Chapter 8
Soft-Switching Techniques
Pursuing of higher switching frequency
Better waveform
• PWM waveform will be closer to expected waveform
• Harmonics is easier to be filtered
Power
Benefits
Smaller volume and weight
• 4.44fNf =4.44fNAB
1
• f =
2 LC
Faster response
2
Disadvantages of higher switching frequency
Disadvantages
Higher power losses on power semiconductor devices
– Power loss=f x Energy loss at each switching
More severe electromagnetic interference (EMI)
Power
– Steeper edges introduce more noises
– Easier to be radiated
Solution
Soft-switching techniques
3
Outline
8.1 Basic concepts on soft-switching
Power
8.2 Classification of soft-switching techniques
8.3 Typical soft-switching circuits and techniques
4
8.1 Basic concepts on soft-switching
Hard-switching
u
i
u
i
O
Power
u
i
t
p
i
u
O
t
p
O
t
a) Turning-on
O
t
b) Turning-off
The process of power semiconductor device
hard-switching
5
The concept of soft-switching
Soft-switching
u
i
u
i
Power
O
u
i
t
p
i
u
O
t
p
O
t
a) Turning-on
O
t
b) Turning-off
The process of power semiconductor device
soft-switching
6
Two types of soft-switching
ZVS—Zero-voltage switching
Power
Specifically means zero-voltage turn-on, i.e., the
voltage across the device is reduced to zero before the
current increases
ZCS—Zero-current switching
Specifically means zero-current turn-off, i.e., the
current flowing through the device is reduced to zero
before the voltage increases
7
8.2 Classification of soft-switching
techniques
Quasi-resonant soft-switching
Power
ZVS PWM (zero-voltage-switching PWM) and ZCS
PWM (zero-current-switching PWM)
ZVT PWM (zero-voltage-transition PWM) and ZCT
PWM (zero-current-transition PWM)
8
The concept of basic switch cell
L
S
L
S
VD
VD
b)
Power
a)
VD
L
S
c)
L
S
VD
d)
9
Quasi-Resonant Converter—QRC
ZVS QRC
ZCS QRC
ZVS MRC (multi-resonant converter)
Resonant DC link converter
Cr1
Power
Cr
Lr
Lr
L
S
S
VD
a) ZVS QRC
Lr
L
S
Cr
VD
b) ZCS QRC
Cr2
L
VD
c) ZVS MRC
Basic switching cells for QRC
10
ZVS PWM converter and ZCS PWM converter
Feature: use of auxiliary switch
Cr
Power
S
S1
Lr
L
Lr
VD
L
S
S1
Cr
a)
VD
b)
Basic switching cells for a) ZVS PWM and b) ZCS PWM
11
ZVT PWM converter and ZCT PWM converter
Feature: auxiliary switch is in parallel with main switch
Lr
Lr
Power
Cr S1
Cr
S1
L
S
VD1
VD
a)
L
S
VD1
VD
b)
Basic switching cells for a) ZVT PWM and b) ZCT PWM
12
8.3 Typical soft-switching circuits
and techniques
ZVS QRC
Power
Resonant DC link converter
Phase-shift full bridge ZVS PWM converter
ZVT PWM converter
13
ZVS QRC
Cr
S
Power
Ui
Lr
VDS
S
L
A
VD
t
O
uS (uCr)
C
R
t
O
iS
t
O
+
uCr
Cr
IL
+
A
uCr
Cr
Ui
Ui
iLr
iLr
uVD
O
to ~ t1
t
O
t
t0t1 t2 t3t4 t5t6
t0
t1 ~ t2
14
Resonant DC link converter
Circuit
Lr
VDS
uCr
Ui
S
Ui
Cr
t
Power
O
Equivalent circuit to RDCL
iLr
IL
Lr
L
Cr
Ui
iLr
S
+
uCr
IL
O
R
t0 t1
t2
t3 t4
t
t0
VDS
15
Summary of QRC
Disadvantages
– Voltage stress increased due to the resonant peak
Power
– Current RMS value increased due to large
circulating energy
– Variable switching frequency due to pulse-frequency
modulation (PFM) control
16
Phase-shift full-bridge ZVS PWM converter
S1
Circuit
O
S2
t
O
S4
t
O
S3
O
S1
Power
A
Ui
S2
CS1 S3
Lr
CS3
B
CS2 S
4
+
VD1 uR L
VD2
CS4
t
uAB
t
O
t
uLr
t
O
C R
iLr
O
t
uT1
O
t
uR
O
iL
t
O
iVD1
t
O
iVD2
t
O
t8 t9 t0
t 1 t 2 t 3 t 4 t5
t6 t7 t8 t9 t 0
t
17
Phase-shift full-bridge ZVS PWM converter
CS1
A
iLr
Lr
iL
kT:1
VD1
Power
Ui
CS2
VDS2 S4
Equivalent circuit during
t1 ~ t2
L
+
Uo
R
iLr
CS 3
iL
VDS 3
VD1
L
Lr
Ui
R
B
S2
CS 4
+
Uo
VD2
Equivalent circuit during
t3 ~ t4
18
Summary of ZVS or ZCS PWM converter
Improvement over QRC
Power
– Voltage and current are basically square-wave,
therefore stresses are greatly reduced
– Constant switching frequency due to pulse-width
modulation (PWM) control
19
ZVT PWM converter
Circuit
S
iV D
IL
S1
L
Lr
VDS
Ui
t
O
Cr
S1
S
O
uS
VD
iLr
+
R
VD1 C Uo
t
t
O
iLr
IL
t
Power
O
iS1
Equivalent circuit during t1 ~ t2
IL
L
Ui
VDS
iLr
Lr
t
O
uS1
t
O
iD
t
O
iS
Cr
S1
t
O
t 0 t1
t2 t3 t4
t5
20
Summary of ZVT or ZCT PWM converter
Improvement over ZVS or ZCS PWM converter
Power
– Soft-switching can be achieved in a wider range of
input voltage and load current
– Circulating energy is reduced to minimum so that
efficiency is increased
21