Is Your IGBT Gate-Driver Power Supply Optimized?

1 ApplicationReportSLLA354 March2015 IsYourIGBTGate-DriverPowerSupplyOptimize d?SanjayPithadiaABSTRACTI nsulatedGateBipolarTransistors(IGBTs)are usedin highcurrentthree-phaseAC theoryand requirementsof gate -drivepowersupplyfor alsodiscussesthe isolationrequirementsand calculationof correctamountof of Figures1 TypicalConfigurationof an (all gate -driversare poweredwith individualisolatedpowersupplies)..33 Three-PhaseInverterWithIsolatedGate-Driv e(lowergate-driversare poweredwith a commonpowersupply)..44 IGBTsWithGateDriveCircuitryfor familiarwith low-powerDC motorsbecausethey are seeneverywhereon a not be seenas muchare all of the largerAC industrialmotorsworkingbehindthe scenestoautomatethe assemblyof automobilesor to lift the elevatorsthat are riddenin everyday. Thesehigh-powermotorsare drivenby electronicswith very differentrequirementsand with usedfor variable-frequencydrivesthat controlthe speedof AC motorsand usedin half-bridgeconfigurationfor eachphaseof the high-sideand a low-sideIGBT switchof the half-bridgeare usedto applypositiveandnegativehigh-voltageDC pulses,respectively,to the motorcoilsin an single,isolatedgatedriverIC drivesthe gateof eachIGBTand galvanicallyisolatesthe high-voltageoutputfromthe collectorof the top (high-side)IGBTis connectedto a very high voltageDCbus.

2 The emitterof that IGBT floatsrelativeto earthgroundto maintainthe transistor s in turn requiresuse of an isolatedgatedriverin orderto isolatethe low-voltagePWMinputsfromthe controlcircuitfromthe high voltagesof the also usedtocontrolthe bottom(low-side) trademarksare the propertyof March2015Is YourIGBTGate-DriverPowerSupplyOptimized? SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedIsolati onIsolationIsolationIsolationIsolationIs olationFromControllerFromControllerFromC ontrollerFromControllerFromControllerFro mControllerVBUSVGNDVBUSVGNDVBUSVGNDS electingthe showstypicalconfigurationin an TypicalConfigurationof an IndustrialMotorDriveAn IGBT gatedriverIC has to performa multitudeof IGBT turn-on,the gatecapacitanceis chargedand,uponreachingthe IGBT thresholdvoltage(VGE_on), the reversetransfercapacitance(calledMillerc apacitance)is also turn off the IGBT,the gatecapacitancehas to be dischargedand,oncethe thresholdvoltage(VGE_off)

3 Is reached,the reversetransfercapacitancealso needsto be ,the turn-onand turn-offvoltageshaveto at leastcrossthethresholdlevel,but practically,thesevalueshaveto be replacedby othervoltagesmorerelevantto ,IGBT sare turnedon with a positivegatevoltageof nominally15 V appliedto the gateis enoughto turn off the ,to preventvoltagechanges(dVCE/dt) acrossthe Millercapacitance(dueto the turningon of the oppositeIGBTin the half bridge)fromturningthe gateof the OFFIGBT backon, a largenegativevoltage(-8 V to -15 V) is oftenappliedto theVEEof the gatedriverIC. It is very importantto selectthe CorrectControlVoltageWhena positivecontrolvoltage(higherthanthe threshold)is appliedbetweengateand emitter,the IGBT turnson. Due to the IGBT trans-conductance,the collectorcurrentis a functionof the also a dependencyon the otherwords,the higherthe gate -emittervoltage,the higherthe possiblecollectorcurrentand the lowerthe achievethelowestpossibleconductionlosses ,whichare determinedby VCEsat= f(IC, VGE), it is desirableto workwithratherhigh the otherhand,it shouldbe notedthat a high gate -emittervoltagemay allowa high shortcircuitcurrentshouldthat fault ,a compromiseneedsto be foundbetweenthe conductionlossesduringnormaloperationand the maximumshortcircuitcurrentin caseof characteristicvaluefor gatevoltageas 15 V, whichis the absolutemaximumvalueshouldnot be exceeded;otherwiseinternaldamageto thedriverIC may occuras well as destructivelyhigh currentmay caseof switchingoff with 0 V, parasiticturn-oncan happendue to eitherof the followingtwo reasons: Due to the feedbackeffectof the Millercapacitance.

4 (Themaincauseof this is the voltagechangebetweencollectorand emitterwhenthe otherIGBTin the half bridgeis turnedon or off). Due to the feedbackeffectof the Emitterstrayinductance.(Themaincauseof this is the changeinload currentdiL/dt).2Is YourIGBTGate-DriverPowerSupplyOptimized? SLLA354 March2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedIsolati onIsolationIsolationIsolationIsolationIs olationFromControllerFromControllerFromC ontrollerFromControllerFromControllerFro mControllerVBUSVGNDVBUSVGNDVBUSVGND+16 V 8 V+16 V 8 V+16 V 8 V+16 V+16 V+16 V 8 V 8 V 8 applyinga negativecontrolvoltage,the IGBTis turnedoff and the gatevoltagerequiredto turn theIGBT backon is muchhigherthancan be achievedby the application,turn-offvoltagesin a range-5 V to -10 V is very mainreasonsare: Lowerrequireddriverpower,whichis directlyproportionalto the voltagelift fromthe negativeto thepositivegatevoltage.

5 Availabilityof driverIC. ManydriverICs are developedon CMOSor BiCMOS technology,whichonlyprovidesa limitedblockingcapabilityof maximum30 V betweenpositiveand accountand sufficientsafetymarginto the maximumvoltagelimits,the usualnegativegatevoltagesprovedby the VEEpowerrail are in the rangeof -5 V to -10 providesmoredetailson isolationrequirementsand Section4 explainsthe calculationof any industrialmotordrive,potentialseparation of the inputcircuit(low-voltage)and the outputcircuit(high-voltage)has to be low-voltageside interfaceswith the controlelectronics,whereas,thehigh-volta geside is connectedto the separationis necessary,becausethe emitterpotentialofthe upperIGBTsis switchedbetweenthe DC+and DC- potentialof the DC-bus,whichcan rangein thehundredsor thousandsof the application,the correspondingstandardsfor clearanceand creepagedistancehaveto be observedas well as compliancewith the test.

6 IEC60664-1,IEC60664-3,IEC61800-5-1,and the simplestcase,it may be sufficientto separateonly the upperIGBTsof a half-bridgefromthe is generallypossibleif the microcontrolleris also referencedto the DC- the interconnectionto the userinterfaceis advisedor required,dependingonthe is mostlyto applybasicisolationfromnoiseand ,separationtakesplaceat everyIGBT,eachdriverwith its own powersupply,asshownin Three-PhaseInverterWithIsolatedGate-Driv e(all gate -driversare poweredwithindividualisolatedpowersuppli es)[1]3 SLLA354 March2015Is YourIGBTGate-DriverPowerSupplyOptimized? SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporated()()2 PQfVCfVDRVGATEINGATEGEINGATE=** D+** D+ 15 V 10 VRGRGECGE+ ACPQfVDRVGATEINGATE=** DIsolationIsolationIsolationIsolationIso lationIsolationFromControllerFromControl lerFromControllerFromControllerFromContr ollerFromControllerVBUSVGNDVBUSVGNDVBUSV GND+16 V V+16 V+16 V+16 V V V VCalculationof complexityfor the powersupplycan be simplifiedfor thoseswitchedthat havetheiremitteron DC-potential,as shownin Three-PhaseInverterWithIsolatedGate-Driv e(lowergate-driversare poweredwitha commonpowersupply)[2]4 Calculationof IGBTD rivePowerWhiledrivingan IGBT,the transitionbetweenthe two gatevoltagelevelsrequiresa certainamountofpowerto be dissipatedin the loop amongthe gatedriver,gateresistorsand is typicallyknownas drivepower- PDRV.

7 This drivepoweris calculatedfromthe gatechargeQGATE, the switchingfrequencyfINand actualdriveroutputvoltageswing VGATE.(1)If thereis an externalcapacitorCGEpresent(auxiliarygat ecapacitor),thenthe gatedriveralso needstochargeand dischargethis capacitor,as shownin IGBTsWithGateDriveCircuitryfor GatePowerCalculationThe valueof RGEis not influencingthe requireddrivepoweras long as CGEis fully chargedand dischargedduringone showsthe requireddrivepowervalue.(2)4Is YourIGBTGate-DriverPowerSupplyOptimized? SLLA354 March2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporated()()2 PQfVCfVPDRVGATEINGATEGEINGATE driver=** D+** D+ the drivepowerdoesnot dependon the valueof the gateresistoror the duty cycleas long asthe switchingtransitiongoesfromfully on to fully off and ,theseequationsare true in is the total drivepowerrequiredby the IGBTbut the gatedriverthat is drivingthe IGBT also powerconsumptionshouldbe addedto get the final valueforgatedrivepower.

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