M139, EN

LowQuiescentCurrentSurgeStopper:RobustAutomotiveSupplyProtectionforISO

7637-2andISO16750-2ComplianceByDanEddleman,SeniorApplicationsEngineer,MixedSignalProducts,LinearTechnology

Automotivepowersuppliesproduceformidabletransientsthatcanreadilydestroyexposedonboardelectronics.Overtime,aselec-tronicshaveproliferatedinvehicles,automotivemanufacturershavedulynotedfailures,compilingarogues’galleryoftheresponsi-blepowersupplytransients.Manufacturershaveindependentlycreatedstandardsandtestproceduresinanefforttopreventsensi-tiveelectronicsfromfallingpreytotheseevents.Recently,though,automotivemanufacturershavecombinedeffortswiththeInter-nationalOrganizationforStandardization(ISO)todeveloptheISO7637-2andISO16750-2standards,whichdescribethepossibletransientsandspecifytestmethodstosimulatethem.

ISO7637�2ANDISO16750-2STANDARDS

ISO7637isentitled“Roadvehicles—Electricaldisturbancesfromconductionandcoupling”andisanelectromagneticcompatibility(EMC)specification.Thisarticleaddressesthesecondofthethreepartsofthisdocument,ISO7637-2“Part2:Electricaltransientconductionalongsupplylinesonly.”

AlthoughISO7637isprimarilyanEMCspecification,priorto2011italsoincludedtransientsrelatedtopowersupplyquality.In2011,thoseportionsrelatedtopowersupplyqualityandnotEMCweremovedtoISO16750,“Roadvehicles—Environmentalcondi-tionsandtestingforelectricalandelectronicequipment”inthesecondoffiveparts,“Part2:ElectricalLoads.”

WhilemostmanufacturersstillmaintaintheirownspecificationsandrequirementsratherthanadoptISO7637-2andISO16750-2verbatim,thereisatrendtowardmorecloselyconformingtotheISOstandards,withmanufacturerspecificationsfollowingthein-ternationalstandardswithminorvariations.

ISO7637-2andISO16750-2providespecificationsforboth12Vand24Vsystems.Forsimplicity,thisarticleonlydescribes12Vspecificationsandpresentsacircuitforprotectingelectronicsconnectedtoanautomotive12Vpowersupply.

LOADDUMP

Loaddumpisthemostchallengingofthepowersupplytransientsbecauseofthesubstantialenergyintheevent.Itoccurswhenthealternatorischargingabattery,andthebatteryconnectionislost.

AlternatorswithoutInternalVoltageClamps

Originally,alternatorsincarswereunclampedandcouldproduceextraordinarilylargevoltagesduringloaddump,about100Vfor12Vsystems.Neweralternatorsareclampedinternallytolimitthemaximumvoltagetoalowervalueduringloaddump.Becauseolderalternators,andsomemodernalternators,donotincludeinternalclamps,theloaddumpspecificationinISO16750-2issplitinto“TestA—withoutcentralizedloaddumpsuppression”and“TestB—withcentralizedloaddumpsuppression.”

Figure1.Astandardalternator’s3-phasestatorwindingsand6-dioderectifierproduceaDCoutputvoltage.

Figure1showsaschematicofanalternator’s3-phasestatorwindingsandthe6-dioderectifierthatconvertsthestator’sACoutputtotheDCthatchargesthebattery.Whenthebatteryconnectionislost,theresultingcurrentflowisasshowninFigure2.Withoutthebatterytoabsorbthestator’scurrent,theoutputvoltagesurgestotheveryhighvoltagesseenduringunclampedloaddump,asshowninFigure3fromtheISO16750-2specification.Thiscorrespondstotheunclampedalternatorscenarioin“TestA—withoutcentralizedloaddumpsuppression.”

Figure2.Unclampedloaddump:Ifthebatteryconnectionislostduringcharging,thealternator’soutputvoltagecansurgeto100V.

Figure3.UnclampedloaddumppulseshapeasdescribedinISO16750-2specification(“TestA…”)

AlternatorswithInternalVoltageClamps

Neweralternatorsuseavalanchediodesthathavewellspecifiedreversebreakdownvoltageswhichlimitthemaximumvoltagedur-ingloaddump.Figure4showscurrentflowduringaloaddumpfaultinaclampedalternatorthatusesavalancherateddiodesinthesixdioderectifier.Whenaclampedalternatorismandatedbytheautomotivemanufacturer,“TestB—withcentralizedloaddumpsuppression”applies.Figure5showstheclampedwaveformfromTestBinISO16750-2.AlthoughISO16750-2specifiesa35Vmax-imumvoltageforthisclampedscenario,beawarethatmanymanufacturersdeviatefromISO16750-2byprovidingtheirownmaxi-mumvoltagespecification.

Figure4.Clampedloaddump:Aninternallyclampedalternatorhasdiodeswithwellspecifiedreversebreakdownvoltagesthatlimittheoutputvoltageto35Vduringloaddump.

Figure5.Clampedalternatorloaddumppulseshape

Also,beawarethatwhenloaddumpwaspartofISO7637-2,onlyonepulsewasspecified,butwhentheloaddumpspecificationmovedtoISO16750-2in2011,theminimumtestrequirementsincreasedtoincludemultiplepulseswithaoneminuteintervalbe-tweenpulses.

TVSProtectionProblems

Theinternalresistance,Ri,ofthealternatorinbothTestAandTestBisspecifiedtobebetween0.5Ωand4ΩinISO16750-2.Thislimitsthemaximumenergythatisdeliveredtoprotectioncircuits.

Nevertheless,onefactisfrequentlyoverlookedbythoseimplementingprotectionfromtheISO16750-2loaddumptransient:theinternalresistance,Ri,doesnotappearinserieswiththe35Vclampedvoltage.Riactuallyappearsbeforetheavalanchediode,asshowninFigure6.

Figure6.IftheonboardelectronicsareprotectedbyTVSdiodesthatbreakdownatalowervoltagethanthealternator’sclampedvoltage,theTVSdiodeswillbeforcedtoabsorballofthealternator’senergy.

IftheonboardelectronicsarelocallyprotectedbyashuntdevicesuchasaTVS(transientvoltagesuppressor)diodewithabreak-downvoltagelessthan35V,theTVSmaybeforcedtoabsorbthealternator’senergy.Inthisscenario,theinternalclampsintheal-ternatorareoflittlebenefit.TheentireloaddumpenergyisdeliveredtotheTVSintheonboardelectronics.

SometimesaseriesresistorisplacedinfrontoftheelectronicsandtheTVSdiode,butunfortunatelythisintroducesavoltagedropandextrapowerdissipationintheresistorevenduringnormaloperation.

ADVANTAGESOFACTIVEPROTECTIONWITHASURGESTOPPER

Abettersolutionistouseaseriesactiveprotectiondevice,suchastheLTC4380lowquiescentcurrentsurgestopper.TheLTC4380blockdiagramisshowninFigure7.AcompleteautomotiveprotectionsolutionisshowninFigure8.

Figure7.BlockdiagramoftheLTC4380surgestopper

Figure8.AnLTC4380-basedcircuitprotectsdownstreamelectronicsfromISO16750-2andISO7637-2transientswhileprovidingupto4Aofoutputcurrent.

Byitsverynature,asurgestopperprotectsthedownstreamelectronicsfromloaddumpaswellastheotherconditionsinISO16750-2andISO7637-2withoutrelyingontheinternalresistanceofthealternator.ThesurgestoppersolutionshowninFigure8providesuninterruptedpowerwhileoperatingfromaclampedalternator.Furthermore,ifitissubjectedtoloaddumpfromanunclampedalternator,itwillnotbedamaged.Intheunclampedscenario,itmayshutofftoprotectitselfandthenautomaticallyreapplypowertotheloadafteracool-downperiod.Itisimportanttonotethatpowerisonlyshutoffinthepresenceofmultiplesimultaneousfaults:animproperunclampedalternatorisinstalledandthebatteryconnectionislostduringcharging.

OPERATIONOFTHESURGESTOPPERPROTECTIONSOLUTION

ThedesigninFigure8protectsdownstreamelectronicsfromISO16750-2andISO7637-2transientswhileprovidingupto4Aofoutputcurrent.Atthesametime,itprotectstheupstreamsystemfromovercurrenteventscausedbyconditionssuchasshort-circuitfaultsinthedownstreamelectronics.Asitdoesthis,itconsumesamiserly35µAofquiescentcurrent,animportantconsiderationinmodernautomobilesfeaturingcountlessbattery-drainingloadswhilethevehicleisnotrunning.

ThisprotectionsolutionisbasedontheLTC4380lowsupplycurrentsurgestopper,limitingtheoutputvoltageto22.7Vfrominputvoltagesashighas100Vattheinput—sufficientprotectionagainstanISO-16750-2loaddumpaswellasISO7637-2pulses1,2a,2b,3a,and3b.Italsopreventscurrentflowduringreversebatteryconditions,andprovidescontinuouspowerduringtheISO16750-2superimposedalternatingvoltagetestatseveritylevel1wherethepeak-to-peakACvoltageis1V.(ItmaytemporarilyshutoffpowerinthepresenceoflargerACvoltages.)Continuouspowerisprovidedtotheloadwhentheinputvoltagedropsaslowas4VtosatisfytheminimumsupplyvoltagerequirementsofISO16750-2.

TheMOSFETsinthiscircuitareprotectedbylimitingthetimespentinhighpowerdissipationconditions,suchaswhentheinputvoltagesurgeshighduringloaddumporwhentheoutputisshortedtoground.IfafaultexceedstheconditionsspecifiedinISO16750-2andISO7637-2,MOSFETM2shutsofftoprotectthecircuit,reapplyingpowerafteranappropriatedelay.

Forexample,asustained100Vinputvoltage,oradownstreamshort-circuitfaultcausesthesurgestoppertoself-protectbylimitingthecurrentinM2andthencompletelyshuttingoffifthefaultpersists.Thismethodhasadistinctadvantageovershunt-typeprotec-tion,whichmustdissipatecontinuouspower—blowingfusesinthebestcase;lightingfiresintheworst.

LoadDumpandOvervoltageProtection

TounderstandtheoperationofthecircuitinFigure8,considerasimplifieddescriptionoftheLTC4380.Duringnormaloperation,theLTC4380’sinternalchargepumpdrivestheGATEpintoenhanceM2.ThevoltageatGATEisclampedtoamaximumof35Vaboveground(whentheSEL=0V),therebylimitingtheoutputvoltageatM2’ssourcetolessthan35V.

ThecircuitinFigure8furtherimprovesonthatvoltagelimitbyaddinga22VavalanchediodeD3,incombinationwithR6,R7,R8,andQ2toregulatetheoutputvoltagetoamaximumoftheavalanchediodevoltage,22V,plusthebase-emittervoltageofQ2,roughly0.7V.Whentheoutputvoltageexceeds22V+0.7V=22.7V,Q2weaklypullsdownonM2’sGATEtoregulateM2’ssourceandtheout-putvoltageat22.7V.

ReverseProtection

MOSFETM1,inconjunctionwithD1,D2,R1,R3,R4,andQ1,protectsthecircuitfromreversevoltageconditions.Whentheinputfallsbelowground,Q1pullsM1’sgatedowntothenegativeinputvoltage,keepingtheMOSFEToff.Thispreventsreversecurrentflowwhenthebatteryisconnectedbackwardandprotectstheoutputfromthenegativeinputvoltages.

D2andR3allowtheLTC4380’sinternalchargepumptoenhanceM1duringnormaloperationwhentheinputispositivesothatM1iseffectivelyasimplepass-throughdevice,dissipatinglessthanI2R=(4A)2•4.1mΩ=66mWofpowerintheNXPPSMN4R8-100BSE.

SOALimit

Whentheinputvoltageishigh,theoutputvoltageofthiscircuitislimitedtoasafelevelbycontrollingMOSFETM2.ThisresultsinsignificantpowerdissipationasvoltageisdroppedacrossM2whilecurrentisdeliveredtotheloadattheoutput.

Iftheinputissubjectedtoasustainedovervoltagecondition,oranovercurrentfaultconditionoccursintheonboardelectronicsatthecircuit’soutput,M2isprotectedbyshuttingoffafteradurationconfiguredbythetimernetworkmadeupofR13,R14,R15,C4,C5,C6,andC14.TheoutputcurrentattheLTC4380’sTMRpinisproportionaltothevoltageacrossMOSFETM2whileM2isincur-rentlimit.

Effectively,theTMRcurrentisproportionaltothepowerdissipatedinMOSFETM2.Theresistor/capacitornetworkattheTMRpinissimilartoanelectricalmodeloftheMOSFET’stransientthermalimpedance.Thisservesto

limitthemaximumtemperatureriseoftheMOSFETtokeepitwithinitsratedsafeoperatingarea.

BecauseallowableMOSFETSOAcurrentfallsoffathighdrain-to-sourcevoltages,the20VavalanchediodeD6,inconjunctionwithR9,R11,andQ3providesextracurrentintothetimernetworkwhentheIN-to-OUTvoltageexceeds20VplusQ3’sbase-emittervolt-age.The4.7VavalanchediodeD7workswithQ4,R12,andC3topreventthisextracurrentfrompullingtheTMRpinaboveitsmaxi-mumratedvoltageof5V.

ThisSOAtrackingcircuitallowstheoutputtoremainsafelypoweredwhentheinputrisestoahighvoltage.But,ifasustainedhighpowerfaultconditionlaststoolong,thecircuitself-protectsbyshuttingoffM2.

ThermalProtection

Theresistor/capacitornetworkontheLTC4380’sTMRpinprotectsagainsteventsthatarefasterthanaboutonesecond.Forslowerevents,thecasetemperatureofM2islimitedbythecircuitconnectedtotheLTC4380’sONpin.

Thethermistor,RPTC,isasmallsurfacemount0402-sizecomponentwitharesistanceof4.7kat115°C.Above115°C,itsresistancerisesexponentiallywithtemperature.Topreventthetimernetworkfromfalselyintegratingoffsetsinthepowermultiplier,theLTC4380doesnotgeneratetimercurrentattheTMRpinuntilM2’sdrain-to-sourcevoltagereaches0.7V.With4Aand0.7V,theMOSFETcoulddissipate0.7V•4A=2.8WcontinuouslywithouttheTMRnetworkdetectingtheMOSFET’stemperaturerise.ThePTCresistor,RPTC,inconjunctionwithresistorsR17–R21andtransistorsQ5A,Q5B,Q6A,Q7A,andQ7BshutsdownthecircuitifMOSFETM2’scasetemperatureexceeds115°C.

Donotbedismayedbythenumberofcomponentsinthethermalprotectioncircuit.Theoverallsolutionisrelativelyeasytoimple-mentandconsistsofsmallcomponentsthatconsumelittleboardarea.Itisaself-biasedcircuitthatisbalancedwhenRPTCequals

ISO7637-2Requirements

R20’s4.75kΩvalue.WhenthetemperatureofRPTC,whichisplacedincloseproximitytoM2,exceeds115°C,itsresistancegrowsandcausesmorecurrenttoflowthroughQ5BthanQ5A.BecausethatresultsinmorecurrentthroughR17thanR18,Q8A’sbasevoltagerisesandQ8A’scollectorpullstheONpinoftheLTC4380low,turningoffM2.Atlowertemperatures,Q5A’scurrentisgreaterthanQ5B’s,andQ8Aremainsoff,allowingtheONpin’sinternalpull-uptokeeptheONpinhigh.NotethattheONpincurrentisusedasthestart-upcurrentofthisself-biasedcircuitthroughthediode-connecteddeviceQ8B.

CONCLUSION

TheISO16750-2andISO7637-2specificationsdescribethechallengingelectricaltransientsthatcanoccurinautomotivesystems.TheLTC4380lowquiescentcurrentsurgestoppercanbeusedtoprotecttheonboardelectronicsfromthesetransients,includingboththeclampedandunclampedloaddumppulses.Thecircuitpresentedinthisarticleprovidesuninterruptedoperationwhenfacedwithloaddumppulsesfromamodern,clampedalternator.Whenfacedwithmoreextremeunclampedloaddumppulses,itshutsofftoprotectthedownstreamelectronics.TheresultisarobustsolutionforISO16750-2andISO7637-2complianceforelec-tronicsthatdrawupto4ampsofsupplycurrent.

SIDEBAR1

WhileloaddumpistypicallythemostdemandingconditiondescribedinISO16750-2,therearemanyadditionalrequirements.

REVERSEBATTERYSection4.7ofISO16750-2describes“ReversedVoltage”orwhatmostautomotiveengineerssimplyrefertoas“ReverseBattery.”Asyouwouldexpect,thisspecificationcoversthehumanerrorscenariowheresomeoneconnectsabatterywiththepolarityreversed.Obviously,thiscanresultindestructionunlessadequateprotectionisprovided.

ISO16750-2requiresthata14Vreversetestvoltagebeappliedatallinputsfor60secondstoensurethatthesystemsurviveswithoutanydamage.Analternativetestconditionof4VreversevoltageisalsoallowedbyISO16750-2ifnofuseispresentinserieswiththealternatorandthealternator’srectifierdiodeslimitthevoltagebyconductingthesubstantialcurrentdeliveredbythereverseconnectedbattery.

MINIMUMANDMAXIMUMSUPPLYVOLTAGESTheminimumandmaximumsupplyvoltagesarespecifiedinsection4.2“Directcurrentsupplyvoltage.”Themaximumsupplyvoltagefor12Vsystemsis16V,andtheminimumisaslowas6V.Forhardwarethatisnotcapableofoperatingaslowas6V,other“codes”areassignedinISO16750-2toclassifytheminimumoperatingvoltageofthedevice.Forthisrequirement,theequipmentisexpectedtooperatecontinuously.

OVERVOLTAGESection4.3ofISO16750-2describes“Overvoltage”requirements.Thefirstrequirementsimulatestheconditionwherethevoltageregulatorhasfailed.Inthistest,18Visappliedfor60minutes.Dependingontheapplication,itmightnotbenecessaryfortheequipmenttooperatenormallywhilethetestisperformed,butitmustreturntonormaloperationafterthetestconditionisremoved.

Thesecondtestconditionsimulatesajump-startwith24Vappliedfor60seconds.Onceagain,itmaynotbenecessaryfortheequipmenttooperatenormallyduringthetest.

SUPERIMPOSEDALTERNATINGVOLTAGESection4.4.2providestestconditionsto“simulatearesidualalternatingcurrentonthedirectcurrentsupply.”ApeaktopeakACvoltageof1V,2V,or4V

(specifiedasa“severitylevel”)issweptfrom50Hzto25kHzmultipletimes.Theupperpeaksofthevoltageareat16Vandtheseriesimpedanceisbetween50mΩand100mΩ.

SUPPLYDIPSSections4.5and4.6ofISO16750-2addressconditionswheretheinputsupplydips,eitherduetothebatterydischarging,anotherdeviceintheautomobilefailingandblowingafuse,orwhenthestartercausesthesupplyvoltagetodip.

Section4.5“Slowdecreaseandincreaseofsupplyvoltage”simulatesabatterybeingslowlydischargedandthenrecharged.Thesupplyvoltageisdischargedto0Voveramatterofminutes,andisthenslowlybroughtbackup.Obviously,itisnotnecessarytooperatecontinuously,butthistestverifiesthatthehardwaredoesnotfailinadestructivemanner,andthatitoperatesnormallywhenpowerisrestored.

Incontrast,Section4.6“Discontinuitiesinsupplyvoltage”isamuchfasterconditionthatattemptstosimulateafailureinanothercircuitthatcausesthesupplytodipuntiltheothercircuit’sfuseblowsopen.Inthisscenario,thesupplydipsto4.5Vfor100msandthenrecoverswitharisetimeandfalltimefasterthan10ms.

ThenextpartofSection4.6specifiesaseriesof5-secondsupplydips,witheachpulseatalowervoltagethanthepreviousone.Thepurposeistoverifythatthedeviceresetsproperlyfollowingasupplydip.

ThethirdandlastpartofSection4.6specifiesawaveformrepresentativeofavehicle’sstartingprofile.Itisappliedtothedevicebeingtested10times.TheexactvoltagesanddurationsrequireddependonthedesiredLevelI,II,IIIorIV,whichisdeterminedbytheapplication.ThelimitsofLevelIareshowninthefigurebelow.

OPENCIRCUITANDSHORT-CIRCUITPROTECTIONSection4.9covers“lineinterruption”testsanddescribesprocedurestoensurethatadeviceresumesnormaloperationafterconnectionisremovedandthenrestored.Section4.10describes“short-circuitprotection”testsandrequiresconnectingeachinputandoutputtothemaximumsupplyvoltageandgroundfor60seconds.

SIDEBAR2

ISO7637-2RequirementsWhilethepowerqualityportionsofISO7637-2movedtoISO16750-2in2011,pulses1,2a,2b,3a,and3barestillcontainedinISO7637-2.

PULSE1Pulse1describesthenegativetransientobservedbyelectronicsconnectedinparallelwithaninductiveloadwhentheconnectiontothepowersupplyisinterrupted.Pulse1beginswiththesupplyvoltagecollapsingto0Vasthesupplyvoltageisremoved.Soonthereafter,a−150Vpulseisappliedwitha2msdecaytime.Theenergyofthenegativepulseislimitedbythe10Ωseriesresistance.

PULSE2APulse2adescribesthepositivevoltagespikethatmayoccurwhencurrentisinterruptedtoacircuitinparallelwiththeelectronicsbeingtested.Ifcurrentisbuiltupinthewiringharness,whenadevicesuddenlystopssinkingcurrent,theenergystoredinthewiringharnessinductancemaycauseavoltagespike.Theenergyofthispositivespikeislimitedbya2Ωseriesresistance.

PULSE2BPulse2bdefinesasituationthatoccurswhentheignitionisswitchedoffandDCmotorsactasgenerators.Forexample,iftheheaterisrunningwhenthedriverturnsoffthecar,forashorttimetheblowermotorcansupplyDCpowertothesystemwhileitspinsdown.

PULSES3AAND3BPulses3aand3barethenegativeandpositivespikesthatmayoccurasaresultofswitchingprocessesincludingarcingacrossswitchesandrelays.Forthisspecification,theenergyislimitedbya50Ωseriesresistance.

INDEPENDENTTESTREPORTTheLTC4380hasbeentestedforISO7637-2andISO16750-2compliancebyanindependenttestfacility.Thefulltestreportisavailableatwww.linear.com/docs/56650.