ZVZCS PWM DC-DC Converter with energy recovery clamp
99
ZVZCS PWM DC-DC CONVERTER WITH ENERGY RECOVERY CLAMP
J. Dudrik, V. Ruš in
Department of Electrical, Mechatronic and Industrial Engineering,
Technical University of Košice, Letná 9, 04200 Košice, Slovak Republic
E-mail: jaroslav.dudrik@tuke.sk, phone: +421 55 6022276, fax: +421 55 6330115
Summary A novel zero-voltage zero-current switching full-bridge PWM DC-DC converter with controlled secondary side
rectifier is presented in this paper. Zero-voltage turn-on and zero-current turn-off for all power switches of the inverter is
achieved for full load range from no-load to short circuit by using turn-off snubber and secondary energy recovery clamp.
Modified PWM control strategy is used for the converter. The principle of operation is explained and analysed and simulation
results are presented.
considerable high involvement in creation of the
IGBT turn-off losses.
1. INTRODUCTION
The conventional phase shifted PWM converters are
often used in many applications because their
topology permits all switching devices to operate
under zero-voltage switching by using circuit
parasitics such as power transformer leakage
inductance and devices junction capacitance.
However, because of phase-shifted PWM control, the
converter has a disadvantage that circulating current
flows through the power transformer and switching
devices during freewheeling intervals.
To decrease the circulating current to zero and thus to
achieve zero-current switching, various snubbers,
auxiliary circuits and/or clamps connected mostly at
the secondary side of power transformer are applied
[1] – [10].
The disconnection of the secondary windings is
usually achieved by application of the reverse bias
for the output rectifier or using controlled rectifier.
The optimal switching for IGBTs is zero-voltage
turn-on and mainly zero-current turn-off due to
elimination of the current tail influence, which has
T
1
D
2. POWER CIRCUITS OF THE PROPOSED
CONVERTER
To achieve the aims mentioned above, the topology of
the following ZVZCS converter was designed.
The DC-DC converter shown in Fig.1 consists of
high-frequency inverter, power transformer, output
rectifier, output secondary switch and output filter.
The main part of the converter includes high
frequency full-bridge inverter consisting of the four
ultrafast IGBT´s T1-T4 and freewheeling diodes D1D4. The secondary winding of the high-frequency
step-down power transformer TR is connected
through a fast recovery rectifier D5, D6 and secondary
switch T5 to output filter consisting of smoothing
choke L0 and capacitor C0.
The converter is controlled by modified pulse-width
modulation (Fig.2), and consequently the zero-voltage
turn-on and zero-current turn-off all of the transistors
T1-T4 in the inverter are reached.
1
u
iP
U
D
3
D
2
L
O
T
3
P
T R
T
D
4
D
5
U
S
U
D
S
6
4
T
2
_
C
U
D
d
A
In p u t
S o u rce
A u x il i a r y
D C /D C
C o n v e r te r
E n e r g y r e c o v e r y c la m p
C
C 1
D
U
I0
O
R
O
C
C1
CC 1
U
O
T
D
5
C
O
D
S 2
L
S1
S 1
S2
D
A
S
S 3
T u rn -o ff s n u b b e r
Fig.1. Scheme of the proposed ZVZCS PWM DC-DC converter
100
Advances in Electrical and Electronic Engineering
The semiconductor switch T5 in the secondary side is
used to reset secondary and consequently also
primary current. The transistor T5 operates with
double switching frequency. At turn-off of the switch
T5 the energy stored in leakage inductance is clamped
by DC1 and CC1 and then transferred trough auxiliary
dc-dc converter to the input source. By using nondissipative snubber to reduce turn-off losses of the
transistor T5, the overall efficiency is increased.
3. OPERATION PRINCIPLE
voltage because at turn-on of the transistor T4 its
freewheeling diode D4 is in on state. Moreover the
rate of rise of the collector current is limited by the
leakage inductance LLP of the transformer.
The transistor turn-off losses are negligible
because transistor T4 turns-off only small magnetizing
current (about 1.5 Amp in this case) as can be seen in
the Fig.3.
1
12A
2
4 00V
3 00V
The basic operation of the proposed soft switching
converter has eight operating modes (intervals)
within each half cycle. The switching diagram and
operation waveforms are shown in Fig. 2.
8A
2 00V
4A
1 00V
0A
T1, T2
0
uP
T3, T 4
td
T5
T 1, T 2
T5
T5
>>
-3A
t
0V
-1 00V
1
168. 0us
17 0.0 us
1 72. 0us
IC(Z 4)- I(D4 ) 2
V(P 1)
17 4.0u s
U
0
t
iP
IP
Iµ
0
176 .0us
1 78.0 us
18 0.0u s
Tim e
0
Fig. 3. Switch (transistor T4 + diode D4) voltage uCE4 and
switch current i C4+i D4.
t
uS
U CC1
US
US
0
t
iD O
iD
iD O
iD 5
iD 6
I D m in
ID m a x
iD 0
iD 5
t
0
I D m in + I L S m ax
IT 5 m
iT 5
0
t
I C S = I T 5 m /2
iCS1
0
iD C 1
t
I L S m ax
I Dm a x
100
0
t
t0 t 1 t2 t3
Collector-emitter voltage uDS and collector current iD
of the secondary transistor T5 (bottom waveforms) is
shown in Fig. 4. The secondary switch (transistor T5)
is turned-on under zero-current due to influence of the
leakage inductance of the transformer LLS reflected to
the secondary side and snubber inductance LS.
The turn-off loss is reduced by snubber capacitors
CS1 and CS1 acting as the non-dissipative snubber as it
is evident in Fig. 4.
t 4 t5 t6
80
t 7 t8
60
Fig. 2 Operation waveforms of the converter
40
4. SIMULATION RESULTS
A simulation model in programme Orcad was created
to verify the properties of the proposed converter.
The simulations were made at input voltage U =
320V.
Parameters:
Transformer TR parameters:
Turns ratio n = 6,
Magnetizing inductance Lm = 800 H,
Leakage inductance LLP = 7 H.
Snubber circuit parameters:
Snubber capacitors CS1 = CS2= 56 nF,
Snubber inductance LS = 5 H.
Set value of clamp voltage UCC1 = 80 V
The following waveforms were obtained at resistive
load.
Fig.3 shows switch voltage uCE4 and switch
current iC4+iD4 during turn-on and turn-off of the
transistor T4 in the converter. The switch (transistor
T4 including diode D4) is turned-on under zero-
20
0
-20
169u s
170us
I(Rco llect or)
1 71us
17 2us
V(M1: d)- V (M1:s )
17 3us
174 us
175u s
176us
1 77us
1 78us
Time
Fig. 4. Collector-emitter voltage uDS and collector
current i D of the secondary transistor T5
Rectified voltage ud and output diode current iD5 is
shown in Fig. 5.
ZVZCS PWM DC-DC Converter with energy recovery clamp
Turn-off snubber is employed to improve turn-off
process of the secondary switch. Using leakage
inductance of the power transformer working as a
turn-on snubber it is ensured zero current turn-on
process of the secondary switch.
Finally, it is possible to say, that IGBTs in the full
bridge inverter operate at almost ideal switching
conditions – ZV turn-on and ZC turn-off.
Soft switching of the secondary switch and leakage
inductance energy transfer to the input source is
ensured by turn-off snubber and energy recovery
clamp containing only non-dissipative components.
100 V
50 V
0V
V( UD)
100 A
50 A
0A
SEL >>
178 us
I( D5)
180 us
182u s
18 4us
18 6us
18 8us
190 us 19 1us
Time
Fig. 5. Rectified voltage ud (upper waveform)
Output diode current i D5 (bottom waveform)
During commutation between secondary diode and
output freewheeling diode the secondary voltage and
accordingly rectified secondary voltage is zero.
At turn-off of the secondary switch T5 the secondary
and also rectified voltage rises as a result of energy
stored in leakage inductance. The over-voltage US
can be maintained on acceptable value by proper
design of the auxiliary dc-dc converter.
Fig. 6 shows the snubber capacitor current at turn-on
and turn-off of the secondary switch T5 (bottom
waveform).
Acknowledgment
This work was supported by Research and
Development Support Agency under project APVV99-031205.
REFERENCES
100
50
0
I(Rcollector)
V(M1:d)- V(M1:s)
50A
25A
0A
SEL>>
-20A
169us
I(Cs1)
170us
171us
172us
173us
174us
175us
176us
177us
Time
Fig. 6. Collector voltage uDS and collector current i D of
the secondary transistor T5 (upper waveforms)
Snubber capacitor current i CS1 (bottom waveform)
5. CONCLUSION
Soft switching and reduction of circulating currents in
the proposed converter are achieved for full load
range using the secondary side turn-off and energy
recovery clamp in combination with modified PWM
control.
At proper design it is possible to utilize the
magnetizing current of power transformer for
charging or discharging output capacitances of the
IGBT switches and thus zero-voltage turn-on of the
IGBTs to achieve.
The IGBT transistors are turned-off almost under zero
current. Only negligible magnetizing current of the
power transformer is turned-off by IGBT transistors.
The main task of the proposed secondary energy
recovery clamp is the transfer the leakage inductance
energy to the input source at turn-off of the secondary
switch.
101
178us
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