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1、APPENDIXAEstimatingMOSFETParametersfromtheDataSheet(EquivalentCapacitances,GateCharge,GateThresholdVoltage,MillerPlateauVoltage,InternalGateResistance,MaximumDv/Dt)Inthisexample,theequivalentCGS,CGD,andCDScapacitances,totalgatecharge,thegatethresholdvoltageandMillerplateauvoltage,appro
2、ximateinternalgateresistance,anddv/dtlimitsofanIRFP450MOSFETwillbecalculated.Arepresentativediagramofthedeviceinagroundreferencedgatedriveapplicationispicturedbelow.VDS,offVDRVIRFP450IDDCGDRHIRRGATEG,IGCDSRLOCGSSThefollowingapplicationinformationaregiventocarryoutthenecessarycalculatio
3、ns:VDS,OFF=380Vthenominaldrain-to-sourceoffstatevoltageofthedevice.ID=5Athemaximumdraincurrentatfullload.TJ=100°Ctheoperatingjunctiontemperature.VDRV=13Vtheamplitudeofthegatedrivewaveform.RGATE=5Wtheexternalgateresistance.RLO=RHI=5Wtheoutputresistancesofthegatedrivercircuit.A1.Capacita
4、ncesThedatasheetoftheIRFP450givesthefollowingcapacitancevalues:Usingthesevaluesasastartingpoint,theaveragecapacitancesfortheactualapplicationcanbeestimatedas:2-40Equations:NumericalExample:VC=2×C×DS,spec25VRSS,aveRSS,specC=2×340pF×=174pFVRSS,aveDS,off380VV25VDS,specCOSS,ave=2×COSS,spec
5、×COSS,ave=2×720pF×=369pFV380VDS,offThephysicalcapacitorvaluescanbeobtainedfromthebasicrelationships:CGD=CRSS,aveCGD=174pFC=C-CC=2600pF-340pF=2260pFGSISSRSSGSC=C-CC=369pF-174pF=195pFDSOSS,aveRSS,aveDSNoticethatCGSiscalculatedfromtheoriginaldatasheetvalues.Withinoneequation,itisimportant
6、tousecapacitorvalueswhicharemeasuredunderthesametestconditions.AlsokeepinmindthatCGSisconstant,itisnotvoltagedependent.Ontheotherhand,CGDandCDScapacitorsarestronglynon-linearandvoltagedependent.Theirhighestvalueisatornear0Vandrapidlydecreasingasthevoltageincreasesacrossthegate-to-drain
7、anddrain-to-sourceterminalsrespectively.A2.GatechargeTheworstcasegatechargenumbersforaparticulargatedriveamplitude,draincurrentlevel,anddrainoffstatevoltagearegivenintheIRFP450datasheet.CorrectingforadifferentgatedriveamplitudeissimpleusingthetypicalTotalGateChargecurveasillustratedo
