MOS DRIVER

IXYSCorporation;3540BassettStreet;SantaClara,CA95054;Tel:[phone];Fax:[phone]IXYSSemiconductorGmbH;Edisonstr.15;D-68623;Lampertheim,Germany;Tel:[phone];Fax:[phone]MOSFET/IGBTDRIVERSTHEORYANDAPPLICATIONSByAbhijitD.Pathak©2001IXYSCorporationAPPLICATIONNOTEAN00021.Introduction1.1.MOSFETandIGBTTechnology.1.2.MOSFETModelsandcriticalparameters1.3.Turn-onandTurn-offphenomenonandtheirexplanations1.4.PowerlossesinDrivers2.TypesofDrivers2.1.ICGateDrivers2.2.Techniquesavailabletoboostcurrentoutputs2.3.Techniquesavailabletogeneratenegativebiasduringturn-off2.4.Needforunder-voltageprotections2.5.OverloadandShortCircuitProtection3.IsolationTechniques3.1.EmployingCharge-pumpandBootstrapTechniques3.2.ExamplesofuseofOpto-couplersinpracticalDriverCircuits3.3.ExamplesusingtransformersinpracticalDriverCircuits4.IXYSLineofMOSFET/IGBTDrivers4.1.TechnicaldetailsofallIXYSDrivers4.2.FeaturesandAdvantagesofIXYSDrivers4.3.ApplyingIXYSDriversinvarioustopologies5.PracticalConsiderations6.ConclusionIXYSCorporation;3540BassettStreet;SantaClara,CA95054;Tel:[phone];Fax:[phone]IXYSSemiconductorGmbH;Edisonstr.15;D-68623;Lampertheim,Germany;Tel:[phone];Fax:[phone].INTRODUCTIONModernPowerElectronicsmakesgeneroususeofMOS-FETsandIGBTsinmostapplicationsand,ifthepresenttrendisanyindication,thefuturewillseemoreandmoreapplicationsmakinguseofMOSFETsandIGBTs.Althoughsufficientliteratureisavailableoncharacteris-ticsofMOSFETsandIGBTs,practicalaspectsofdrivingtheminspecificcircuitconfigurationsatdifferentpowerlevelsandatdifferentfrequenciesrequirethatdesignen-gineerspayattentiontoanumberofaspects.Anattemptismadeheretoreviewthissubjectwithsomeillustrativeexampleswithaviewtoassistbothexperienceddesignengineersandthosewhoarejustinitiatedintothisdiscipline.1.1MOSFETANDIGBTTECHNOLOGYDuetotheabsenceofminoritycarriertransport,MOS-FETscanbeswitchedatmuchhigherfrequencies.Thelimitonthisisimposedbytwofactors:transittimeofelec-tronsacrossthedriftregionandthetimerequiredtochargeanddischargetheinputGateand‘Miller’capacitances.IGBTderivesitsadvantagesfromMOSFETandBJT.ItoperatesasaMOSFETwithaninjectingregiononitsDrainsidetoprovideforconductivitymodulationoftheDraindriftregionsothaton-statelossesarereduced,especiallywhencomparedtoanequallyratedhighvolt-ageMOSFET.AsfarasdrivingIGBTisconcerned,itresemblesaMOS-FETandhenceallturn-onandturn-offphenomenacom-ments,diagramsandDrivercircuitsdesignedfordrivingMOSFETapplyequallywelltoanIGBT.Therefore,whatfollowsdealsonlywithMOSFETmodels.N+CoCpCCGDN+GATEElectrodeSOURCEMETALIZATIONP-BASERBDBCN-DRIFTDepletionboundariesFig.(1A)MOSFETcellinternalstructureFig.(1B)CrosssectionalviewofN-ChannelMOSFETshowingvariousinter-junctioncapacitancesCCCGSDSGDN+PN-N+N+PSOURCEGATEPARASITICBJTDRAINDRAINBODYDEPLETIONLAYERINTERNALBODYDIODEOVGSActualLinearizedgm=IDDRAINCURRENTGS(th)VGSVIDIDVGSFig.(2)TransfercharacteristicsofapowerMOSFET1.2MOSFETMODELSANDCRITICALPARAMETERSFig.(1A)showstheinternalcellstructureofaDMOSMOSFET.Ascanbeseen,theGatetoSourceCapaci-tanceconsistsofthreecomponents,namely,Cp,thecom-ponentcreatedbytheGateElectrodeovertheP-baseregion;CN+,duetotheoverlapoftheGateElectrodeabovetheN+sourceregionand,CO,arisingduetotheproximityoftheGateElectrodetothesourcemetallization.Infact,alltheseareaddedtoyieldCGS,whichwecallGate-to-SourceCapacitance.Itisthistotalvalueofcapacitancethatneedstobefirstchargedtoacriticalthresholdvolt-agelevelVGS(th),beforeDrainCurrentcanbegintoflow.TheGate-to-Draincapacitance,CGD,istheoverlapca-pacitancebetweentheGateelectrodeandtheN-driftDrainregion.CGDissometimesreferredtoasthe‘Miller’ca-pacitanceandcontributesmosttotheswitchingspeedlimitationoftheMOSFET.Thejunctioncapacitancebe-tweenthedraintotheP-BaseregionisCDS.TheP-BaseregionoftheMOSFETisshortedtotheN+source.Fig.(2)showscurveofID(DrainCurrent)versusVGS(GateSourceIXYSCorporation;3540BassettStreet;SantaClara,CA95054;Tel:[phone];Fax:[phone]IXYSSemiconductorGmbH;Edisonstr.15;D-68623;Lampertheim,Germany;Tel:[phone];Fax:[phone]Voltage).Thegraphhasaslope(ID/VGS)equaltogm,whichiscalledtransconductance.PleasenotethattheactualrelationshipbetweenVGSandIDisshownbydottedlineanditcanbeobservedthatinthevicinityofVGS(th),therelationshipbetweenVGSandIDisparabolicinnature:ID=K[VGS-VGS(th)]2Eq.1.1However,forPowerMOSFETs,itisappropriatetocon-sidertherelationshiptobelinearforvaluesofVGSaboveVGS(th).Themanufacturer’sdatasheetvalueofVGS(th)isspeci-fiedat25oC.Fig.(3A)showsasymbolofN-ChannelMOSFETandanequivalentmodelofthesamewiththreeinter-junctionpara-siticcapacitances,namely:CGS,CGDandCDS.Ihaveshownalltheseasvariableastheyindeedare.ForexampletheCGD,decreasesrapidlyastheDraintoSourcevoltagerises,asshowninFig.(3B).InFig.(3B),thehighvalueofCGDiscalledCGDh,whilethelowvalueofCGDistermedCGDl.Fig.(1B)showsanothercross-sectionalviewofaMOSFETwithallthesecapacitances.Inaddition,ItalsoshowstheinternalbodydiodeandtheparasiticBJT.1.3Turn-onandTurn-offPhenomena1.3.1Turn-onPhenomenonTounderstandTurn-onandTurn-offphenomenaofthePow