60W-SiC-MOSFET辅助电源官方评估板设计

1Subjecttochangewithoutnotice.(SiC)MOSFETtoreplaceconventionaltwo-switchFlybackconverterforhighvoltageinputauxiliarypowersupplyofthreephaseapplications.ThedemonstrationboardisnotdesignedtobeaproductandistobeonlyusedasatooltoevaluatetheperformanceCreeswitchingdevices.2.INTRODUCTIONThree-phaseapplications,suchasmotordrive,UPSandPVinverter,haveafrontendAC/DCorDC/DCconvertertoboosttheDClinkvoltageupto600Vdcto800Vdc.Factoringinadesignmargin,themaximumDClinkvoltageisupto1000V.Tosupportsuchsystemsinpractice,anauxiliarypowersupplyisusedtogeneratepowerforcoolingfans,displays,controllogicandsystemprotectionfunctionswiththeDClinkvoltageasitsinput.Forsuchlowpowerapplications,Flybacktopologyisthemostcommontypeintheindustry;however,theconventionalsingleendFlybacktopologyhasdifficultyinmeetinghighinputvoltage.Thefirstdifficultyiscausedbythehighinputvoltage(1000Vdc);thesingle-endFlybacktopologywouldrequirehighblockingvoltageswitchingdevices.Currently,theSiliconMOSFETonlyhas1500Vblockingvoltage,whichhaslowvoltagestressdesignmarginandthusaffectsthereliabilityofthepowersupply.Thesecondchallengeisthatmostofthe1500VSiMOSFETshaveverylargeon-stateresistance,andthiswillleadtohigherlosses,higherthermalandlowerefficiency,especiallywhenthewholethree-phasesystemisoperatingatlightoutputloadandauxiliarypowerlossesoccupymostoftotalsystemlosses.Lastly,tosupportawideinputvoltagerange,apureresistancestartupcircuitisnormallyused.However,thestart-upresistancewillleadtolossesathighinputvoltage.Largerstart-upresistancewillhavelesslossesbutleadtolongstart-uptimeatlowinputvoltage.Inordertoovercometheseauxiliarypowersupplydesignchallengestosupplyhighinputvoltage,two-switchFlybackconverterwasproposedtousehighsideandlowside800VSiMOSFETsasshowninFig.1,butithastheadditionalisolationgatedrivecircuitwhichincreasescomponentcountsandcomplicatesthedesign.Thisapplicationnoteproposesasingle-endFlybackconvertertoreplacecomplicatedtwo-switchFlybackconverterbyusing1700VSiCMOSFET.Anactivestart-upcircuitisalsointroducedtoachievelessstart-uplosseswithfasterstartuptime.The60Wexperimentalreferencedesigndemonstratesthatthe1700VSiCMOSFETcanreducetotalcostandsimplifythedesignofauxiliarypowersupply.Figure1:Aconventionaltwo-switchFlybackconverterwith800VSiMOSFET2CPWR-AN14,REV-60WAuxiliaryPowerSupplyDemoboardThisdocumentisprovidedforinformationalpurposesonlyandisnotawarrantyoraspecification.Forproductspecifications,[e-mail]1700VSiCMOSFETToday,SiCdevicesarecharacterizedbyanumberofpromisingpropertieslikehighratingvoltages,lowswitchinglosses,lowon-stateresistance,higheroperatingtemperature,andhighradiationhardness.Acommerciallyavailable1700VTO-247packagedSiCMOSFET,C2M1000170D,fromCreeIncisusedforawideinputauxiliarypowersupplyapplication.ThetablecomparesthekeyparametersbetweenSiCMOSFETandSiMOSFETwithcommonTO-247package.Fromthiscomparison,SiCMOSFETcansupportmuchhigherblockingvoltageto1700Vandavalanchevoltageabove1800V,whileSiMOSFETonlyhas1500Vblockingvoltagewithloweravalanchevoltage.Fortheon-stateresistanceandparasiticcapacitance,theSiCMOSFEThaslowervaluethanSiMOSFETtohavelowconductionlossesandlowswitchinglosses.Thiskeydifferencewillvalue1700VSiCMOSFETtohavehighefficiencyandhighreliabilityreplacing1500VSiMOSFET.Table1:Parametercomparisonsof1700VSiCMOSFETand1500VSiMOSFETParametersSiCMOSFETC2M1000170DSiMOSSTW4N150SiMOS2SK2225DSV(BR)DSS1700V1500V1500VAvalanche1800VN/AN/AId@Tc=25°C5A4A2ARdson@150°C2ohm9ohm20ohmCoss14pF120pF60pFTjmax150°C150°C150°CPackageTO-247TO-220,TO-247TO-3PF4.ACTIVESTART-UPCIRCUITInthisdesign,anon-dissipative,activestart-upcircuithasbeenimplementedtooptimizeconverterefficiencyandfaststart-uptime.Thealternativeistouseapureresistivestart-upcircuitwhichsignificantlyaffectsconverterefficiencyandstartuptimesatlowinputvoltagesinanegativeway.Figure2showstheproposedactivestart-upcircuit.Wheninputvoltageisincreasing,Q6isturnedonbyVbasefrompathR31toR36.TheVCCvoltagecomesfrompathR22toR25whenU1(UCC28C44)isturningon.OnceU1startsoperating,theVCCsupplycomesfromtheprimaryauxiliarywinding.WhenVCCreachesthestartupthresholdofU1,theVREF(+5V)goestohighandQ7isturnedon.AndthenQ6isturnedoff,whichdisconnectsthestart-upcurrentpathtoVCC.TheR31toR36resistorswithlargevalueareusedasthevoltagebalancingforinputcapacitorsC1toC3.ThestartupresistorsR22toR25feedsthePWMcontrollerofU1untiltheauxiliarysupplyvoltagerisesandisdisconnectedfromVCCofU1andthentherearenomorelossesfromstart-upresistors.Sotheactivestart-upcircuitcanreducethestartuppowerdissipation,especiallyathighlineinputvoltageandimprovetheefficiency.Theadditionalpowerdissipationundersuchnormalsteadystateconditionsisduetothebalanceresistances,andtheycanbesetatveryhighvalues(6Moh