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DC-DC Power Supply Topologies
DC
• Buck Converter (also known as the
forward converter)

• Boost Converter (also known as the
“fly back converter)

• Buck/Boost Converters
• Ćuk Converter

DC

Switched-mode converters vs linear dc-dc converters
S
Vin

Vin
Vout

Vout

Linear dc-dc converters
S
Vin

Vin
Vout

Switched-mode converters

Vout

Applications:
• Power Supplies

Properties:
• High efficiency (ideally 100%)

• Robust and withstand extreme conditions (shortcircuiting & open circuiting)
• Allow energy to flow in either direction (in some cases)

• To provide electrical isolation (in some cases)

The Buck Converter
The buck converter is a step-down converter, in the sense
that the output voltage can never be greater than the input
voltage
...
But if the C has 13Vdc, then when the switch
closes, the source current spikes to a huge value and
burns out the switch
...
But
now, if the L has current when the switch attempts to open,
the inductor’s current momentum and resulting Ldi/dt
burns out the switch
...

With high-frequency switching, the load voltage ripple
can be reduced to a small value
...
If L is finite, the result
will be a small increase in the current through it,  I(on)
...
Again applying: vL = L di/dt we find that:
 Ioff = – Vout (1 – D) T / L
In the steady state:
Ion +  Ioff = 0

(Vin – Vout ) DT / L = Vout (1 – D) T / L
Vout = D Vin
The value of the capacitor is determined by the amount of voltage ripple
acceptable at the output
...

• The losses in the practical switch and diode increase somewhat as
Imax increases, so L should be large
...
For
a given filtering effect there is a trade-off between L and C
...


• To ensure continuous mode of operation, L should be large
...


IL, min = – (Vout / L) (1 – D)T + IL, max

 I = IL, min – IL, max = Vout (1 – D) T / L
The average current in the inductor must be equal to the dc
through the load:
IL, avg = I0 = Vout / R
iL

IL, max

I
IL, avg = I0

IL, min
ton

toff
T

t

IL, max = IL, avg +  IL/2 = (Vout / R) + (Vout / 2L) (1 – D)T

IL, min = IL, avg –  IL/2 = (Vout / R) – (Vout / 2L) (1 – D)T
The current supplied by the source varies from IL, min to IL, max
during the time the switch is closed and is zero otherwise
...
The minimum current in
the continuous conduction mode can be zero
...

(Vout / R) – (Vout / 2Lmin) (1 – D)T = 0
Hence:
Lmin= (1-D)(RT)/2 = (1-D)R / (2f)

In practice, a value of inductance greater than Lmin is
desirable to ensure continuous current
...
Assuming that all of the
ripple component in iL flows through the capacitor and its average
component flows through the load resistor, the shaded area
represents an additional charge  Q
...
T

T

t

d
...
T

t

T

Drive S1

y1
d
...
T

T

t

d
...
T

T

t

 IL

Effect of raising and lowering Iout while
holding Vin, Vout, f, and L constant

ΔI

iL

Raise Iout
ΔI
Lower Iout
ΔI

• ΔI is unchanged

• Lowering Iout (and, therefore, Pout ) moves the
circuit toward discontinuous operation
23

Effect of raising and lowering f while holding
Vin, Vout, Iout, and L constant

iL

Lower f

Raise f

• Slopes of iL are unchanged

• Lowering f increases ΔI and moves the circuit
toward discontinuous operation
24

Effect of raising and lowering L while
holding Vin, Vout, Iout and f constant

iL

Lower L

Raise L

• Lowering L increases ΔI and moves the circuit
toward discontinuous operation

25

Buck converter for solar applications
The panel needs a ripple-free current to stay on the max power point
...

ipanel

+ vL –
iL

Iout

L
V panel

C

iC

+
V out
–

Put a capacitor here to provide the ripple current
required by the opening and closing of the MOSFET
In that way, the panel current can be ripple free and
the voltage spikes can be controlled

26

Example:
In a step-down converter, consider all components
to be ideal
...
Calculate the minimum
inductance ‘L’ required to keep the converter operation in a

continuous-conduction mode under all conditions if Vd
...
is
10-40V, Pout  5W, and fs = 50 kHz
...


•

HEVs became widely available to the public in the 1990s with the introduction of the
Honda Insight and Toyota Prius
...
Without a
boost converter, the Prius needs almost 417 batteries to power the motor
...


•

Find boost converters in:
• Cell Phones
• Laptops
• MP3 Players
• Car Navigation Systems
• Digital Camera

The Boost Converter
The boost converter is a step-up converter
...

iin

L

iout

D

Vs

Vin

Vout

C

Load

S

Vin

Vout

Vs
t

t

t

The Boost Converter
Vin

VS
V

VS

Vout

Vout
Vin

t
iL

I
Iavg
ton

toff

t

D = ton / (ton + toff)
ton = DT
toff = (1 – D) T

&

T = ton + toff

‘S’ is Closed:
A voltage, v in appears across ‘L’ during ton
...


 Ion = Vin DT / L

‘S’ is Open:
A voltage, v in – vout (negative voltage) appears across ‘L’ during
toff
...

Vout can be varied by PWM (changing ‘D’)
The choice of the capacitor is theoretically determined by the
amount of voltage ripple acceptable at the output
...


• The losses in the practical switch and diode increase somewhat as
Imax increases, so L should be large
...
For a
given filtering effect there is a trade-off between L and C
...

• To ensure continuous mode of operation, L should be large
...


Vin D
L
 iL f
D (1  D) 2 R
Lmin 
2f

D
C
  Vout
R f 
 V
 out






Buck vs
...
Design for
continous inductor current and an output ripple
voltage of less than one percent
...
Assume ideal components
for the design
...

It is also labeled as indirect converter because the
source is never directly connected to the load
...

i in

Vin

+
-

i out

D

S
+
VL
-

L
iL

C

-

-

+ +

Vout
+

Load

The Buck / Boost Converter
i

V in

in

-

Vin

S

+
VL
-

+

i

D
-

L
i

C
+ +
L

VL

t
-Vout
iL

I
Iin + Iout
ton

toff

-

t

out

V out
+

Load

‘S’ is Closed:
A voltage, v in appears across ‘L’ during ton and  Ion = Vin DT / L

‘S’ is Open:
A voltage,– vout (negative voltage) appears across ‘L’ during toff and
 Ioff = - Vout (1 – D) T / L

In the steady state:

 Ion +  Ioff = 0
Vin DT / L
When
When
When

- Vout (1 – D) T / L = 0
Vout = Vin D / (1 – D)
0 < D < 0
...
5
Vout = Vin
0
...
It will need some form of drive circuit
...


• Diode must be a fast type; low stored charge
...

• Inductor design – already discussed
...


Example
A buck-boost circuit has the following parameters:
Vs = 24,
D = 0
...


Example
Design a buck-boost converter to supply a load of 75 W at
50 V from a 40 V source
...
Specify the duty ratio, switching
frequency, inductor size, and capacitor size
...
The load is a 15 W
resistor
...
The output is regulated at -12 V
...
The output voltage ripple must be
less than 1 percent of any operating condition
...
Specify values of
the inductor and capacitor, and explain how you made your
design decisions

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