Chapter 5
DC to DC Converters
(Choppers)
Outline
5.1 Basic DC to DC converters
5.2 Composite DC/DC converters and
Power
connection of multiple DC/DC converters
5.3 Isolated DC to DC converters (Indirect DC
to DC converters )
2
5.1 Basic DC to DC converters
5.1.1 Buck converter (Step-down converter)
Power
5.1.2 Boost converter (Step-up converter)
5.1.3 Buck-Boost converter (Step-down/stepup converter) and Cuk converter
5.1.4 Sepic converter and Zeta converter
3
5.1 Basic DC to DC converters
Introduction—Buck converter
Power
SPDT switch changes
dc component
Switch output
voltage waveform
Duty cycle D:
0 D 1
complement D’:
D’ = 1 - D
4
Power
Dc component of switch output voltage
Fourier analysis: Dc component = average value
5
Power
Insertion of low-pass filter to remove switching
harmonics and pass only dc component
6
Basic operation principle of buck converter
Power
Buck converter
with ideal switch
Realization
using power
MOSFET and
diode
7
Thought process in analyzing basic
DC/DC converters
Basic operation principle (qualitative analysis)
– How does current flow during different switching states
– How is energy transferred during different switching states
Power
Verification of small ripple approximation
Derivation of inductor voltage waveform during different
switching states
Quantitative analysis according to inductor volt-second
balance or capacitor charge balance
8
Actual output voltage waveform of
buck converter
Power
Buck converter
containing practical
low-pass filter
Actual output voltage
waveform
v(t) = V + vripple(t)
9
The small ripple approximation
Power
v(t) = V + vripple(t)
In a well-designed converter, the output voltage
ripple is small. Hence, the waveforms can be
easily determined by ignoring the ripple:
10
Power
Buck converter analysis:
inductor current waveform
11
Power
Inductor voltage and current
subinterval 1: switch in position 1
12
Power
Inductor voltage and current
subinterval 2: switch in position 2
13
Power
Inductor voltage and current waveforms
14
Power
Determination of inductor
current ripple magnitude
15
Power
Inductor current waveform
during start-up transient
16
Power
The principle of inductor
volt-second balance: Derivation
17
Power
Inductor volt-second balance:
Buck converter example
18
Power
The principle of capacitor charge
balance: Derivation
19
Power
Boost converter example
20
Power
Boost converter analysis
21
Power
Subinterval 1: switch in position 1
22
Power
Subinterval 2: switch in position 2
23
Power
Inductor voltage and capacitor current
waveforms
24
Power
Inductor volt-second balance
25
Power
Conversion ratio M(D) of
the boost converter
26
Power
Determination of inductor current
dc component
27
Continuous-Conduction-Mode (CCM) and
Discontinuous-Conduction-Mode (DCM) of buck
L
V
R
io
iG
+
E
EM
VD uo
M
Power
iG
ton
O
io
toff
iG
t
T
i1
I10
iG O
io
i2
I20
t1
O
uo
t
O
uo
toff
ton
t
Tt
x
i1
E
i2
t1
I20
t2
t
E
E
O
O
CCM
t
DCM
EM
t
28
Continuous-Conduction-Mode (CCM) and
Discontinuous-Conduction-Mode (DCM) of boost
L
M
VD
uo
V
EM
E
Power
a)
uo
t
O
i
i1
i2
ton
O
io
t
T
i2
I20
I10
toff
t
O
ton
T
b)
CCM
E
i1
I20
I10
O
uo
E
t1 tx
t2
toff
t
c)
DCM
29
5.2 Composite DC/DC converters and
connection of multiple DC/DC converters
5.2.1 A current-reversible chopper
Power
5.2.2 Bridge chopper (H-bridge DC/DC
converter)
5.2.3 Multi-phase multi-channel DC/DC
converters
30
5.2.1 A current reversible chopper
V1
VD2
E
L
R
io
V2
Power
VD1
uo
M
EM
Can be considered as a
combination of a Buck and
a Boost
Can realize two-quadrant ( I & II)
operation of DC motor:
forward motoring,
forward braking
31
Bridge chopper (H-bridge chopper)
V1
VD1
E
V2
Power
VD2
V3
uo
L
R
io
+
VD3
M
EM
V4
VD4
Can be considered as the combination of two
current-reversible choppers.
Can realize 4-quadrant operation of DC motor.
32
Multi-phase multi-channel
DC/DC converter
L
E
C
V1
i1 L1
V2
i2 L2
V3
i3 L3
VD3 VD2 VD1
Power
u3
u2
u1
io
M
O
u2
t
uo O
t
O
uo
t
O
i1
t
O
i2
t
O
i3
t
O
io
t
O
t
u3
u1
Current output capability is
increased due to multi-channel
paralleling.
Ripple in the output voltage and
current is reduced due to multichannel paralleling.
Ripple in the input current is
reduced due to multi-phase
paralleling.
33
5.3 Isolated DC to DC converters
(Indirect DC to DC converters )
DC input
Inverter
High frequency
AC
Transformer
AC
DC output
Rectifier
Filter
Isolation
Power
Reasons to use indirect DC to DC structure
Necessary isolation between input and output
In some cases isolated multiple outputs are needed
The ratio of input and output voltage is far away from 1
Power semiconductor devices usually used
Inverter part: Power MOSFETs, IGBTs
Rectifier part: Fast recovery diodes, Schottky diodes, Synchronous
rectifiers
34
Classification of isolated DC to DC converters
Power
According to whether transformer current is
uni-direction or bi-directional
Isolated DC to DC
converters
Single-ended converters
• Forward converter
• Flyback converter
Double-ended converters
• Half bridge
• Push-pull
• Full bridge
35
5.3.1 Forward converter
VD1
Power
Ui +
S
N3 N
1
N2
L
W1
W3
VD3 S
W2
VD2
Uo
N 2 t on

Ui
N1 T
Simple, low cost
+
Uo
O
uS
Ui
O
iL
O
iS
O
t
t
t
t
Uni-polar transformer current, low power applications
36
5.3.2 Flyback converter
Power
Ui
+
N1
N2 VD +
W1
W2
S
Uo
S
O
uS
Ui
to n
to ff
O
iS
O
Uo
N 2 t on

Ui
N 1 t off
iVD
Simple, low cost
O
t
t
t
t
Uni-polar transformer current, low power applications
37
5.3.3 Half bridge converter
S1
to n
t
O
S2
+
C1
S1
N1
Ui
Power
+
C2
S2
W2 VD1 +
ud L
N2
N3
W1 W
3
O
t
T
uS1
+
Uo
Ui
t
Ui
O
uS2
t
O
VD2
iS1
t
O
iS2
Uo
N 2 ton

Ui
N1 T
O
iD1
O
iS2
t
iL
t
iL
O
t
Cost higher than forward and flyback converter
Bi-polar transformer current, up to several kilowatts
38
5.3.4 Push-pull converter
to n
S1
S2
S1
Power
Ui
+
S2
O
VD1
N1
N2
N1'
N2'
t
O
t
T
uS1
L
+
Uo
VD2
t
2Ui
O
uS2
t
O
i S1
t
O
iS2
O
Uo
N 2 2t on

Ui
N1 T
2Ui
i D1
t
iL
O
i S2
O
t
iL
t
Cost higher than forward and flyback converter
Center-tapped transformer
39
5.3.5 Full-bridge converter
S1
+
Ui
+
Power
S2
VD1
S3
uT
N1
+
ud
VD3
L
+
Uo
N2
W1 W2
S4
VD2
S1(S4)
O
S2(S3)
O
VD4
-
to n
t
t
T
uS1(uS4)
Ui
O
uS2(uS3)
t
Ui
t
O
iS1(i S4)
t
O
iS2(iS3)
Uo
N 2 2t on

Ui
N1 T
Cost is even higher
O
iD1(iD4)
O
iS2(iS3)
O
t
iL
t
iL
t
Bi-polar transformer current, up to several hundreds of
kilowatts
40
5.3.6 Rectifier circuits in
the isolated DC to DC converters
VD1
L
VD3 L
VD1
+
+
VD2
Full-wave rectifier
Power
VD4
VD2
Full-bridge rectifier
V1
L
+
V2
Synchronous rectifier
41
5.3.7 Configuration of switching power
supply
Linear power supply
Line
frequency
AC input
Line frequency
DC
Transformer
Rectifier
Filter
Series Pass
Regulator
Regulated
DC output
Power
Isolation
Switching power supply
High
frequency
AC
Line
frequency
AC input
Rectifier
DC
Filter
Inverter
High
frequency
AC
Transformer
Rectifier
Filter
Regulated
DC output
Isolation
Indirect DC to DC converter
42