Bootstrap Circuitry Selection for Half Bridge Configurations

1 1 SLUA887 August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurationsApplicationRepo rtSLUA887 August2018 BootstrapCircuitrySelectionfor half -BridgeConfigurationsMamadouDiallo,H ighPowerDriversABSTRACTD rivingMOSFETsin half -bridgeconfigurationspresentmanychal lengesfor thosechallengesis generatingbiasfor the bootstrapcircuittakescareof this ,TI's 620 Vhalf-bridgegatedriverwith interlockto presentthe differentcomponentsin a bootstrapcircuitand how to properlyselectthemin orderto ensurepredictableswitchingof the of to to BootstrapDiodeReverseRecoveryTime(Zoomed Out)..55HB_HSRingingEffectson (Rboot= 0 Ohms)..77 VDD/HB-HSFastRampUp (Rboot= ).. of Tables1 IntroductionWhenusinghalf-bridgeconfigur ations,it is necessaryto generatehigh-sidebias to drivethe gateof thehigh-sideFET referencedto the of the mostpopularand cost effectiveway fordesignersto do so is the use of a bootstrapcircuitwhichconsistsof a capacitor,a diode,a resistorand applicationreportwill explainhow this circuitworks,the key componentsof the bootstrapcircuitsand theirimpactin the app notewill put emphasison half -bridgegatedrivesusingdriverswith no built-inbootstrapdiode,whichgivesdesigne rsflexibilityand reducespowerdissipationin the gatedriverIC.

2 Additionally,it will discussthe layoutconsiderationsfor the differentcomponentsof this August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurations2 BasicOperationof BootstrapCircuitA bootstrapcircuitis usedin half -bridgeconfigurationsto supplybias to the showsthe chargingpathof a bootstrapcircuitin a simplifiedhalf-bridgeconfigurationusingU CC27710,TI's 620 Vhalf-bridgedriverwith low-sideFET is on (high-sideFET is off), the HS pin and theswitchnodeare pulledto ground;the VDDbias supply,throughthe bypasscapacitor,chargesthebootstrapcapac itorthroughthe bootstrapdiodeand BootstrapChargingPathWhenthe low-sideFET is turnedoff and the high-sideis on, the HS pin of the gatedriverand the switchnodeare pulledto the high voltagebus HV; the bootstrapcapacitordischargessomeof the storedvoltage(accumulatedduringthe chargingsequence)to the high-sideFET throughthe HO and HS pins of the gatedriveras shownin August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurations3 BootstrapComponentsSelectionThis sectiondiscusseseachcomponent'srole and its impactin the designperspective,this is the mostimportantcomponentbecauseit providesa low impedancepathto sourcethe high peakcurrentsto chargethe a generalrule of thumb,thisbootstrapcapacitorshouldbe sizedto haveenoughenergyto drivethe gateof the high-sideMOSFET withoutbeingdepletedby morethan10%.

3 This bootstrapcap shouldbe at least10 timesgreaterthanthegatecapacitanceof the reasonfor that is to allowfor capacitanceshift fromDC biasand temperature,and also skippedcyclesthat occurduringload gatecapacitancecan bedeterminedusingEquation1:(1)Oncethe gatechargedetermined,the minimumvaluefor the bootstrapcapacitorcan be estimatedusingEquation2:(2)Alternatively ,a moredetailedcalculationof the minimumbootstrapcapacitorvaluecan be doneusingEquation3:(3)It is importantto notethat valuesbelowthe minimumrequiredbootstrapcapacitorvalueco uldlead toactivationof the driver'sUVLO thereforeprematurelyturningoff the the flip side,highervaluesof the bootstrapcapacitorlead to lowerripplevoltageand longerreverserecoverytime in someconditions(wheninitiallychargingthe bootstrapcap or with a narrowbootstrapchargingperiod)as well ashigherpeakcurrentthroughthe relatesthe bootstrapcap and the peakcurrentsthroughthe bootstrapdiode.

4 (4)It is generallyrecommendedto use low ESRand ESL surfacemountmulti-layerceramiccapacitors (MLCC)with goodvoltageratings(2xVDD),temperaturecoe fficientsand August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor chargeto replenishthe bootstrapcapacitormustcomefromsomelarger bypasscapacitor,usuallythe a rule of thumb,this bypasscapacitorshouldbe sizedto be at least10timeslargerthanthe bootstrapcapacitorso that it is not completelydrainedduringthe chargingtime ofthe allowsthe bootstrapcapacitorto be 10x ratioresultsin 10%maximumrippleon the VDDcapacitorin worstcaseconditions.(5) orderto minimizelossesassociatedwith the reverserecoverypropertiesof the diodeand groundnoisebouncing,a fast recoverydiodeor Schottkydiodewith low forwardvoltagedropand low junctioncapacitanceis risk associatedwith chargesuppliedbackto the gatedriversupplyfromthe bootstrapcapacitorand , showsthe reverserecoverylosseswhenusingdiodeswith reverserecoverytimeson HB-HS(Ch1).

5 We canobservelargeamountof overand undershooton the HB-HSpin whichcan triggerthe driver'sUVLO andshutdownthe HS pin (switchnode)is pulledto a highervoltage,the diodemustbe able reversebias fastenoughto blockany chargesfromthe bootstrapcapacitorto the bootstrapdiodeshouldbe carefullychosensuchthat it is capableof handlingthe peaktransientcurrentsduringstart-up;ands uchthat its voltageratingis higherthanthe systemDC-linkvoltagewith ReverseRecoveryLossesdue to BootstrapDiodeReverseRecoveryTimeFigure4 belowshowsa reverserecoveryconditioncreated(channel1 ) by settingup the timingtospecificallyforcethe switchnodehigh with the August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurationsFigure4. ReverseRecoveryLossesdue to BootstrapDiodeReverseRecoveryTime(Zoomed Out)Figure5 showsthe effectsof the losseson the HB-HSpin whichcan triggerthe switchnodeandpotentiallydamagethe August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurationsFigure5.

6 HB_HSRingingEffectson role of the bootstrapresistoris to limit the peakcurrentsat the bootstrapdiodeduringstart-up,itshouldthe reforebe carefullyselectedas it introducesa time constantwith the bootstrapcapacitorgivenby Equation6:(6)This time constantoccuringduringthe high-sideoff time explainsthe dependencyon duty dutycyclebeingconstant,the bootstrapresistorand bootstrapcapacitorshouldbe tunedappropriatelytoachievethe bootstrapresistorvalueswill increasethe time constantleadingto ,the bootstrapresistorchosenmustbe able to withstandhigh powerdissipationduringthe firstchargingsequenceof the energycan be estimatedby Equation7:(7)This energyis dissipatedduringthe chargingtime of the bootstrapcapacitorand can be estimatedusingEquation8:(8) August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurationsThis resistoris essentialin limitingthe peakcurrentsthroughthe bootstrapdiodeat start-upand limitingthe dv/dtof HB-HS(high-sidefloatingsupplyto the returnhigh-sidefloatingsupply).

7 The peakcurrentthroughthis resistorcan be calculatedusingEquation9:(9)Figure6 showsthe fast rampup on VDD(CH4)and HB-HS(CH1)whenusinga 0-Ohmresistorwhichleadsto undesiredchangein voltageon LO(CH3)and HO(CH2).Figure6. VDD/HB-HSFastRampUp (Rboot= 0 Ohms)Figure7 showshow usingslightlyhigherresistorvalue(Rboot= )solvethis is importanttonotethat the bias risingrate observedin Figure7 doesnot applyto all August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurationsFigure7. VDD/HB-HSFastRampUp (Rboot= )4 LayoutConsiderationsfor BootstrapComponentsOnceall bootstrapcomponentsappropriatelyselected ,it is importantto carefullyplacethesecomponentsin orderto minimizeparasiticinductancesand reducehigh high currentpathincludesthe bootstrapcapacitor,the bootstrapdiode,the ground-referencedVDDbypasscapacitorof thedriver,and the is thereforeimportantto reducethat pathand keepthat loop assmallas bootstrapcapacitorand bypasscapacitorshouldbe placedas closedas possibleto the belowshowsa goodlayoutexampleusingUCC27710with allbootstrapcomponentslocatednearthe gatedriverIC minimizingany effectsof parasiticinductancesandreducingthe high peakcurrentspathof the is also importantto separatehigh voltagepowerand low August2018 SubmitDocumentationFeedbackCopyright 2018.

8 TexasInstrumentsIncorporatedBootstrapCir cuitrySelectionfor half -BridgeConfigurations5 SummaryThis applicationreportusedUCC27710,TI's 600 Vfamilyof half -bridgedriversto discussthe basicoperationof a bootstrapcircuitin a also discussedthe role and importanceofeachbootstrapcomponentsrequi redto generatebias for the high-sideFET in detailedcalculationmethodas well as a generalrule of thumbestimationfor the ,it discussedhow to properlyplacethesecomponentson a PCBlayoutonceall thecomponentsare UCC27710 ProductFolder UCC27710 Datasheet UCC27710 EvaluationModule half -bridgeDriverProducts Fundamentalsof MOSFETand IGBTGateDriverCircuitsIMPORTANTNOTICEFOR TIDESIGNINFORMATIONANDRESOURCEST exasInstrumentsIncorporated( TI ) technical,applicationor otherdesignadvice,servicesor information,including,but not limitedto,referencedesignsand materialsrelatingto evaluationmodules,(collectively, TI Resources ) are intendedto assistdesignerswho aredevelopingapplicationsthat incorporateTI products.

9 By downloading,accessingor usingany particularTI Resourcein any way,you(individuallyor, if you are actingon behalfof a company,yourcompany)agreeto use it solelyfor this purposeand subjectto the termsofthis s provisionof TI Resourcesdoesnot expandor otherwisealterTI s applicablepublishedwarrantiesor warrantydisclaimersfor TIproducts,and no additionalobligationsor liabilitiesarisefromTI providingsuchTI reservesthe rightto makecorrections,enhancements,improvement sand otherchangesto its TI understandand agreethat you remainresponsiblefor usingyourindependentanalysis,evaluationa nd judgmentin designingyourapplicationsand that you havefull and exclusiveresponsibilityto assurethe safetyof yourapplicationsand complianceof yourapplications(andof all TI productsusedin or for yourapplications)with all applicableregulations,lawsand ,with respectto yourapplications,you haveall the necessaryexpertiseto createand implementsafeguardsthat (1)anticipatedangerousconsequencesof failures,(2) monitorfailuresand theirconsequences,and (3) lessenthe likelihoodof failuresthatmightcauseharmand take agreethat priorto usingor distributingany applicationsthat includeTI products,youwill thoroughlytest suchapplicationsand the functionalityof suchTI productsas usedin has not conductedanytestingotherthanthat specificallydescribedin the publisheddocumentationfor a particularTI are authorizedto use, copyand modifyany individualTI Resourceonly in connectionwith the developmentof applicationsthat includethe TI product(s)

10 Identifiedin suchTI OTHERLICENSE,EXPRESSOR IMPLIED,BY ESTOPPELOR OTHERWISETOANYOTHERTI INTELLECTUALPROPERTYRIGHT,ANDNO LICENSETO ANYTECHNOLOGYOR INTELLECTUALPROPERTYRIGHTOF TI OR ANYTHIRDPARTYIS GRANTEDHEREIN,includingbut not limitedto any patentright,copyright,maskworkright,orot herintellectualpropertyrightrelatingto any combination,machine,or processin whichTI productsor servicesare referencingthird-partyproductsor servicesdoesnot constitutea licenseto use suchproductsor services,or a of TI Resourcesmay requirea licensefroma thirdpartyunderthe patentsor otherintellectualpropertyof thethirdparty,or a licensefromTI underthe patentsor otherintellectualpropertyof RESOURCESAREPROVIDED AS IS ANDWITHALL DISCLAIMSALL OTHERWARRANTIESORREPRESENTATIONS,EXPRESS OR IMPLIED,REGARDINGTI RESOURCESOR USETHEREOF,INCLUDINGBUTNOTLIMITEDTOACCUR ACYOR COMPLETENESS,TITLE,ANYEPIDEMICFAILUREWAR RANTYANDANYIMPLIEDWARRANTIESOFMERCHANTAB ILITY,FITNESSFORA PARTICULARPURPOSE,ANDNON-INFRINGEMENTOF SHALLNOTBE LIABLEFORANDSHALLNOTDEFENDOR INDEMNIFYYOUAGAINSTANYCLAIM,INCLUDINGBUT NOTLIMITEDTO ANYINFRINGEMENTCLAIMTHATRELATESTO OR IS BASEDON ANYCOMBINATIONOF PRODUCTSEVENIFDESCRIBEDIN TI RESOURCESOR NO EVENTSHALLTI BE LIABLEFORANYACTUAL,DIRECT,SPECIAL,COLLAT ERAL,INDIRECT,PUNITIVE,INCIDENTAL,CONSEQ UENTIALOR EXEMPLARYDAMAGESIN CONNECTIONWITHORARISINGOUTOF TI RESOURCESOR USETHEREOF,ANDREGARDLESSOF WHETHERTI HASBEENADVISEDOF THEPOSSIBILITYOF agreeto fully indemnifyTI and its representativesagainstany damages,costs,losses,and/orliabilitiesar isingout of yournon-compliancewith the termsand provisionsof this Noticeappliesto TI the use and purchaseof certaintypesof materials,TI productsand.