Features:  All the benefits of Zero-Voltage-Zero-Current-Switched (ZVZCS) converters.  All switches operate with Zero-Voltage-Switching (ZVS).  Can be Implemented with MOSFETs hence operating in high switching frequencies.

1. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
A ZVS-PWM Full-Bridge Converter with Reduced
Conduction Losses (T22)
IEEE Applied Power Electronics Conference and Exposition (APEC)
Fort Worth, TX, USA
Dunisha Wijeratne & Gerry Moschopoulos
University of Western Ontario, London, ON, Canada
2011 March 9th

2. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Outline
• Introduction

3. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Outline
• Introduction
• Proposed Converter

4. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Outline
• Introduction
• Proposed Converter
• Steady-State Operation

5. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Outline
• Introduction
• Proposed Converter
• Steady-State Operation
• Converter Features

6. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Outline
• Introduction
• Proposed Converter
• Steady-State Operation
• Converter Features
• Design Guidelines

7. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Outline
• Introduction
• Proposed Converter
• Steady-State Operation
• Converter Features
• Design Guidelines
• Experimental Results

8. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Outline
• Introduction
• Proposed Converter
• Steady-State Operation
• Converter Features
• Design Guidelines
• Experimental Results
• Conclusion

9. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Outline
• Introduction
• Proposed Converter
• Steady-State Operation
• Converter Features
• Design Guidelines
• Experimental Results
• Conclusion

10. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Introduction
• Generally there is a need to design the fastest, most efficient and compact
power converter.
• With soft switching in the switches (e.g. ZVS) it is possible to operate
the converter with high switching frequencies.
• Under light load conditions, MOSFETs cannot turn on with ZVS as there
is insufficient current to discharge Coss .
• Many researchers, therefore, have proposed variations on the basic
ZVS-PWM-FB topology to extend the load range.

11. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Introduction
• Some topologies use extra passive components to generate current in the
converter’s primary side to discharge Coss of MOSFETs;
• But any efficiency gain is offset by additional conduction losses.

12. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Introduction
• Another approach is to add active components to the standard topology.
• Current needed to discharge Coss of MOSFET at light loads is generated
without increasing conduction losses because the auxiliary circuit conducts
only for a shorter duration.
• None of these approaches decrease conduction losses.

13. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Introduction
• Therefore, zero-voltage-zero-current-switching (ZVZCS) PWM FB
converters have been proposed.
• A passive auxiliary circuit extinguishes circulating current in the
transformer primary.
• ZCS, however, prevents the lagging leg switches turning on with ZVS.
• As a result, IGBTs (which have lower Coss than MOSFETs) are preferred
over MOSFETs.
• But IGBTs are slower than MOSFETs - switching frequency must be
limited.

14. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Proposed Converter
• The ZVZCS FB section consists of switches S1 -S4 , main transformer
(Tm ), secondary side rectifier diodes, Aux. 2 and the output filter.
• Aux. 1: Switches Sa , Sb , blocking diodes DSa , DSb , resonant components
Lr , Cr and transformer (Ta ).
• Aux. 2: Capacitor Cx and diodes Dc , Dd .

15. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Proposed Converter
• Aux. 1: Becomes active just before a lagging leg switch is turned on and
lasts till iLr = 0.
• Aux. 1: Provides a path for Coss of the lagging leg switches to discharge,
so that they can be turned on with ZVS.

16. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Steady-State Operation
Mode 1 (t0 < t < t1 )
• Converter behaves as a standard
ZVZCS FB converter.
• Mode forms part of the power
transfer mode.
• iin flows through Llk and Llk
resonates with Cx .
• Cx reaches its peak voltage at the
end of the mode.

17. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Steady-State Operation
Mode 2 (t1 < t < t2 )
• Vin is applied entirely across the
primary winding of Tm .
• Cx remains at its peak voltage.

18. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Steady-State Operation
Mode 3 (t2 < t < t3 )
• S1 is turned off at t = t2 .
• ip charges and discharges switch
capacitors CS1 and CS3
respectively.
• Discharging and charging is linear
until the primary voltage of Tm
drops to a level that equals the
reflected Cx voltage.

19. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Steady-State Operation
Mode 4 (t3 < t < t4 )
• The non-linear dead time between
S1 and S3 .
• Tm ’s primary voltage decreases but
the secondary side rectifier voltage
is held by Cx .
• The difference of Vin and Tm
primary voltage is applied across
Llk .
• ip starts to decrease.
• When ip falls below the reflected
load current, Cx starts to discharge
to bridge the gap.

20. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Steady-State Operation
Mode 5 (t4 < t < t5 )
• S3 can be turned on with ZVS.
• Converter starts to freewheel.
• Towards the end of the mode, ip
= 0 and S2 can be turned off
with ZCS at t = t5 .

21. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Steady-State Operation
Mode 6 (t5 < t < t6 )
• After S2 is turned off, Sa can be
turned on with ZCS.
• Sa allows CS4 to discharge into
Aux. 1.
• Part of iin charges CS2 and the
remainder enters Tm .
• CS4 is fully discharged at t = t6 .

22. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Steady-State Operation
Mode 7 (t6 < t < t7 )
• At t = t6 , iin = ip .
• Vin is applied across Llk ; thus ip
increases linearly until it equals
the reflected load current.
• Current in the secondary side of
Tm freewheels.
• In Aux. 1, iLr comprises of the
Tm ’s primary current ip and the
current of DS4 .

23. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Steady-State Operation
Mode 8 (t7 < t < Ts /2)
• ip starts to increase beyond Io /nm .
• Resonance of Cr and Lr decreases
iLr .
• As ip is increasing while iLr is
decreasing, the window of
opportunity for the lagging leg
switches to turn on with ZVS lasts
till ip <= iLr .
• At t = Ts /2, iLr = 0, Sa is turned
off with ZCS sometime thereafter.

24. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Converter Features
• Main switches:
• S1 -S4 turn on with ZVS.
• S2 and S4 turn off with ZCS.
• Aux. switches:
• Sa and Sb turn on and off with ZCS.

25. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Converter Features
• Freewheeling current in Tm is extinguished so that conduction losses are
reduced.
• Llk in Tm can be minimized; thus duty cycle loss is minimized.

26. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Converter Features
• Aux. 1 conducts for a very short duration.
• Its components can be implemented with lower power rated devices.
• When Aux. 1 is implemented with a small transformer, energy in
the circuit can be transferred to the load.

27. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Design Guidelines

28. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Design Guidelines
Main transformer (Tm ) turns ratio nm
• Since the input to output voltage conversion ratio is fixed, nm should be
selected simultaneously with the duty ratio D.
• nm should not be too low as that will increase the current in Tm and the
converter will need to be in the freewheeling mode for a longer time to
completely remove iLlk .
• A higher nm can have a negative influence on the voltage regulation of
the converter.

29. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Design Guidelines
Main transformer (Tm ) Leakage Llk
• Lo aids ZVS operation in S1 and S3 .
• Therefore Llk can be minimized.
• A lower Llk will decrease the duty ratio loss from the primary to the
secondary side.
• A low Llk also aids the ZCS.

30. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Design Guidelines
Aux. 2 Capacitor(Cx )
• Main function of Cx is to create a counter voltage across Llk to ensure
that the primary current decreases to zero within the freewheeling time.
• If Cx is too small, it will not have sufficient energy stored in it to
discharge Llk .
• If Cx is too big, then unnecessary conduction losses will occur in Aux. 2.

31. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Design Guidelines
Aux. 1 Inductor(Lr )
• Higher Lr increases, the characteristic impedance in Aux. 1 and thus
reduces the peak current stress.
• This allows use of lower current rated switches in Aux. 1.
• A lower Lr in Aux. 1 means the time at which iLlk = iLr in Mode 2 gets
closer to t2 ; hence the window of opportunity narrows, making the ZVS
operation difficult.

32. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Experimental Results
Converter Design
Design Specification Converter Parameters
Input voltage: 380 Vin Tm turns ratio: nm = 4
Output voltage: Vo = 48Vdc
Ta turns ratio: na = 0.1
Output power: 500 kW
Aux 2 capacitor: Cx =0.1 µF
Switching frequency: fs =125 kHz
Aux 1 capacitor: Cr =100 nF
Maximum power: Po,max =500 W
Aux 1 inductor: Lr =1 µH

33. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Experimental Results
Lagging Leg Switching
• Lagging leg switches turn on with ZVS and turn off with ZCS.
• Current in the switch is negative so that it has a ZVS turn on.
• Current in the switch is zero as the freewheeling current is extinguished
before it is turned off, so that it has a ZCS turn off.

34. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Experimental Results
Leading Leg Switching
• Leading leg switches turn on with ZVS.

35. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Experimental Results
Aux. 2 Diode Waveforms
(a) Dc voltage and current (b) Dd voltage and current.
• Voltage and current waveforms of the two diodes in Aux. 2.
• Both diodes Dc and Dd turn on and off softly.

36. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Experimental Results
Aux. 1 Switch Waveform
• The top figure is the switch voltage, the middle is the gate pulse and the
bottom is the switch current.
• Switch turns on and off with ZCS.
• Aux. 1 conducts current for only a very short duration.

37. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
Conclusion
• A novel dc-dc PWM ZVS FB converter was proposed.
• The converter is a ZVZCS PWM converter with fewer conduction losses
than the standard ZVS-PWM FB converter, but with an extended ZVS
load range.
• Features:
• All the benefits of ZVZCS converters.
• All switches operate with ZVS.
• Can be implemented with MOSFETs hence operating in high switching
frequencies.
• The operation of the converter, general design guidelines and features
were explained and its feasibility is proved with experimental results
obtained from a lab prototype.

38. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
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39. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
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40. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
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41. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
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42. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
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43. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
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44. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
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45. Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion
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