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Chapter 8
Composite Converters
Power Electroni
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The concept of composite converter
Composite converter:
Combination of two or more converters in cascaded
connection
Indirect AC to AC converters
(AC-DC-AC converters)
Composite converters
Indirect DC to DC converters
(Isolated DC to DC converters)
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8.1 Indirect AC to AC converters
(AC-DC-AC converters)
Classifications
According to type of the DC links:
Voltage-source type
AC-DC-AC converters
Current-source type
Variable voltage variable frequency
(VVVF)
(AC-DC-AC frequency converters)
According to whether output voltage and frequency is
variable:
AC-DC-AC converters
Constant voltage constant frequency
(CVCF)
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8.1.1 Configurations of AC-DC-AC converters
Configurations with one-direction power flow
AC
- P B E
AC
4 P V S D F
AC
- P B E
AC
4 P V S D F
Voltage-source type Current-source type
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Configuration with regenerative energy
dissipating circuit
AC
- P B E
AC
4 P V S D F
V
0
R
0
Braking transistor and braking resistor
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Configurations with regenerative power feedback
through inversion-mode thyristor rectifier circuit
AC
- P B E
AC
4 P V S D F
AC
- P B E
AC
4 P V S D F
I
d
U
d
U
L
Voltage-source type Current-source type
AC
- P B E
AC
4 P V S D F
U
V
W
a
b
c
4 P V S D F - P B E
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Configurations realizing bi-directional power
flow through double-sided PWM converters
Voltage-source type
Current-source type
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8.1.2 Major Applications of AC-DC-AC
frequency converters (VVVF converters)
Adjustable speed AC motor drives
Advantages of AC motors over DC motors
Energy saving on AC motors
High-performance AC motor drives
Control of AC motors driven by AC-DC-AC frequency
converters (VVVF converters)
Constant voltage frequency ratio control
Slip frequency control
Vector control
Direct torque control
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8.1.3 Major Applications of CVCF converters
Uninterruptible power supplies (UPS)
Basic configuration of UPS
Source
Electricity
utility
Rectifier Inverter
Load
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UPS with back-up energy source
Source
Electricity
utility Rectifier Inverter
Load
Diesel
Engine
S
1
2
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UPS with back-up energy source
and bypass lines
Source
Electricity
utility
Rectifier
Inverter
Load
Diesel
Engine
1
2
3
4
Bypass lines
S
1
S
2
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8.2 Indirect DC to DC converters
` (isolated DC to DC converters)
Inverter Transformer Rectifier Filter
DC input
AC
AC
DC output
High frequency
Isolation
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
Reduction of transformer and filter volume and weight
Power semiconductor devices usually used
Inverter part: Power MOSFETs, IGBTs
Rectifier part: Fast recovery diodes, Schottky diodes, Synchronous
rectifiers
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Classification of isolated DC to DC converters
According to whether transformer current is
uni-direction or bi-directional
Single-ended converters
? Forward converter
? Flyback converter
Isolated DC to DC
converters
Double-ended converters
? Half bridge
? Push-pull
? Full bridge
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8.2.1 Forward converter
Simple, low cost
Uni-polar transformer current, low power applications
S
u
S
i
L
i
S
O
t
t
t
t
U
i
O
O
O
+
+
U
i
S
VD
1
VD
2
L
N
1
N
3
N
2
VD
3
U
o
W
2
W
1
W
3
T
t
N
N
U
U
on
1
2
i
o
=
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8.2.2 Flyback converter
Simple, low cost
Uni-polar transformer current, low power applications
+
+
U
i
S
VD
N
1
N
2
U
o
W
1
W
2
S
u
S
i
S
i
VD
t
on
t
off
t
t
t
t
U
i
O
O
O
O
off
on
1
2
i
o
t
t
N
N
U
U
=
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8.2.3 Half bridge converter
Cost higher than forward and flyback converter
Bi-polar transformer current, up to several kilowatts
+
+
S
1
S
2
VD
1
VD
2
L
U
i
N
1
N
2
N
3
+
u
d
U
o
+
C
1
C
2
W
1 W
3
W
2
S
1
S
2
u
S1
u
S2
i
S1
i
S2
i
D1
i
S2
t
T
t
t
t
t
t
t
t
t
on
U
i
U
i
i
L
i
L
O
O
O
O
O
O
O
O
T
t
N
N
U
U
on
1
2
i
o
=
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8.2.4 Push-pull converter
++
S
1
S
2
VD
1
VD
2
L
U
i
N
1
N
1
'
N
2
N
2
'
U
o
S
1
S
2
u
S1
u
S2
i
S1
i
S2
i
D1
i
S2
t
on
T
t
t
t
t
t
t
t
t
2U
i
2U
i
i
L
i
L
O
O
O
O
O
O
O
O
T
t
N
N
U
U
on
1
2
i
o
2
=
Cost higher than forward and flyback converter
Center-tapped transformer
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8.2.5 Full-bridge converter
Cost is even higher
Bi-polar transformer current, up to several hundreds of
kilowatts
+
+
N
1
S
2
S
3
S
4
VD
1
VD
2
VD
4
VD
3
L
S
1
N
2
+
+
u
d
u
T
U
i
U
o
W
2
W
1
S
1
(S
4
)
S
2
(S
3
)
u
S1
(u
S4
)
u
S2
(u
S3
)
i
S1
(i
S4
)
i
S2
(i
S3
)
i
D1
(i
D4
)
i
S2
(i
S3
)
t
on
T
t
t
t
t
t
t
t
t
U
i
U
i
i
L
i
L
O
O
O
O
O
O
O
O
T
t
N
N
U
U
on
1
2
i
o
2
=
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8.2.6 Rectifier circuits in
the isolated DC to DC converters
+
VD
1
VD
2
L
+
VD
1
L
VD
3
VD
2
VD
4
+
V
1
V
2
L
Full-bridge rectifier
Synchronous rectifier
Full-wave rectifier
Linear power supply
? Switching power supply
20
Line
frequency
AC input
Rectifier Filter
Series Pass
Regulator
Regulated
DC output
Transformer
DC
InverterFilter Transformer
High
frequency
AC
Rectifier Rectifier Filter
AC
High
frequency
Regulated
DC output
Isolation
Indirect DC to DC converter
Line frequency
Isolation
8.2.7 Configuration of witching power supply
Line
frequency
AC input
DC
