A REVIEW OF SOFT-SWITCHING TECHNIQUES IN HIGH PERF

AREVIEWOFSOFT-SWITCHINGTECHNIQUESINHIGHPERFORMANCEDCPOWERSUPPLIESRobertL.SteigerwaldGECorporateR&DSchenectady,NY12301(518)387-5076AbstractAreviewojsomeojthemorepracticalsof-switchingtechniquesindc-dcconvertersisgivenwithaviewtowardsactualandpotentialapplicationareas.Abriejreviewofzero-voltageswitching(ZVS)andzero--Currentswitching(ZCS)isfollowedbyadiscussionofvariousso@-switchedcircuittopologicalcategories,theiradvantagesanddisadvantagesandtheirareasofapplication.Applicationojtheseso@-switchingtechniquestohighpowerjactorsupplies,distributedpowersystemsandbothlow-voltageandhigh-voltagepowersupplieswillbereviewed.Adesigncomparisonofseveralapproachesisgivenfora100-kWdc-dcconverter.INTRODUCTIONWhiletherehavebeenmanypapersinrecentyearsdescribing“soft-switching”circuittopologies,therehasbeenlittlediscus-sionofwherethevarioustopologiescouldbeapplied.Alsocom-parisonsofvarioustopologiesforagivenapplicationarerarelydiscussed.Areviewofsomeofthemorepracticalsoft-switch-ingtechniquesindc-dcconverterswillbegivenwithaviewto-wardsactualandpotentialapplicationareas.Abriefreviewofzero-voltageswitching(ZVS)andzero-currentswitching(ZCS)willbefollowedbyadiscussionandcomparisonofvari-ouscircuittopologicalcategorieswhichtakeadvantageofsoftswitching.Loadresonantconverters(usuallyfrequencycon-trolled)willbecomparedwith“transitionresonant”converters(usuallyPWMcontrolled)astotheiradvantagesanddisadvan-tagesandtheirareasofapplication.Theemphasisinthispaperisonhigherpowerconvertersrequiringbridgetypecircuits.Also,itisassumedthatelectricalisolationbetweensourceandload(i.e.,atransformer)isneeded.Applicationofthesesoft-switchingtechniquestohighpowerfactorsupplies,distributedpowersystemsandbothlowandhigh-voltagepowersupplieswillbereviewed.SOFTSWITCHING-ZCSANDZVSFig.1illustratestypicalpowerdevicewaveformsresultingfromzerc-currentswitching(ZCS)operation.WhileapowerFETisshowninFig.1,thediscussionisalsovalidforIGBTsandGTOs.Aninductiveelementisinserieswiththedevicesothatattumon,thedrain(orcollectorforIGBToranodeforGTO)voltage,vd,fallstonearzerobeforecurrentrisesinthedevice.Becauseofthisthereislittletum-onswitchinglossduetoloadcurrent.Note,however,thatenergystoredonthedrain-sourcecapaci-tanceislostinthedevice.Thiscapacitancemaybethedevicesownparasiticdrain-sourcecapacitanceoritmaybeanexternalsnubbercapacitancewh,ichistypicallydischargedthroughare-sistor(notshown).Thisinternal(parasitic)devicecapacitancelossissignificantinconvertersusingpowerFETsasfrequenciesapproachandexceed1MHzbutisusuallynegligibleattypicaloperatingfrequenciesofpowerGTOsandIGBTs.However,inthesehigherpowerdevices,snubberlossesbecomeanissue.AttumoffinZCScircuits,thedrain-sourcevoltageisreducedtozeroandreversed(flowsintheinverseparalleldiode)byexter-vdFddvldtohbffFig.1Zero-currentswitchingnalcircuitmeans(suchasthereversa1ofcurrentinaresonantcir-cuit)asseeninFig.1.Duringthecurrentreversal,thedeviceisgatedoffsothatwhenvoltageisreappliedthedeviceisintheoffstate.Thus,ideallythereisnotum-offlossinthedevice.Inpracticalimplementations,however,therateofreapplicationofvoltage(dv/dt)mustbelimitedtopreventretriggering(GTOs)orlatch-up(IGBTs),andreduceEMI.Inaddition,thetimedur-ingwhichthecurrentisintheinverseparalleldiodemustbesuf-ficienttoallowmuchofthedevicestoredcharges(forbipolarde-vices)torecombine.Otherwise,substantialdevicecurrentwillbepresentduringthereapplicationofvoltageresultinginhighenergylossduringtumoff.Itwillbenotedthatallthyristorforcedcommutatedcircuitspopularduringthe1960’sand1970’s(beforetheintroductionofgatecontrolleddevices)areoftheZCStype[1,2].Zerc-voltageswitching(ZVS)isillustratedinFig.2.Thistypeofswitchingrequirespowerdeviceswithgateturnoffcapability.AsillustratedinFig.2low-losstumoffisachievedduetoaca-pacitanceconnecteddirectlyacrossthedevice.(Thisisincon-torinserieswiththedevice.)ThecapacitancemaybetheparasiticdrainsourcecapacitanceofapowerMOSFEToradis-trasttoZCSwherebylow-losstumonisachievedwithaninduc-0-7803-3026-9195$4.0001995IEEE1iCIIonOffFig.2Zero-voltageswitchingCretecapacitance(whichisthenormalcaseforbipolartypede-vicessuchasIGBTsandGTOs).Thedrainsourcecapacitanceisnotdischargedthroughthedeviceattumon-ratherattumonthedrain-sourcevoltageisreducedtozerothroughextemalcircuitmeans(suchasthereversalofvoltageinaresonantcir-cuit)ascurrent,ic,reversesandeventuallyflowsthroughthein-verseparalleldiode.Thedeviceisgatedonduringthediodeconductiontime.Astheextemalcircuitforcesacurrentreversal(positiveic)thedevicepicksupthecurrentinthefonvarddirec-tion.Thusverylowtumonlossisincurred.NotethatinZVS,thecapacitiveenergyisnotlostbutisretumedtothecircuitthroughresonantactionwhileintheZCScasetheenergystoredinthecapacitanceislost.ThusZVScircuitsarenormallythetopologiesofchoiceforveryhighswitchingfre-quencieswhenFETsareused(IMHz).EitherZCSorZVStechniquescanbeappliedforhigherpowerapplicationsatlowerswitchingfrequencies.Thechoicewilldependonswitchingfre-quency,sizerequirements,switchinglosses(typeofswitchingdeviceused),andcontrolcomplexity.Normally,ZVSap-proachesaresimplertocontrolth