Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

A ZVS-PWM Full-Bridge Converter with ReducedConduction 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

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Outline

• Introduction

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Outline

• Introduction

• Proposed Converter

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Outline

• Introduction

• Proposed Converter

• Steady-State Operation

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Outline

• Introduction

• Proposed Converter

• Steady-State Operation

• Converter Features

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Outline

• Introduction

• Proposed Converter

• Steady-State Operation

• Converter Features

• Design Guidelines

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Outline

• Introduction

• Proposed Converter

• Steady-State Operation

• Converter Features

• Design Guidelines

• Experimental Results

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

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

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Introduction• Generally there is a need to design the fastest, most efficient and compactpower converter.

• With soft switching in the switches (e.g. ZVS) it is possible to operatethe converter with high switching frequencies.

• Under light load conditions, MOSFETs cannot turn on with ZVS as thereis insufficient current to discharge Coss.

• Many researchers, therefore, have proposed variations on the basicZVS-PWM-FB topology to extend the load range.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Introduction• Some topologies use extra passive components to generate current in theconverter’s primary side to discharge Coss of MOSFETs;

• But any efficiency gain is offset by additional conduction losses.

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 generatedwithout increasing conduction losses because the auxiliary circuit conductsonly for a shorter duration.

• None of these approaches decrease conduction losses.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Introduction• Therefore, zero-voltage-zero-current-switching (ZVZCS) PWM FBconverters have been proposed.

• A passive auxiliary circuit extinguishes circulating current in thetransformer primary.

• ZCS, however, prevents the lagging leg switches turning on with ZVS.• As a result, IGBTs (which have lower Coss than MOSFETs) are preferredover MOSFETs.

• But IGBTs are slower than MOSFETs - switching frequency must belimited.

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 componentsLr, Cr and transformer (Ta).

• Aux. 2: Capacitor Cx and diodes Dc, Dd.

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 andlasts 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.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Steady-State OperationMode 1 (t0 < t < t1)

• Converter behaves as a standardZVZCS FB converter.

• Mode forms part of the powertransfer mode.

• iin flows through Llk and Llkresonates with Cx.

• Cx reaches its peak voltage at theend of the mode.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Steady-State OperationMode 2 (t1 < t < t2)

• Vin is applied entirely across theprimary winding of Tm.

• Cx remains at its peak voltage.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Steady-State OperationMode 3 (t2 < t < t3)

• S1 is turned off at t = t2.

• ip charges and discharges switchcapacitors CS1 and CS3

respectively.

• Discharging and charging is linearuntil the primary voltage of Tmdrops to a level that equals thereflected Cx voltage.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Steady-State OperationMode 4 (t3 < t < t4)

• The non-linear dead time betweenS1 and S3.

• Tm’s primary voltage decreases butthe secondary side rectifier voltageis held by Cx.

• The difference of Vin and Tmprimary voltage is applied acrossLlk.

• ip starts to decrease.

• When ip falls below the reflectedload current, Cx starts to dischargeto bridge the gap.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Steady-State OperationMode 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 offwith ZCS at t = t5.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Steady-State OperationMode 6 (t5 < t < t6)

• After S2 is turned off, Sa can beturned on with ZCS.

• Sa allows CS4 to discharge intoAux. 1.

• Part of iin charges CS2 and theremainder enters Tm.

• CS4 is fully discharged at t = t6.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Steady-State OperationMode 7 (t6 < t < t7)

• At t = t6, iin = ip.

• Vin is applied across Llk; thus ipincreases linearly until it equalsthe reflected load current.

• Current in the secondary side ofTm freewheels.

• In Aux. 1, iLr comprises of theTm’s primary current ip and thecurrent of DS4 .

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Steady-State OperationMode 8 (t7 < t < Ts/2)

• ip starts to increase beyond Io/nm.

• Resonance of Cr and Lr decreasesiLr .

• As ip is increasing while iLr isdecreasing, the window ofopportunity for the lagging legswitches to turn on with ZVS laststill ip <= iLr.

• At t = Ts/2, iLr = 0, Sa is turnedoff with ZCS sometime thereafter.

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.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Converter Features

• Freewheeling current in Tm is extinguished so that conduction losses arereduced.

• Llk in Tm can be minimized; thus duty cycle loss is minimized.

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 inthe circuit can be transferred to the load.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Design Guidelines

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Design GuidelinesMain transformer (Tm) turns ratio nm

• Since the input to output voltage conversion ratio is fixed, nm should beselected simultaneously with the duty ratio D.

• nm should not be too low as that will increase the current in Tm and theconverter will need to be in the freewheeling mode for a longer time tocompletely remove iLlk .

• A higher nm can have a negative influence on the voltage regulation ofthe converter.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Design GuidelinesMain 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 thesecondary side.

• A low Llk also aids the ZCS.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Design GuidelinesAux. 2 Capacitor(Cx)

• Main function of Cx is to create a counter voltage across Llk to ensurethat the primary current decreases to zero within the freewheeling time.

• If Cx is too small, it will not have sufficient energy stored in it todischarge Llk.

• If Cx is too big, then unnecessary conduction losses will occur in Aux. 2.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Design GuidelinesAux. 1 Inductor(Lr)

• Higher Lr increases, the characteristic impedance in Aux. 1 and thusreduces 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 getscloser to t2; hence the window of opportunity narrows, making the ZVSoperation difficult.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Experimental ResultsConverter Design

Design SpecificationInput voltage: 380 Vin

Output voltage: Vo = 48Vdc

Output power: 500 kW

Switching frequency: fs=125 kHz

Maximum power: Po,max=500 W

Converter ParametersTm turns ratio: nm= 4

Ta turns ratio: na= 0.1

Aux 2 capacitor: Cx=0.1 µF

Aux 1 capacitor: Cr=100 nF

Aux 1 inductor: Lr=1 µH

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Experimental ResultsLagging 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 extinguishedbefore it is turned off, so that it has a ZCS turn off.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Experimental ResultsLeading Leg Switching

• Leading leg switches turn on with ZVS.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Experimental ResultsAux. 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.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

Experimental ResultsAux. 1 Switch Waveform

• The top figure is the switch voltage, the middle is the gate pulse and thebottom is the switch current.

• Switch turns on and off with ZCS.

• Aux. 1 conducts current for only a very short duration.

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 lossesthan the standard ZVS-PWM FB converter, but with an extended ZVSload 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 featureswere explained and its feasibility is proved with experimental resultsobtained from a lab prototype.

Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

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Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

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Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

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Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

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Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

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Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

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Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

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Outline Introduction Proposed Converter Operation Features Design Guidelines Experimental Results Conclusion

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