International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014
Abstract —A new active soft switching circuit for Zero
Voltage Switched Pulse Width Modulated (ZVS-PWM) full bridge converter is presented in this paper. The proposed circuit has two auxiliary circuit cells (Auxiliary circuit cell-1,
Auxiliary circuit cell-2), one for each ground referred active switch. Auxiliary circuit cell consists of an active switch, a diode, a resonant inductor and a capacitor, and a coupled winding derived from main power transformer. Auxiliary circuit when gated properly creates zero voltage across the main switch during its turn-on. Winding coupled to the power transformer helps in resetting auxiliary inductor current to zero and hence turn-off of auxiliary switch is lossless. Steady state operation of proposed circuit with necessary analytical expressions is presented. Circuit simulation results of the proposed active soft switched ZVS-PWM full bridge converter are presented.
Index Terms —Zero voltage switching, active soft switching,
ZVS PWM full bridge.
I NTRODUCTION
The constant demand in Power Processing Systems (PPS) for applications like water pumps, air conditioner fed from
Photo Voltaic (PV) systems (as shown in Fig. 1) is towards higher efficiency and power density with low EMI. PPS for example in applications like air conditioners has to convert
48 VDC input from PV to 230 VAC. This paper focuses on the front end DC-DC converter required to meet such load demands.
I.
auxiliary switches are not soft switched. A new active soft switching circuit for non-isolated and isolated converters is proposed in [6]-[9]. The novelty of these circuits lies in achieving soft switching for both main, auxiliary switches and gating to auxiliary switches are ground referred.
Proposed active soft switched ZVS PWM full bridge converter has two identical auxiliary cells connected to conventional full bridge as shown in Fig. 2. S
1
to S
4
are main switches and D
1
to D converter.C
1
to C
4
4
are output rectifier diodes of the
are capacitors and D parallel diodes across the main switches S
1
B1
to D
to S
4
B4
are anti
respectively.
Auxiliary cell-1 consists of an active switch S
4a resonant inductor L
4r
, resonant capacitor C
4r
, a diode D
4a
, a winding (L
4T
,
) coupled to the primary of power transformer. Auxiliary cell-1 when operated properly achieves ZVS to main switch S
4
.
Gating sequence to main (S
1
S
4a
to S
4
) and auxiliary switches (S
2a
,
) are as shown in Fig. 3. Auxiliary switch S
2a
should be gated immediately when main switch S before S
2
is gated. Auxiliary switch S
4a
3
is turned off and
is also gated in the similar way as that of S
2a
. Turns ratio between primary and secondary of power transformer is n. Turns ratio between primary and coupled windings (L
2T
, L
4T
) is k
T
.
Fig. 1. PV system feeding solar pumps, air conditions systems etc.
Full bridge DC-DC converters are conventional choice for medium and high power applications. Isolation transformer provides high voltage gains apart from providing isolation.
Switching at high frequency provides better power densities but overall system efficiency reduces because of increased switching losses. Soft switched full bridge converters such as
ZVS-PWM full bridge converters addresses this issue [1]-[5].
Salient features of the circuits proposed in these papers are wide range of ZVS, complexity in implementation; higher conduction losses during freewheeling intervals, some of the
Manuscript received June 10, 2013; revised August 27, 2013.
The authors are with Department of Electrical Engineering, Indian
Institute of Technology Madras, Chennai 600036, India (e-mail: gorla.indra@gmial.com, lakshmin@ee.iitm.ac.in).
Fig. 3. Gating pulses (G
(G
2a
, G
4a
) to auxiliary switches (S
2a
, S
4a
) respectively.
1
-G
4
) to main switches (S
1
-S
4
) and gating signals
II.
S TEADY S TATE A NALYSIS
Steady state analysis of the proposed active soft switched
ZVS PWM full bridge converter is presented in this section.
To reduce the complexity of analysis, following assumptions are made. y All the devices are assumed ideal (no R dson
for switches, no forward voltage drop for diodes). y Output filter inductor is large enough to treat it as a constant current source. y Parasitics of transformer such as inter winding capacitance, leakage inductance are neglected.
DOI: 10.7763/IJESD.2014.V5.444
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International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014
Fig. 2. Proposed active soft switching circuit for pulse width modulated full bridge DC-DC convertersSteady state operation of the circuit is divided into twelve intervals (I
1
-I
12
).
Interval1 (I
1
) (t
0
< t < t
1
): This is a power transfer interval and circuit conditions during this interval are as shown in Fig.
4. Devices S interval.
1
, S
2
, D
1 and D
2 are in conduction during this
Circuit conditions during this interval are as shown in the Fig.
6. All rectifier diodes in the secondary D
1
-D
4 are in conduction sharing full load current equally.
L m
 m dt

0; ( ) i
D 1
( )
 i
D 3
( )

I
2 p

I m
Fig. 4. Positive power transfer interval.
L m di m dt

V dc
( )
 m
( )

V dc
L m
( t
 t
0
) p
( )
 i' + i (t) = nI + i (t)
This interval ends when gating to the main switch S2 is turned off.
Interval 2 (I
2
): (t
1
< t < t
2
) Next switch to turn-on is S
3 by the end of this transition interval capacitor C
3 across S
3
, has to discharge completely and body diode D conduction. Gating S
3 while body diode D
B3
B3 should be in in conduction ensures ZVS for main switch S
3
. Circuit conditions during this interval are as shown in Fig. 5.
i
Fig. 6. Freewheeling interval.
This interval ends when S
1 is turned off.
Interval 4 (I
4
) (t
3
< t < t
4
): This interval starts when auxiliary switch S4a is gated immediately when S
1 is turned off. Circuit conditions during this interval are depicted in Fig.
7. Resonant inductor current raises linearly during this interval until it reaches reflected load current in the primary.
All the rectifier diodes (D
1
-D
4
) are in conduction during this interval. Equations governing this interval are as follows.
L
4 r
( )

V dc
L
4 r
( t
 t
3
)
( )

(
D1
( )
 i
D 3
( ))
Fig. 5. Transition from positive power transfer to freewheeling interval.
v dv t
C 3
( )
C 2 dt t


 p
2 C nI

I m
2C
; ( ) p
 nI

I m
( t
 t
1
); v
C 3
( )

V dc
 nI

I m
2C
( t
 t
1
)
Interval 3 (I
3
) (t
2
< t < t
3
): This is a freewheeling interval.
Fig. 7. Linear charging interval in auxiliary cell-1.
This interval ends when resonant inductor current reaches reflected load current.
Interval 5 (I
5
) (t
4
< t < t
5
): Resonant inductor (L
4r
) resonates with resonant capacitor (C
4r
) during this interval.
Voltage across the main switch S
4 decreases from V dc to zero
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International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014 during resonance. Circuit conditions during this interval are as shown in Fig. 8. Equations governing this interval are as follows. By the end of this interval, body diode of main switch S
4
will be in conduction. Gating S
4
while D
B4
in conduction ensures ZVS during turn-on.
This interval ends when main switch S
4
is turned off.
Interval 8 (I
8
): Next switch to conduct is S
1
, hence capacitor C diode D
1
has to discharge completely and anti parallel
B1
should conduct to ensure ZVS turn-on for S
Circuit conditions during this interval are shown in Fig. 11.
1
.
Fig. 8. Resonant interval in auxiliary cell-1. i
L
4 r
( ) = + dc
C
4 r
L
4 r
Sin
( ω ( t t
4
) ) v
C
4 r
( ) = V cos
( ω ( t - t
4
) )
, where ω =
1
L C
4 r
Interval 6 (I
6
) (t
5
< t < t
6
): Circuit conditions during this interval are as shown in Fig. 9. Voltage across resonant capacitor is − V dc
/k
T
. This capacitor is clamped by the body diode of S4 to resets the resonant inductor current to zero by the end of this interval. Turning off S
4a
after complete reset of auxiliary inductor current ensures ZCS during its turn-off.
Equations governing this interval are as follows.
Fig. 11. Transition from negative power transfer to freewheeling interval.
Interval 9 (I
9
): This is a freewheeling interval. Main switch S
3
and anti parallel diode D
B1
are in conduction during this interval. All the rectifier diodes D
1
-D
4
are in conduction since voltage applied across the primary of transformer is zero during this interval.
Fig. 12. Freewheeling interval.
Fig. 9. Resetting interval in auxiliary cell-1. i
L
4 r
( ) = i
L
4 r
( ) −
V dc
K L
4 r
( −
5
)
=
⎛
⎜
⎝ nI V dc
C
L
4
4 r r
⎛
⎜
K
K
2
T
T
− 1
⎞
⎟
⎞
⎟ −
V dc
K L
4 r
( −
5
)
Interval 7 (I
7
): This is a negative power transfer interval.
Main switches S
3
and S
4
, output rectifier diodes D
3
, D
4
are in conduction during this interval. Circuit conditions during I
7 are as shown in Fig. 10.
Fig. 10. Negative power transfer interval.
Circuit conditions during I
9 are as shown in Fig. 12. This interval ends when gating to active switch S
3 is removed.
Interval 10 (I
10
): Auxiliary switch S
2a is gated immediately when S
3 is turned off as shown in Fig. 3. The circuit condition during this interval is as shown in Fig. 13.
Current through resonant inductor (L
2r
) raises linearly. All the rectifier diodes D
1
-D
4 continue to conduct during this interval.
Fig. 13. Linear charging interval in auxiliary cell-2.
This interval ends when resonant inductor current reaches reflected load current.
Interval 11 (I
11
): Resonant inductor L
2r capacitor C
2r and resonant resonates during this interval. Output rectifier diodes D
3 and D
4 gets reverse biased and D
1 and D
2 conducts full load current. By the end of this interval, capacitor C
2 across the main switch S
2 diode D
B2 discharges completely and body starts conducting. Gating S
2
, while D
B2 in conduction ensures ZVS during turn-on. Circuit conditions during this interval are depicted in Fig. 14.
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International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014
Fig. 14. Resonant interval in auxiliary cell-2.
Interval 12 (I
12
): The circuit condition during this interval is as shown in Fig. 15. The resonant capacitor voltage is clamped to –V dc
/k
T
since D
B2
is in conduction during this interval. This resets resonant inductor current linearly to zero.
Turning off S
2a
when resonant inductor current is zero ensures ZCS during its turn-off.
Fig. 15. Resetting interval of auxiliary cell-2.
Next interval is positive power transfer interval i.e., I
1
.
Equations governing interval 1 to interval 6 also holds good for interval 7 to interval 12.
Modes of operation along with interval description are shown in Table I. Theoretical waveforms for proposed converter is shown in Fig. 16 & Fig. 17. Auxiliary circuit intervals i.e., linear charging interval (t interval (t
4
3
< t < t
4
), Resonant
< t < t
5
) and resetting interval (t
5
< t < t
6
) are indicated in Fig. 17. As mentioned earlier, voltage across the coupled winding helps in resetting the resonant inductor current to zero. Turning off the auxiliary switch after resetting the resonant inductor current reduces the switching losses in the auxiliary switches as shown.
TABLE I: M ODES OF O PERATION OF P ROPOSED A CTIVE S OFT S WITCHED
Z VT P WM F ULL B RIDGE D C -D C C ONVERTER
Fig. 16. Theoretical waveforms of proposed active soft switched ZVS PWM full bridge DC-DC converter.
Fig. 17. Theoretical waveforms showing gating to auxiliary switch, corresponding auxiliary inductor current, gating and drain to source voltage of main switch.
III.
R ESULTS AND D ISCUSSIONS
Circuit simulations results of proposed active soft switched ZVS-PWM full bridge DC-DC converter is presented in this section. Gating to the main switches are
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International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014 shown in Fig. 18. It can be seen from the figure that gating to the ground referred switches S
2
, S
4
are PWM controlled.
Main switches S dead time between switches of same legs i.e., between S
S
4
or S
2
and S
3
1
, S seen from this figure that auxiliary switch S
4a immediately after S
1
3
are gated complementary with proper
. Gating to auxiliary switch S cell-1 and main switches S
1
, S
4
are shown in Fig. 19. It can be is turned off and before S
4
1
and
4a
of auxiliary
is gated
is turned on.
Gating to auxiliary switch S
4a
, current through auxiliary inductor L
4r
, gating to main switch S voltage of main switch S
4
4
and drain to source are shown in Fig. 20. Similarly gating to auxiliary switch S
2a inductor L
2r
, current through auxiliary
, gating to main switch S
2
and drain to source .
Voltage of main switch S
2
is shown in Fig. 21. The following inferences are drawn from these figures.
• Drain to source voltage of main switches S
2
and S
4
is zero during their turn-on because of auxiliary cell-1 and auxiliary cell-2. Hence main switches are turn-on with
ZVS.
• Auxiliary switch current which is same as auxiliary inductor current is made zero before its turn-off. This ensures ZCS turn-off for auxiliary switches.
• Auxiliary switches conduct for smaller duration of time
(apx 7-10%) of total switching period. Hence conduction losses due to additional circuit will be less.
Fig. 18. (from top) Gating to main switches S
1
-S
4
.
Fig. 19. (from top) Gating to main switches S
1
, Gating to auxiliary switch S
4a
,
Gating to main switch S
4
.
Fig. 22. (from top) Gating to the main switches S
1 voltage of S
1
.
and Drain to source
Fig. 20. (from top) Gating to auxiliary switches S
4a
, Resonant inductor current I
L4r
, Gating to main switch S
4
, Drain to source voltage of S
4
.
Fig. 23. (from top) Gating to the main switches S
3 voltage of S
3
.
and Drain to source
Gating and drain to source voltage of main switches S
1 and
S
3 are shown in Fig. 22 and Fig. 23 respectively. It can be seen from these figures that drain to source voltage has come down to zero before gating to main switches are given. It can be concluded that additional auxiliary cells did not disturbed the ZVS mechanism for main switches S
1 and S
3
.
Fig. 21. (from top) Gating to auxiliary switches S
2a
, Resonant inductor current I
L2r
, Gating to main switch S
2
, Drain to source voltage of S
2
.
24
IV. C ONCLUSION
A new active soft switched ZVS-PWM full bridge DC-DC converter is proposed in this paper. The proposed auxiliary circuit achieves zero voltage during turn-on for main switches without affecting the zero voltage turn-on conditions for other switches. Proposed auxiliary circuit operates for small duration of time (10% of Ts) and hence additional conduction losses would be less. Current through the auxiliary switch is made zero during turn-off which reduces the turn-off losses in the auxiliary switches.

International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014
R EFERENCES
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[7] G. N. B. Yadav and N. Lakshminarasamma, “Novel soft transition pushpull converter: Analysis, modeling, design and implementation,” in Proc. IECON 2011 - 37th Annual Conference on IEEE Industrial
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G. N. B. Yadav and N. Lakshminarasamma, “Analysis and Design of a
Novel Active Soft Switched Phase-Shifted Full Bridge Converter,”
World Academy of Science, Engineering and Technology
Naga Brahmendra Yadav Gorla received B.Tech degree in Electrical and electronics Engineering from
Acharya nagarjuna university, India in 2010. He is currently working towards MS (Research) in Power
Electronics at Indian Institute of Technology Madras,
Chennai, India. His areas of interest include DC-DC conversion, inverters, parasitic effects at higher switching frequencies etc.
N. Lakshmi Narasamma
73, 2013.
obtained her Ph.D. degree in Electrical Engineering from the Indian
Institute of Science and joined the faculty of
Electrical Engineering at the Indian Institute of
Technology, Madras as an Assistant Professor in the year 2009. She has coauthored four journal papers in peer-reviewed journals, including the
IEEE Transactions on Power Electronics and several premier conferences. Her research interests are in the areas of Power Electronics and drives.
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