Adaptive off-time control for variable-frequency, soft-switched flyback converter at light loads

Adaptive off-time control for variable-frequency, soft-switched flyback converter at light loads

596IEEETRANSACTIONSONPOWERELECTRONICS,VOL.17,NO.4,JULY2002

AdaptiveOff-TimeControlforVariable-Frequency,Soft-SwitchedFlybackConverteratLightLoads

´,Fellow,IEEEYuriPanovandMilanM.Jovanovic

Abstract—Thesoftswitchingofaflybackconvertercanbe

achievedbyoperatingthecircuitinthecriticalconductionmode.However,thecritical-modeoperationatlightloadscannotbemaintainedduetoaveryhighswitchingfrequencyandthelossoftheoutputvoltageregulation.Acontrolwhichregulatestheoutputdowntothezeroloadandmaintainssoftswitchingatlightloadsisproposed.Theproposedcontrolschemewasimplementedinthe380-V/19-V,65-Wflybackdc/dcconverter.

IndexTerms—Adaptivecontrol,flybackconverter,softswitching,variablefrequency.

I.INTRODUCTION

D

UEtotheminimumnumberofsemiconductorandmag-neticcomponents,aflybackconverterisaveryattractivecandidateforoff-linelow-costpowersupplieswhichrequireinput/outputisolation.Sincethevoltageontheprimaryswitchattheturn-oninstantishighandtheassociatedcapacitiveturn-onlossissubstantial,theswitchsoftturn-onishighlydesirable.ThesoftswitchingisalsohelpfulinreducingthesizeandlossoftheEMIfilter.Thesoftturn-onoftheprimaryswitchisaccom-plishedintheflybackconverteroperatinginthecriticalconduc-tionmode,i.e.,attheboundaryofcontinuousanddiscontinuousconductionmodes(CCM/DCM).Thecriticalmodeofoperationrequiresavariablefrequency(VF)controltoregulatetheoutputvoltageagainstinput-voltageandloadvariations.OperationoftheVFflybackconverterinthecriticalmodeisdiscussedin[1]–[6].However,theVFcritical-modecontrolsuffersfromawidefrequencyrangeandlossoftheoutputvoltageregulationatlightload.Namely,forthiscontrol,theswitchon-timeshoulddecrease,astheloadcurrentdecreases.However,sincethemin-imumon-timeofthecontrollerislimitedbyitsinternaldelays,theconverteroperatinginthecriticalmodecannotmaintaintheoutputregulationatlightload.

Thisproblemcanbealleviatedbylimitingtheminimumcon-trolleroff-timetoafixedvalue,sothatconverterswitchesfromthecriticaloperationmodeathighloadtotheDCMwithacon-stantoff-timeatlightloads,asimplementedin[3],[4].How-ever,itwasfoundexperimentallythattheconstantoff-timecon-trolcouldnotmaintaintheoutputregulationastheloadcurrentapproacheszerovalue.Thecontrolimplementationsin[3],[4]alsodonotprovidesoftswitchingatlightloads.

TheobjectiveofthepaperistoproposeasimpleVFconvertercontrolwhichmaintainstheoutputregulationandsoftswitchingintheentireloadrange.

ManuscriptreceivedSeptember3,1999;revisedMarch15,2002.Recom-mendedbyAssociateEditorM.E.Elbuluk.

TheauthorsarewiththeDeltaProductsCorporation,PowerElectronicsLab-oratory,ResearchTrianglePark,NC27709USA.

PublisherItemIdentifier10.1109/TPEL.2002.800958.

II.CONVENTIONALOFF-TIMECONTROLOFVFFLYBACK

CONVERTERATLIGHTLOADSA.PrincipleofOperation

ThefunctionaldiagramoftheVFflybackconverterwithaconstantoff-timecontrolatlightloadsanditskeywaveformsare

showninFig.1.Theswitchingcyclestartsattheinstant

,whenswitchSWturnson.Duringtheinterval

[

increaseslinearly.Output

signal

].Whensecondary

current

reacheszeroattheinstant,transformermagne-tizing

inductance

andsecondary

rectifier

acrosssensing

winding.V

oltageresonates,

asshowninFig.1(b).When

voltage

goesnegativeat,detectoroutputsignalBbecomeshigh.ThewaytheRSlatchissetdependsontheload.Atheavyloads[seeFig.1(b)],

secondarycurrentfall

time

islongerthanthedurationoftheoutput

pulse

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