I
2N6545
MJE13005
MJE12007
200 ns
to
500 ns
20 k
SWITCHMODE II
MJ13081
100 ns
100 k
SWITCHMODE III
MJ16010
50 ns
200 k
TMOS
MTP5N40
20 ns
500 k
Family
Table 11–2 is a chart of the transistor voltage requirements for the various off–line converter circuits
...
Bridge circuits, on the other hand, turn on and off from the dc bus and their most
critical voltage is the turn–on or VCEO(sus) rating
...
Power Transistor Voltage Chart
Circuit
Line
Voltage
220
120
Flyback, Forward or Push–Pull
Half or Full–Bridge
VCEV
VCEO(sus)
VCEO(sus)
VCEV
850 kV to 1
...
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83
Most switchmode transistor load lines are inductive during turn–on and turn–off
...
This inductance effectively snubs most turn–on load lines so that the rectifier
recovery (or short circuit) current and the input voltage are not applied simultaneously to the transistor
...
Turn–off transients due to this same leakage inductance, however, are almost always
a problem
...
If the resulting
inductive load line exceeds the transistor’s reverse bias switching capability (RBSOA) then an RC network
may also be added across the primary to absorb some of this transient energy
...
Resistance values
of 100 Ω to 1000 Ω in this RC network are generally appropriate
...
For single transistor converters, the circuits
shown in Figure 11–1 are generally used
...
In most of today’s designs snubber elements are small or nonexistent and voltage spikes from energy
left in the leakage inductance a more critical problem depending on how good the coupling is between the
primary and clamp windings and how fast the clamp diode turns on
...
Figure 11–1
...
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84
RC
Snubber
RC
Network
Zener and Mosorb Transient Suppressors
If necessary, protection from voltage spikes may be obtained by adding a zener and rectifier across
the primary as shown in Figure 11–1
...
0 W zener lines with ratings up to 200
V, and 10 W TO–220 Mosorbs with ratings up to 250 V can provide the clamping or spike limiting function
...
),
and standard zener/avalanche diodes used for reference, low–level regulation and low–level protection
purposes
...
The basic semiconductor
technology and processing are identical
...
Mosorb devices are intended specifically for transient protection purposes and are
designed, therefore, with a large effective junction area that provides high pulse power capability while
minimizing the total silicon use
...
MOVs, like Mosorbs, do have the pulse power capabilities for transient suppression
...
The main attributes of such devices are low manufacturing cost, the
ability to absorb high energy surges (up to 600 joules) and symmetrical bidirectional “breakdown”
characteristics
...
These limitations restrict the use of MOVs primarily to the protection
of insensitive electronic components against high energy transients in applications above 20 V, whereas,
Mosorbs are best suited for precise protection of sensitive equipment even in the low voltage range the
same range covered by conventional zener diodes
...
This is where the all–important rectifier comes into play
...
) The input rectifier is generally a standard recovery bridge that operates off the ac line and
into a capacitive filter
...
0 V output windings and for the higher voltage, 12 V to 15 V outputs, the more economical
fast recovery or ultrafast diodes are used
...
Switchmode Power Supply Flyback or Boost Design
D4
12V Output
D2
D5
D1
AC Line
Cont
IC
5V Output
D3
Q1
TMOS1
OC
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...
To reduce cost, most earlier approaches of using choke input filters, soft start relays (Triacs), or SCRs to
bypass a large limiting resistor have been abandoned in favor of using small limiting resistors or thermistors
and a large bridge
...
The procedure for finding the right component and checking its fit is as follows:
1
...
2
...
4 Vin
Ip =
t = RSC
RS
Where Vin is the RMS input voltage; RS is the total series resistance; and C is the filter capacitor size
...
Choosing Input Rectifiers
Filter
Cap
AC
Line
Bridge
RS
Load
C
3
...
If the curve
of IS versus time is not given on the data sheet, the approximate value for IS at a particular pulse
width (t) may be calculated knowing:
• IFSM — the single cycle (8
...
• I2 √ t = K, which applies when the diode temperature rise is controlled by its
thermal response as well as power (i
...
, T = K′P √ t for t < 8
...
This gives:
1/4
IS2 √ t = I2FSM √ 8
...
3 ms ) , t is in milliseconds
...
If IS < IP, consider either increasing the limiting resistor (RS) or utilizing a larger diode
...
In addition to the Schottky (SBR) and fast recovery (FR), there is also an ultrafast recovery
(UFR)
...
The
obvious point here is that lower forward voltage improves efficiency and lower recovery times reduce
turn–on losses in the switching transistors, but the tradeoff is higher cost
...
0 V outputs and fast recovery and ultrafast devices for 12 V outputs and greater
...
Ten years ago Schottkys were very
fragile and could fail short from either excessive dv/dt (1
...
0 V per nanosecond) or reverse
avalanche
...
Table 11–3
...
5 V to 0
...
9 V to 1
...
2 V to 1
...
2 V to 1
...
0 µs
Soft
Soft
Soft
Soft
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SECTION 12
BASIC SWITCHING POWER SUPPLY
CONFIGURATIONS
The implementation of switching power supplies by the non–specialist is becoming increasingly easy
due to the availability of power devices and control ICs especially developed for this purpose by the
semiconductor manufacturer
...
Flyback and Forward Converter Switching Power Supplies (50 W to 250 W)
•
•
•
•
Input line variation: Vin + 10%, – 20%
Converter efficiency: η = 80%
Output regulation by duty cycle (δ variation: δ(max) = 0
...
0 Pout
η δ(max) Vin(min) √ 2
Pout
η δ(max) Vin(min) √ 2
=
=
5
...
25 Pout
Vin
• Maximum transistor working voltage: Vw = 2 Vin(max)
• Working frequency: f = 20 kHz to 200 kHz
(Forward)
√ 2 + guardband
Basic Flyback Configuration
Output
Rectifier
DC Output
Power
Inverter
Line
Input
Control
Circuitry
Input
Rectifier
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Flyback and Forward Converter Semiconductor Selection Chart
Output Power
50 W
100 W
175 W
250 W
Input Line Voltage (Vin)
120 V
220 V
240 V
120 V
220 V
240 V
120 V
220 V
240 V
120 V
MOSFET Requirements:
Max Working Current (Iw)
Max Working Voltage (Vw)
2
...
2 A
750 V
4
...
5 A
750 V
8
...
4 A
750 V
11
...
4 A
MDA104A
0
...
4 A
MDA206
0
...
35 A
MDA970
1
...
6 A
MDA3506
Output Rectifiers:
Recommended types for
Output Voltage of: 5
...
Each circuit has a practical power range
or capability associated with it as follows:
Circuit
Power Range
Parts Cost
DC Converter
5
...
00
Converter w/30 V Transformer
10 W
7
...
00
Flyback
50 W
15
...
00
Half–Bridge
200 W
30
...
00
First to be discussed will be the low power (20 W to 200 W) converters which are dominated by the
single transistor circuits shown in Figure 12–1
...
This means that transformer size is sacrificed for circuit simplicity
...
A clamp winding is usually present to allow
energy stored in the leakage reactance to return safely to the line instead of avalanching the switching
transistor
...
The flyback is the lowest
cost regulator because output filter chokes are not required since the output capacitors feed from a current
source rather than a voltage source
...
However, it is an excellent choice when multiple output voltages are required and does tend to
provide better cross regulation than the other types
...
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...
Low Power Popular (20 to 200 W) Converter Configurations
(a) Flyback (Clamp Winding is Optional)
(b) Forward (Clamp Winding is Necessary)
(c) Two Transistor Forward or Flyback
(Clamp Winding is not Needed)
A 120/220 Vac flyback design requires transistors that block twice the peak line plus transients or about
1
...
ON Semiconductor’s MJE13000 and 16000A series with ratings of 750 V to 1000 V are normally
used here
...
The recent availability of 900 V and 1000 V TMOS FETs allows designers to
operate in the next higher range (50 kHz to 80 kHz) and some have even gone as high as 300 kHz with
square wave designs and FETs
...
0 kV bipolar transistors are also planned in the future and will
provide another design alternative
...
With this circuit a designer can use the faster 400 V to 500 V FET transistors and push operating
frequencies considerably higher
...
A subtle variation in the method of operation can be Figure 12–2
...
The difference is referred
800 V
to as operation in the discontinuous or continuous mode
and the waveform diagrams are shown in Figure 12–2
...
0 A
1
...
If the transistor is turned on while
IC
0 A IC
energy is still being dumped into the load, the circuit is
Discontinuous Mode
Continuous Mode
operating in the continuous mode
...
In many instances, the same transformer
transformer may be used with only the gap reduced to provide more inductance
...
In dealing with
the continuous mode, it should also be noted that the transistor must now turn on from 500 V to 600 V rather
than 400 V level because there no longer is any deadtime to allow the flyback voltage to settle back down
in the input voltage level
...
The flyback converter stands out from the others in its need for a low inductance, high current primary
...
The industry needs something better that will provide
permeabilities of 60 to 120 instead of 2000 to 3000 for this application
...
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90
The single transistor forward converter is shown in Figure 12–1(b)
...
The operating model for this circuit is actually the buck regulator discussed
earlier
...
The
additional output rectifier is used as a freewheeling diode for the LC filter and the third winding is actually
a reset winding
...
However, its main function is to return energy stored in the magnetizing
inductance to the line and thereby reset the core after each cycle of operation
...
This also is a very popular
low power converter and like the flyback is practically immune from transformer saturation problems
...
Forward Converter
that the voltage requirements are identical to the
Transistor Waveforms
flyback
...
0 kV blocking devices like the MJE13000
800 V
and MJE16000 transistors are required
...
With this circuit,
operation in the discontinuous mode refers to
1
...
” This means that choke current
0A
starts at and returns to zero during each cycle of
operation
...
Standard ferrite cores work fine here and in the high power converters as well
...
And, zeners or RC clamps may be used to reset the core in lieu of the clamp
winding to lower the voltage stress on the switching transistors
...
Push–Pull Converter
(200 W to 1
...
Because the transformers operate in this mode they
tend to be almost half the size of the equivalent single
transistor converters and thereby provide a cost
advantage over their counterparts at power levels of
200 kW to 1
...
The push–pull converter shown in Figure 12–4 is
one of the oldest converter circuits around
...
Because these converters are free running rather than driven and operate from low
voltages, transformer saturation problems are minimal
...
The transistors are also subjected to twice the peak line
voltage which requires the use of high voltage (1
...
Both of these drawbacks have tended
to discourage designers of off–line switchers from using this configuration until current mode control ICs
were introduced
...
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...
8
Maximum transistor working current:
Pout
1
...
Push–Pull Semiconductor Selection Chart
Output Power
100 W
250 W
500 W
Input Line Voltage (Vin)
120 V
220 V
240 V
120 V
220 V
240 V
120 V
220 V
240 V
MOSFET Requirements:
Max Working Current (Iw)
Max Working Voltage (Vw)
1
...
6 A
750 A
2
...
6 A
750 V
5
...
1 A
750 V
MTM2N50
MTP2N45
—
MTM2N90
MTP2N90
—
MTM4N45
MTP4N45
—
MTM2N90
MTP2N94
—
MTM7N45
—
MTH7N45
MTM4N90
—
—
Input Rectifiers:
Max Working Current (Iw)
Recommended Types
0
...
5 A
MDA210
2
...
25 A
MDA210
4
...
5 A
MDA3510
Output Rectifiers:
Recommended types for
output voltages of: 5
...
com
92
MBR20035CT
MUR10010CT
MUR10015CT
MUR3015PT
MUR840A
Half–Bridge/Full–Bridge Switching Power Supplies (100 W to 500 W/500 W to 1000 W)
•
•
•
•
Input line variation: Vin + 10%, – 20%
Converter efficiency: η = 80%
Output regulation by duty cycle (δ) variation: δ(max) = 0
...
8 Pout
=
Iw =
Vin
η δ(max) Vin(min) √ 2
Pout
=
=
(Half–Bridge)
1
...
Half–Bridge Semiconductor Selection Chart
Output Power
100 W
350 W
500 W
Input Voltage (Vin)
120 V
220 V
240 V
120 V
220 V
240 V
120 V
220 V
240 V
MOSFET Requirements:
Max Working Current (Iw)
Max Working Voltage (Vw)
2
...
25 A
380 V
5
...
1 A
380 V
11
...
25 A
380 V
MTM5N35
MTP3N40
—
MTM2N45
MTP2N45
—
MTM8N40
—
MTH8N40
MTM4N45
MTP4N45
—
MTM10N25
MTP10N25
—
MTM7N45
—
MTH7N45
Input Rectifiers:
Max Working Current (Iw)
Recommended Types
0
...
5 A
MDA210
2
...
25 A
MDA210
4
...
5 A
MDA3510
Output Rectifiers:
Recommended types for
output voltage of: 5
...
com
93
MBR20035CT
MUR10010CT
MUR10015CT
MUR3015PT
MUR840A
Half and Full–Bridge
The most popular high power converter is the half–bridge (Figure 12–6)
...
First, the transistors never see more than the
peak line voltage and the standard 400 V fast switchmode transistors that are readily available may be
used
...
0 µF to 5
...
Because the
primary winding is driven in both directions, a full–wave output filter, rather than half, is now used and the
core is actually utilized more effectively
...
This still allows the inverter transformer to operate from a nominal
320 V bus when the circuit is connected to either 120 V or 220 V
...
The designer’s dream, of course, is for
fast transistors that can handle a clamped inductive load line at rated current
...
With the improved RBSOA that
these transistors feature, less snubbing is required and this improves both the cost and efficiency of
these designs
...
Half–Bridge Converter
with Split Windings
Figure 12–6
...
0 kW)
+ Vin
+ Vin
+ Vout
CC
+ Vout
CC
Basic Full–Bridge Configuration
Output
Filter
Output
Rectifier
DC Output
Control
Circuitry
Line
Input
Power
Inverter
Input
Rectifier
http://onsemi
...
Full–Bridge Semiconductor Selection Chart
Output Power
500 W
750 W
1000 W
Input Voltage (Vin)
120 V
220 V
240 V
120 V
220 V
240 V
120 V
220 V
240 V
MOSFET Requirements:
Max Working Curren (Iw)
Max Working Voltage (Vw)
5
...
1 A
380 V
8
...
7 A
380 V
11
...
25 A
380 V
Power MOSFETs Recommended:
Metal (TO–204AA) (TO–3)
Plastic (TO–220AB)
Plastic (TO–218AC)
MTM8N20
MTP8N20
—
MTM4N45
MTP4N45
—
MTM10N25
MTP10N25
—
MTM7N45
MTP4N45
MTH7N45
MTM15N20
MTP12N20
MTH15N20
MTM7N45
—
MTH7N45
Input Rectifiers:
Max Working Current (Iw)
Recommended Types
4
...
5 A
MDA3510
7
...
8 A
9
...
0 A
Output Rectifiers:
Recommended types for
output voltage of: 5
...
The effective current limit of today’s low cost TO–218 discrete transistors (250 mil die) is somewhere
in the 10 A to 20 A area
...
Because full line rather than half is applied to the primary winding,
the power out can be almost double that of the half–bridge with the same switching transistors
...
Another variation of the half–bridge
is the split winding circuit, shown in Figure 12–5
...
Because
both emitters are at an ac ground point, expensive drive transformers can now be replaced by lower cost
capacitively–coupled drive circuits
...
Full–Bridge Converter
(200 W to 1
...
com
95
SECTION 13
SWITCHING REGULATOR DESIGN EXAMPLES
In addition to the application materials in this data book, ON Semiconductor publishes several
application notes which contain basic information on the design of power supplies using a variety of ON
Semiconductor Analog ICs
...
Several converter design examples and numerous
applications circuits with test data are included in this application note
...
Operating details of the MC34129 Current Mode Switching Regulator Controller, and examples of its
use with ON Semiconductor SENSEFET™ products, are provided in AN976
...
The TL594
can be used in this same application
...
Many data sheets also
include printed circuit board layouts for some key applications so that the designer can evaluate the
integrated circuits in an actual power supply
...
http://onsemi
...
In light of the recent
advances in LSI circuitry, this technique has taken on added importance
...
If this supply were
to fail due to component failure or other accidental shorting of higher voltage supply busses to the low
voltage bus, several hundred dollars worth of circuitry could literally go up in smoke
...
A
...
As the name implies, the SCR is used
much like a crowbar would be, to short the dc supply when an overvoltage condition is detected
...
This method is very effective in eliminating
the destructive overvoltage condition
...
Figure 14–1
...
http://onsemi
...
Crowbar SCR Surge Current Waveform
I
Ipk
di
dt
Surge Due to
Output Capacitor
Current Limited
Supply Output
t
B
...
This large current surge, illustrated in Figure
14–2, can cause SCR failure or degradation by any one of three mechanisms: di/dt, peak surge current,
or I2 t
...
To aid in the selection of devices for this application, ON
Semiconductor has characterized several devices specifically for crowbar applications
...
This significantly
reduces the amount of empirical testing that must be done by the designer
...
Table 14–1
...
0 µs, exponentially decaying
** All devices available with 25, 50, and 100 V ratings
1
...
Since the anode current flows through
this turned–on gate region, very high current densities can occur in the gate region if high anode currents
appear quickly (di/dt)
...
The value of di/dt that an SCR can safely handle is influenced by its construction and the characteristics
of the gate drive signal
...
0 µs rise time signal will maximize its di/dt
capability
...
0 µs rise time
...
com
98
the SCR, as shown in Figure 14–3
...
2
...
The surge capability of the SCR is directly proportional
to its die area
...
This may result in the average current capability of the SCR
exceeding the steady state current requirements imposed by the dc power supply
...
Circuit Elements Affecting SCR Surge & di/dt
RLead
Output
Capacitor
ESR
LLead
R*
L*
ESL
*R and L
empirically
determined
(For additional information on SCRs in crowbar applications refer to Characterizing the SCR for Crowbar Applications,
Al Pshaenich, ON Semiconductor AN789)
...
The Sense and Drive Circuit
In order to maximize the crowbar SCR’s di/dt capability, it should receive a fast rise time
high–amplitude gate–drive signal
...
Also important is the sense circuitry’s noise immunity
...
If the sensing
circuit has low immunity and is operated in a noisy environment, nuisance tripping of the OVP circuit can
occur on short localized noise spikes, which would not normally damage the load
...
This results in excessive
system down time
...
These can be classified into three types: zener, discrete, and “723
...
The Zener Sense Circuit — Figure 14–4 shows the use of
a zener to trigger the crowbar SCR
...
Additionally, this method does not allow the trip point to be
adjusted except by zener replacement
...
The Discrete Sense Circuit — A technique which can
provide adequate gate drive and an adjustable, low
temperature coefficient trip point is shown in Figure 14–5
...
In addition, this method is not particularly noise immune and often suffers from nuisance
tripping
...
com
99
3
...
This is illustrated in
Figure 14–6
...
5 V
...
The Discrete Sense Circuit
Figure 14–6
...
The MC3423 — To fill the need for a low cost, low complexity method of implementing crowbar
overvoltage protection which does not suffer the disadvantages of previous techniques, an IC has been
developed for use as an OVP sense and drive circuit, the MC3423
...
In addition, its features include:
1
...
5 V to 40 V supply voltages
...
Adjustable low temperature coefficient trip point
...
Adjustable minimum overvoltage duration before actuation to reduce nuisance
tripping in noisy environments
...
Remote activation input
...
Indication output
...
Block Diagram — The block diagram of the MC3423 is shown in Figure 14–7
...
6 V reference, two comparators and a high current output
...
6 V on Pin 3 or by a TTL/5
...
The circuit also has a comparator–controlled current source which can be used in conjunction with and
external timing capacitor to set a minimum overvoltage duration (0
...
0 ms) before actuation occurs
...
Figure 14–7
...
com
100
VCC
1
Current
Source
4
2
-
+
+
Vsense 1
-
Vref
2
...
ACT
...
Basic Circuit Configuration — The basic circuit configuration of the MC3423 OVP is shown in
Figure 14–8
...
The shortest possible propagation delay is thus obtained
...
Their values can be determined by the equations given in Figure 14–8 or
by the graph shown in Figure 14–9
...
Otherwise, the power supply, across which the SCR is connected, must be shut down to reset
the crowbar
...
Figure 14–8
...
6 V (1 + 1 )
R2
R2
(-)
R2 ≤ 10 kΩ for minimum drift
*Needed if supply is not current–limited
7
...
To prevent false tripping of the OVP circuit by noise which would not normally harm the load,
http://onsemi
...
To implement this feature, the circuit configuration of
Figure 14–10 is used
...
The value of this capacitor determines the
minimum duration of the overvoltage condition (tD) which is necessary to trip the OVP
...
The circuit operates in the following manner: when VCC rises above the trip
point set by R1 and R2, the internal current source begins charging the capacitor, CD, connected to Pins
3 and 4
...
If the overvoltage condition disappears before this occurs, the capacitor is
discharged at a rate 10 times faster than the charging rate, resetting the timing feature until the next
overvoltage condition occurs
...
Indication Output — An additional output for use as an indicator of OVP activation is provided by the
MC3423
...
It will remain in a saturated state until the SCR crowbar pulls the supply voltage, VCC, below
4
...
This output can be used to clock an edge triggered flip–flop whose output inhibits
or shuts down the power supply when the OVP trips
...
http://onsemi
...
R1 versus Trip Voltage
for the MC3423 OVP
(+)
30
R1, RESISTANCE (kΩ )
Figure 14–10
...
7 k
20
R3
R1
Power
Supply
Min
1 6
2
MC3423
3
R2
10
8
RG
5
4
Indication
Out
7
CD
0
0
5
...
Remote Activation Input — Another feature of the MC3423 is its Remote Activation Input, Pin 5
...
7 V, the MC3423 operates normally
...
0 V, the OVP output is activated independent of whether or not an
overvoltage condition is present
...
In addition, the Indication Output of one MC3423 can be used to activate
another MC3423, if a single transistor inverter is used to interface the former’s Indication Output to the
latter’s Remote Activation Input
...
MC3425 Power Supply Supervisory Circuit
In addition to the MC3423 a second IC, the MC3425 has been developed
...
The block diagram is shown below in
Figure 14–12
...
5 V regulator
...
5 µA current sink (IH)
which is used for programming the input hysteresis voltage (VH)
...
5 × 10–6 RH
...
CD versus Minimum Overvoltage Duration, tD for The MC3423 OVP
C D, CAPACITANCE (µ F)
1
...
1
0
...
001
0
...
001
0
...
1
tD, DELAY TIME (ms)
http://onsemi
...
0
10
Separate Delay pins (OV DLY, UV DLY) are provided for each channel to independently delay the Drive
and Indicator outputs, thus providing greater input noise immunity
...
A capacitor connected
from these Delay pins to ground, will establish a predictable delay time (tDLY) for the Drive and Indicator
outputs
...
5 V regulator
...
5 V
...
5 CDLY
tDLY =
=
= 12500 CDLY
IDLY(source)
200 µA
Figure 14–13 provides CDLY values for a wide range of time delays
...
The sink current
IDLY(sink) capability of the Delay pins is ≥1
...
The Overvoltage Drive Output is a current–limited emitter–follower capable of sourcing 300 mA at a
turn–on slew rate of 2
...
The Undervoltage Indicator Output is an
open–collector NPN transistor, capable of sinking 30 mA to provide sufficient drive for LEDs, small relays
or shutdown circuitry
...
The MC3425 has an internal 2
...
0% for the basic devices
...
Block Diagram
VCC
8
+
+
200µA
OV Sense
+
3
-
Input
Comp
...
OV
+
200µA
+
UV Sense
-
4
1
+
Output
Comp
...
UV
6
+
2
...
5µA
2
5
INPUT SECTION
UV OV
DLY DLY
Note: All voltages and currents are nominal
...
com
104
7
Gnd
OUTPUT SECTION
OV Drive
UV Indicator
Figure 14–13
...
0
0
...
01
0
...
0001
2
...
001
0
...
1
1
...
0
E
...
They offer the designer an
economical solution for low voltage detection with a single external resistor
...
The two families of undervoltage sensing circuits,
taken together, cover the needs of the most
Input
2
commonly specified power supplies used in
1
MCU/MPU systems
...
The table summarizes critical
parameters of both families
...
0 V ± 5% system and features a tighter
hysteresis specification
...
0 V ± 10% and 3
...
2 Vref
ideal for applications where extended battery life
is required such as consumer products or hand
held equipment
...
0 V
MPU/logic power supply used in appliance,
= Sink Only Positive True Logic
+
automotive, consumer, and industrial equipment
...
________________________________
REFERENCES
1
...
(Out of Print)
2
...
3
...
Pierce Jr
...
27, 1974
...
Transient Thermal Response — General Data and Its Use, Bill Roehr and Brice Shiner, ON Semiconductor AN569
...
com
105
SECTION 15
HEATSINKING
A
...
The operating junction temperature is given by:
TJ = TA + PD θJA
where:
(15
...
1), can be expressed as a sum of
thermal resistances as shown below:
where: θJC
θJA = θJC + θCS + θSA
= junction–to–case thermal resistance
(15
...
2) applies only when an external heatsink is used
...
θJC depends on the device and its package (case) type, while θSA is a property of the heatsink and θCS
depends on the type of package/heatsink interface employed
...
Table 15–1
...
5°C/W
0
...
36°C/W
0
...
2°C/W
1
...
6°C/W
2 mil MICA
*Typical values; heatsink surface should be free of oxidation, paint, and anodization
Examples showing the use of Equations (15
...
2) in thermal calculations are as follows:
Example 1: Find required heatsink θSA for an MC7805CT, given:
TJ(max) (desired) = +125°C
TA(max) = +70°C
PD = 2
...
com
106
Mounted directly to heatsink with silicon thermal grease at interface:
1
...
From Table 15–1
...
6°C/W
3
...
1) and (15
...
0 – 1
...
9°C/W required)
2
θSA =
Example 2: Find the maximum allowable TA for an unheatsinked MC78L15CT, given:
TJ(max) (desired) = +125°C
PD = 0
...
From MC78L15CT data sheet, θJA = 200°C/W
2
...
1), find TA:
TA = Tj – PD θJA
= 125 – 0
...
Selecting a Heatsink
Usually, the maximum ambient temperature, power being dissipated, the TJ(max), and θJC for the
device being used are known
...
1)
and (15
...
The designer may elect to use a commercially available heatsink, or if
packaging or economy demands it, design his own
...
Commercial Heatsinks
As an aid in selecting a heatsink, a representative listing is shown in Table 15–2
...
Table 15–2
...
3–1
...
0–3
...
0–5
...
0–7
...
0–10
Wakefield — 672
Thermalloy — 6001, 6016, 6051, 6105, 6601
IERC — LA µP
Staver — V1–3, V1–5, V3–3, V3–5, V3–7
10–25
Thermalloy — 6013, 6014, 6015, 6103, 6104, 6105, 6117
*All values are typical as given by the manufacturer or as determined from characteristic curves supplied by the manufacturer
...
com
107
Table 15–2
...
0 to 10
IERC H P3 Series
Staver — V3–7–225, V3–7–96
10 to 15
Thermalloy — 6030, 6032, 6034
Staver — V4–3–192, V–5–1
20 to 30
Wakefield — 295
Thermalloy — 6025, 6107
15 to 20
Thermalloy — 6106
Staver — V4–3–128, V6–2
TO–226AA (TO–92)
θSA*(°C/W)
46
50
57
65
72
80 to 90
85
Manufacturer/Series or Part Number
Staver F5–7A, F5–8
IERC AUR
Staver F5–7D
IERC RU
Staver F1–8, F2–7
Wakefield 292
Thermalloy 2224
DUAL–IN–LINE–PACKAGE ICs
20
30
32
34
45
60
Thermalloy — 6007
Thermalloy — 6010
Thermalloy — 6011
Thermalloy — 6012
IERC — LIC
Wakefield — 650, 651
*All values are typical as given by the manufacturer or as determined from characteristic curves supplied by the manufacturer
...
, Inc
...
Saxon Ave
...
Magnolia Blvd
...
O
...
Valley View Ln
...
com
108
2
...
The design of forced air
cooled heatsinks is usually done empirically, since it is difficult to obtain accurate air flow measurements
...
It
must be emphasized, however, that any custom heatsink design should be thoroughly tested in the actual
equipment configuration to be certain of its performance
...
Obviously, the basic goal of any heatsink design is to produce a heatsink with an adequately low
thermal resistance, θSA
...
Unfortunately,
a precise calculation method for θSA is beyond the scope of this book
...
2
...
4
...
The heatsink will not be located near other heat radiating surfaces
...
Unrestricted convective air flow
...
3)
A = area of the heatsink surface
η = heatsink effectiveness
Fc = convective correction factor
hc = convection heat transfer coefficient
∈ = emissivity
Hr = normalized radiation heat transfer coefficient
The convective heat transfer coefficient, hc, can be found from Figure 15–1
...
The fin temperature
rise (TS – TA) is given by:
(15
...
Convection Coefficient (hc)
10
9
...
0
7
...
0
5
...
0
*If greater precision is desired, or
more information on heat flow and
heatsinking is sought, consult the
references list at the end of this
section
...
0
2
...
com
109
200
The significant heatsink dimension (L) is dependent on the heatsink shape and mounting place and
is given in Table 15–3
...
Table 15–3
...
0
Horizontal
length × width
length + width
Horizontal Plane
both surfaces
exposed
1
...
9
Circular Plane
The normalized radiation heat transfer coefficient (Hr) is dependent on the ambient temperature (TA) and
the heatsink temperature rise (TS – TA) given by Equation (15
...
Hr can be determined from Figure 15–2
...
Normalized Radiation Coefficient (Hr)
2
...
5
1
...
9
0
...
7
TA = 100°C
75°C
50°C
0
...
5
0
...
Table 15–4
...
com
110
0
...
7 to 0
...
05
0
...
70
0
...
85 to 0
...
92 to 0
...
89 to 0
...
Use of the
nomograph is as follows:
a) Find hT = Fchc + ∈Hr from Figures 15–1, 15–2 and Tables 15–3 and 15–4,
and locate this point on the nomograph
...
c) Determine D for the heatsink shape as given in Figure 15–4 and draw a line
from this point through α, which was found in (b), to determine η
...
7 (for vertically mounted plates only)
...
3), it is necessary to know the heatsink size
...
This process is iterated until the smallest heatsink is
obtained that has the required θSA
...
Figure 15–3
...
0
0
...
0
Fin Thickness
For
Aluminum
0
...
0
For
Copper
0
...
8
0
...
6
0
...
4
0
...
01
0
...
0
4
...
0
1
...
1
0
...
0
0
...
0
1
...
0
η
Fin Effectiveness
10
10
10
0
...
4
0
...
0
hT = FChC+∈Hr
94
0
...
001
0
...
01
75
Inches
70
65
10
...
1
60
0
...
Determination of D for Use in η Nomograph of Figure 15–3
a
b
D^
d
s
√ ab , if a,b ơ S & b ≤ 2a
π
D]
http://onsemi
...
Heatsink θSA = 25°C/W
2
...
0 W
3
...
Heatsink to be constructed from 1/8″ (0
...
a) First, a trial heatsink is chosen: 2″ × 3″ (experience will simplify this selection and
reduce the number of necessary iterations
...
3) are evaluated by using the Figures and Tables given:
A = 2″ × 3″ = 6 sq
...
L = 6/5″ = 1
...
(from Table 15–3)
TS – TA = 50°C (from Figure 15–4)
hc = 5
...
9 (from Table 15–3)
Hr = 6
...
9 (from Table 15–4)
hT = Fchc + Hr∈ = 10
...
13 (from Figure 15–3)
D = 1
...
94 ]1 (from Figure 15–3)
c) Using Equation (15
...
66°C/W < 25°C/W
Aη (Fchc + ∈Hr)
d) Since 2″ × 3″ is too large, try 2″ × 2″
...
SOIC MINIATURE IC PLASTIC PACKAGE
Thermal Information
The maximum power consumption an integrated circuit can tolerate at a given operating ambient
temperature, can be found from the equation:
TJ(max) – TA
PD(T =
A)
RθJA (typ)
where: PD(TA) = power dissipation allowable at a given operating ambient temperature,
TJ(max) = maximum operating junction temperature as listed in the maximum ratings section,
TA = desired operating ambient temperature,
RθJA (typ) = typical thermal resistance junction–to–ambient
...
com
112
THERMAL CHARACTERISTICS OF SOIC PACKAGES
Measurement specimens are solder mounted on a Philips SO test board #7322–078, 80873 in still air
...
As thermal resistance varies inversely with die area, a given
package takes thermal resistance values between the max and min curves shown
...
Figure 15–5
...
Die Size
2K Mils2
140 _
120
_
Max
...
2
150
JUNCTIONĆTOĆAIR (° C/W)
160
_
R θ JA, THERMAL RESISTANCE
°C/W
θJA
170
_
130
2
...
The SOP–8 and SOP–16L packages have external
dimensions which are identical to the standard SO–8 and SO–16L surface
mount devices, but the center four leads of the packages are all connected
to the leadframe die flag
...
This advantage is fully realized when the package is mounted on a printed
circuit board with a single pad for the four center leads
...
JUNCTIONĆTOĆAIR (° C/W)
2
...
0 oz
...
0
ÎÎÎ
ÎÎÎ ÎÎÎ
Î
ÎÎÎÎÎÎÎ
Î ÎÎÎ
R θ JA, THERMAL RESISTANCE
80
30
2
...
0 mm
30
40
1
...
2
0
...
4
0
50
L, LENGTH OF COPPER (mm)
http://onsemi
...
SOP–16L Thermal Resistance and Maximum
Power Dissipation versus P
...
B
...
0 mm
RθJA
0
2
...
6
2
...
Copper
L
50
30
Data taken using Philips SO test board #7322-078, 80873
*SOP-8 using standard SO-8 footprint Ċ minimum pad size
2
...
2
0
...
4
50
PD, MAXIMUM POWER DISSIPATION (W)
_
200
180
Figure 15–6
...
C
...
Copper Length
THERMAL CHARACTERISTICS OF DPAK AND D2PAK PACKAGE
The evaluation was performed using an active device (4900 square mils) mounted on 2
...
Measurements were made in still air and no auxiliary
thermal conduction aids were used
...
The curve shown in Figure 15–8 is a plot of
junction–to–air thermal resistance versus the length of the square copper pad in millimeters
...
0 ounce copper it has a thermal
resistance which is comparable to a TO–220 device mounted vertically without additional heatsinking
...
0
2
...
Copper
L
Minimum
Size Pad
60
50
40
RθJA
0
5
...
6
L
80
70
2
...
2
ÎÎÎÎ
ÎÎÎÎ
ÎÎÎÎ
R θ JA, THERMAL RESISTANCE
100
20
25
30
0
...
4
0
PD, MAXIMUM POWER DISSIPATION (W)
Figure 15–8
...
C
...
Copper Length
L, LENGTH OF COPPER (mm)
The thermal characteristics of the D2PAK are shown in Figure 15–9
...
0 oz
...
C
...
The maximum power dissipation was measured with a junction
temperature of 150°C
...
0
Free Air
Mounted
Vertically
60
2
...
Copper
L
Minimum
Size Pad
50
2
...
0
L
1
...
5
PD(max) for TA = +50°C
ÎÎÎÎ
ÎÎÎÎ
ÎÎÎÎ
ÎÎÎÎ
R θ JA, THERMAL RESISTANCE
80
0
5
...
com
114
25
30
1
...
3–Pin and 5–Pin D2PAK
Thermal Resistance and Maximum Power
Dissipation versus P
...
B
...
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...
e
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com
116
HB206/D
Rev
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com
N
...
com
HB206/D
Title: Linear Switching Voltage Regulator
Description: In most electronic systems, voltage regulation is required for various functions. Today’s complex electronic systems are requiring greater regulating performance, higher efficiency and lower parts count
Description: In most electronic systems, voltage regulation is required for various functions. Today’s complex electronic systems are requiring greater regulating performance, higher efficiency and lower parts count