功率MOSFET的驱动电路设计

2-1DesignAndApplicationGuideforHighSpeedMOSFETGateDriveCircuitsByLaszloBaloghABSTRACTThemainpurposeofthispaperistodemonstrateasystematicapproachtodesignhighperformancegatedrivecircuitsforhighspeedswitchingapplications.Itisaninformativecollectionoftopicsofferinga“one-stop-shopping”tosolvethemostcommondesignchallenges.Thusitshouldbeofinteresttopowerelectronicsengineersatalllevelsofexperience.Themostpopularcircuitsolutionsandtheirperformanceareanalyzed,includingtheeffectofparasiticcomponents,transientandextremeoperatingconditions.ThediscussionbuildsfromsimpletomorecomplexproblemsstartingwithanoverviewofMOSFETtechnologyandswitchingoperation.Designprocedureforgroundreferencedandhighsidegatedrivecircuits,ACcoupledandtransformerisolatedsolutionsaredescribedingreatdetails.AspecialchapterdealswiththegatedriverequirementsoftheMOSFETsinsynchronousrectifierapplications.Several,step-by-stepnumericaldesignexamplescomplementthepaper.I.INTRODUCTIONMOSFET–isanacronymforMetalOxideSemiconductorFieldEffectTransistoranditisthekeycomponentinhighfrequency,highefficiencyswitchingapplicationsacrosstheelectronicsindustry.Itmightbesurprising,butFETtechnologywasinventedin1930,some20yearsbeforethebipolartransistor.ThefirstsignallevelFETtransistorswerebuiltinthelate1950’swhilepowerMOSFETshavebeenavailablefromthemid1970’s.Today,millionsofMOSFETtransistorsareintegratedinmodernelectroniccomponents,frommicroprocessors,through“discrete”powertransistors.ThefocusofthistopicisthegatedriverequirementsofthepowerMOSFETinvariousswitchmodepowerconversionapplications.II.MOSFETTECHNOLOGYThebipolarandtheMOSFETtransistorsexploitthesameoperatingprinciple.Fundamentally,bothtypeoftransistorsarechargecontrolleddeviceswhichmeansthattheiroutputcurrentisproportionaltothechargeestablishedinthesemiconductorbythecontrolelectrode.Whenthesedevicesareusedasswitches,bothmustbedrivenfromalowimpedancesourcecapableofsourcingandsinkingsufficientcurrenttoprovideforfastinsertionandextractionofthecontrollingcharge.Fromthispointofview,theMOSFETshavetobedrivenjustas“hard”duringturn-onandturn-offasabipolartransistortoachievecomparableswitchingspeeds.Theoretically,theswitchingspeedsofthebipolarandMOSFETdevicesareclosetoidentical,determinedbythetimerequiredforthechargecarrierstotravelacrossthesemiconductorregion.Typicalvaluesinpowerdevicesareapproximately20to200picosecondsdependingonthesizeofthedevice.ThepopularityandproliferationofMOSFETtechnologyfordigitalandpowerapplicationsisdrivenbytwooftheirmajoradvantagesoverthebipolarjunctiontransistors.OneofthesebenefitsistheeaseofuseoftheMOSFETdevicesinhighfrequencyswitchingapplications.TheMOSFETtransistorsaresimplertodrivebecausetheircontrolelectrodeisisolatedfromthecurrentconductingsilicon,thereforeacontinuousONcurrentisnotrequired.OncetheMOSFETtransistorsareturned-on,theirdrive2-2currentispracticallyzero.Also,thecontrollingchargeandaccordinglythestoragetimeintheMOSFETtransistorsisgreatlyreduced.Thisbasicallyeliminatesthedesigntrade-offbetweenonstatevoltagedrop–whichisinverselyproportionaltoexcesscontrolcharge–andturn-offtime.Asaresult,MOSFETtechnologypromisestousemuchsimplerandmoreefficientdrivecircuitswithsignificanteconomicbenefitscomparedtobipolardevices.Furthermore,itisimportanttohighlightespeciallyforpowerapplications,thatMOSFETshavearesistivenature.ThevoltagedropacrossthedrainsourceterminalsofaMOSFETisalinearfunctionofthecurrentflowinginthesemiconductor.ThislinearrelationshipischaracterizedbytheRDS(on)oftheMOSFETandknownastheon-resistance.On-resistanceisconstantforagivengate-to-sourcevoltageandtemperatureofthedevice.Asopposedtothe-2.2mV/°Ctemperaturecoefficientofap-njunction,theMOSFETsexhibitapositivetemperaturecoefficientofapproximately0.7%/°Cto1%/°C.ThispositivetemperaturecoefficientoftheMOSFETmakesitanidealcandidateforparalleloperationinhigherpowerapplicationswhereusingasingledevicewouldnotbepracticalorpossible.DuetothepositiveTCofthechannelresistance,parallelconnectedMOSFETstendtosharethecurrentevenlyamongthemselves.ThiscurrentsharingworksautomaticallyinMOSFETssincethepositiveTCactsasaslownegativefeedbacksystem.Thedevicecarryingahighercurrentwillheatupmore–don’tforgetthatthedraintosourcevoltagesareequal–andthehighertemperaturewillincreaseitsRDS(on)value.Theincreasingresistancewillcausethecurrenttodecrease,thereforethetemperaturetodrop.Eventually,anequilibriumisreachedwheretheparallelconnecteddevicescarrysimilarcurrentlevels.InitialtoleranceinRDS(on)valuesanddifferentjunctiontoambientthermalresistancescancausesignificant–upto30%–errorincurrentdistribution.A.DeviceTypesAlmostallmanufacturershavegottheiruniquetwistonhowtomanufacturethebestpowerMOSFETs,butallofthesedevicesonthemarketcanbecategorizedintothreebasicdevicetypes.TheseareillustratedinFig.1.n+n+n+Substraten-EPIlayerGATESOURCEDRAINppn+n+n+Substraten-EPIlayerGATESOURCEDRAINp+p+(a)(b)n+n+SubstratepGATESOURCEDRAINpn(c)OXIDEFig.1.PowerMOSFETdevicetypes.Double-diffusedMOStransistorswereintroducedinthe1970’sforpowerapplicationsandevolvedcontinuouslyduringtheyears.Usingpolyc