外文文献翻译（原文）-基于白色光源的LED寿命

IEEE/OSAJOURNALOFDISPLAYTECHNOLOGY,VOL.1,NO.1,SEPTEMBER2005167TLifeofLED-BasedWhiteLightSourcesNadarajahNarendranandYiminGuAbstractEventhoughlight-emittingdiodes(LEDs)mayhaveaverylonglife,poorlydesignedLEDlightingsystemscanexperi-enceashortlife.Becauseheatatthep-n-junctionisoneofthemainfactorsthataffectthelifeoftheLED,byknowingtherelation-shipbetweenlifeandheat,LEDsystemmanufacturerscandesignandbuildlong-lastingsystems.Inthisstudy,severalwhiteLEDsfromthesamemanufacturerweresubjectedtolifetestsatdifferentambienttemperatures.Theexponentialdecayoflightoutputasafunctionoftimeprovidedaconvenientmethodtorapidlyestimatelifebydataextrapolation.ThelifeoftheseLEDsdecreasesinanexponentialmannerwithincreasingtemperature.Inasecondex-periment,severalhigh-powerwhiteLEDsfromdifferentmanufac-turerswerelife-testedundersimilarconditions.Resultsshowthatthedifferentproductshavesignificantlydifferentlifevalues.IndexTermsDegradation,life,lightsource,whitelight-emit-tingdiode(LED).I.INTRODUCTIONHEinterestforusinglight-emittingdiodes(LEDs)fordisplayandilluminationapplicationshasbeengrowingsteadilyoverthepastfewyears.Thepotentialforlonglifeandreducedenergyusearetwokeyattributesofthisrapidlyevolvingtechnologythathavegeneratedsomuchinterestforitsuseintheabovementionedapplications.Traditionally,thelamplifeoflightsourcescommonlyusedinilluminationap-plicationsisdeterminedbysubjectingthemtoapredeterminedon/offcycleuntilhalfthenumberoflightsourcesceasetoproducelight1.Unlikethesesources,LEDsrarelyfailcat-astrophically;instead,theirlightoutputslowlydegradesovertime.EvenifanLEDistechnicallyoperatingandproducinglight,atsomepointtheamountoflightproducedbytheLEDwillbeinsufficientfortheintendedapplication.Therefore,thelifeofanLEDshouldbebasedontheamountoftimethatthedevicecanproducesufficientlightfortheintendedapplication,ratherthancompletefailure.Basedonthisargument,arecentpublicationfromanindustrygroupdefinesthelifeofanLEDdeviceorsystemforuseingenerallightingapplicationsastheoperatingtime,inhours,forthelightoutputtoreach70%ofitsinitialvalue2.ThemostwidelyusedwhiteLEDsincorporatealayerofphosphoroveraGaN-based,short-wavelengthlightemitter3.Usually,thephosphorisembeddedinsideanepoxyresinthatsurroundstheLEDdie.Someportionoftheshort-wave-lengthradiationemittedbytheLEDisdown-convertedbythephosphor,andthecombinedradiationcreateswhitelight.ManuscriptreceivedJanuary26,2005;revisedApril20,2005.ThisworkwassupportedinpartbytheU.S.DepartmentofEnergyunderCooperativeAgreementDE-FC26-01NT41203andbytheUniversityofCalifornia,SantaBarbara.TheauthorsarewiththeLightingResearchCenteratRensselaerPolytechnicInstitute,Troy,NY12180USA(e-mail:;).DigitalObjectIdentifier10.1109/JDT.2005.852510EarlywhiteLEDswerepackagedsimilartotheindicator-stylecoloredLEDs,specifically5mmandSMD(surfacemountdevices).Althoughtheseproductsdemonstratedtheconceptofawhitelightsource,theydidnotproducesufficientlightfordisplayandilluminationapplications.Furthermore,theseindi-cator-stylewhiteLEDshadarelativelyshortlife,500010000htoreach70%lightlevelundernormaloperatingconditions4.Toaddressthehigherluminousfluxrequirements,manu-facturershavestartedtocommercializehigh-powerilluminatorLEDsthatarepresentlyproducingoveronehundredtimesthefluxcomparedtoindicator-stylewhiteLEDs.Thehigherlightoutputisachievedbyusinglargerdies,higherdrivecurrents,andimprovedheatextractionmethods5,6.Inaddition,somemanufacturersareusingbetterencapsulantstoimprovethelifeofwhiteLEDs6.ThereareseveralstudiesthathaveinvestigatedtheagingmechanismsofGaN-basedLEDs710.Duringthe1990s,Bartonetal.investigatedthedegradationofGaN-basedblueLEDsandshowedthatlightoutputreductionovertimeoccurredprimarilyduetotheyellowingoftheepoxysurroundingthedie7.In2001,Narendranetal.observedthatindicator-stylewhiteLEDpackagesdegradedveryrapidly,withtheLEDsreachingthe50%lightoutputlevelwithin6000h4.Inthatsamestudy,itwasshownthatthechromaticityvaluesofthewhiteLEDsshiftedtowardyellowovertime,anditwasspec-ulatedthattheyellowingoftheepoxywasthemaincauseforlightoutputdegradation4.Therefore,basedonpaststudies,theprimaryreasonforthedegradationofindicator-stylewhiteLEDpackagesistheyellowingoftheepoxythatiscausedbyexcessiveheatatthep-n-junctionoftheLED10.Someofthenewerilluminator-stylewhiteLEDsuseencapsulantmaterialsthathavelowerphotodegradationcharacteristics5,andthereforehavealowerdegradationrate.However,therearefactorssuchasthedegradationofthedieattachepoxy,discolorationofthemetalreflectorsandtheleadwires,anddegradationofthesemiconductingelementthatareinfluencedbyheat,andtheseallcontributetotheoveralldegradationofthewhiteLED.Althoughthenewerhigh-powerwhiteLEDswouldhavealowerdegradationratecomparedtotheearlyindicator-styledevices,itistheheatatthep-n-junctionthatmostinfluencesthedegradation.Theheatatthep-n-junctioniscausedbytheambienttemperatureandtheohmicheatingatthebandgap.Asstatedearlier,longlifeisonekeyfeatureofLEDtech-nologythathasattractedsomanyend-usecommunities.Toben-efitfromthelong-lifefeature,itisthefinalsystemthathastooperateforalongtime,notjusttheindividualLED.Asnotedinpaststudies,heatatthep-n-junctionisoneofthekeyfac-torsthatdeterminethelifeofthewhiteLED.Therefore,ifsys-temsarenotproperlydesignedwithgoodthermalmanagement1551-319X/$20.002005IEEE168IEEE/OSAJOURNALOFDISPLAYTECHNOLOGY,VOL.1,NO.1,SEPTEMBER2005Fig.1.ExamplesofT-pointsfortwodifferenttypesofLEDs.techniques,eveniftheyuselong-lifewhiteLEDsthelifeofthefinalsystemwouldbeshort.Developingtherelationshipbe-tweenjunctiontemperatureandlifewouldbeveryusefulforproducinglong-lifesystems.AlthoughtherearedifferentmethodsavailableforestimatingthejunctiontemperatureofLEDs,theyarenotveryconvenient,especiallyoncetheLEDsareintegratedintoasystem11.Fur-thermore,thesemethodsarenotdirect;consequently,theyarepronetoerroneousresults.Alternatively,itismuchmoreconve-nientanddirecttomeasuretheheatatalocationexternaltotheLEDpackagethatissufficientlyclosetothejunctionandwhereatemperaturesensorcanbedirectlyattached.Thetemperatureofthispointshouldhaveagoodrelationshiptothejunctiontem-perature.Thepointwhereatemperaturesensorcanbeattachedforthismeasurementcouldbetheleadwire(cathodeside)fortheindicator-styleLEDsandtheboardforhigh-powerLEDs(seeFig.1).Mostmanufacturerscanrecommendsuchapoint,andwerefertothisastheT-pointinthismanuscript.SincewhiteLEDsinthemarketplacearepackageddiffer-ently,theirabilitytotransferheatfromthedietothesurroundingenvironmentisdifferentfromproducttoproduct.Therefore,itisreasonabletoassumethatdifferentproductshavedifferentdegradationratesasafunctionofheat.AgraphthatshowsthelifeoftheLEDasafunctionofT-pointtemperatureisextremelyusefulforsystemmanufacturerstobuildreliable,long-lastingsystems.Byknowinghowmuchimpactheathasonthedegra-dationrateorlifeoftheLED,thesystemmanufacturercanse-lectcomponentsanddriveparameters,includingtheamountofheatsinkanddrivecurrent,foraproductbeingdesignedforagivenapplication.Therefore,theobjectiveofthestudypresentedinthisman-uscriptwastoinvestigatetherelationshipbetweentheT-pointtemperatureandlifeofawhiteLED.Asecondobjectivewastounderstandthedegradationrateofdifferenthigh-powerwhiteLEDproductspresentlyavailableinthemarketplace.II.EXPERIMENTTounderstandtherelationshipbetweentheT-pointtempera-tureandlife,onetypeofhigh-powerwhiteLEDthatiscom-monlyavailableinthemarketplacewasselected.SeveraloftheseLEDsweresubjectedtoalifetestunderdifferentam-bienttemperatures.Thedetailsoftheexperimentalsetuparede-scribedinthefollowingparagraphs.BecausethedifferentLEDarrayshavetooperateatapar-ticularambienttemperature,thearrayswereplacedinsidespe-ciallydesigned,individuallife-testchambers,showninFig.25.Thetestchambershadtwodifferentfunctions:1)tokeepFig.2.LEDlife-testchamber.theambienttemperatureconstantfortheLEDarraysand2)toactaslight-integratingboxesformeasuringlightoutput.EachindividualLEDarraywasmountedatthecenteroftheinsidetopsurfaceofalife-testchamber.Aphotodiodeattachedtothecenteroftheleftpanelcontinuouslymeasuredthelightoutput.Asmallwhitebaffleplacedoverthephotodiodeshieldeditfromthedirectlight,allowingonlythereflectedlighttoreachthephotodiode.Aresistancetemperaturedetectorplacedontopofthebafflemeasuredthechambersambienttemperatureandcontrolledtheheaterthatprovidedthenecessaryheattothechamberthroughatemperaturecontroller.Thetemperaturein-sidetheboxremainedwithinC.Theheaterwasattachedtoaraisedaluminumplatewithamatte-whitecoverthatsatonthechamberfloor.ThetemperaturewasestimatedusingaJ-typethinwirethermocouplesolderedtotheT-pointofonewhiteLED.Foreachchamber,anexternalLEDdrivercon-trolledthecurrentflowthroughtheLEDs.Alllife-testchamberswereplacedinsideatemperature-controlledroom,asshowninFig.3.Thelife-testchamberswerestaggeredverticallyandhor-izontallytoensurethatheatrisingfromthebottomchambersdidnotaffectthechambersabovethem.A.Experiment1Thegoalofthefirstexperimentwastodeterminetheeffectofheatonthelifeofhigh-powerwhiteLEDs.Tensimilarhigh-powerwhiteLEDsfromthesamemanufacturingbatchwereacquiredinearly2004.Fivearrayswerecreatedbyconnectingtwohigh-powerLEDsinseriesperarray,andthesearrayswerelife-tested.TheLEDswereoperatedattheirratedcurrentof350mAbutatdifferentambienttemperatures.AftertheinitialbatchofLEDs,arrays15,weretestedforseveralthousandhours,additionalhigh-powerwhiteLEDsfromthesamemanufacturerwereacquiredduringthelatterpartof2004andweresubjectedtosimilarlife-testing(arrays67).TheoperatingconditionsofthearraysaresummarizedinTableI.Twoadditionallifetestdata(arrays89),obtainedusingsimilarhigh-powerwhiteLEDspurchasedin2002andoperatedundersimilarconditions,arealsoincluded.TheonlydifferenceisthatNARENDRANANDGU:LIFEOFLED-BASEDWHITELIGHTSOURCES169Fig.3.LEDlife-testlaboratory.TABLEITESTINGCONDITIONSFORTHEHIGH-FLUXWHITELEDARRAYSINEXPERIMENT1arrays8and9hadsixwhiteLEDsinsteadoftwo,asusedintheotherarrays.Usuallyduringtheinitialperiod,thelightoutputoftheLEDsincreasesandthendecreases.Thisismostlikelyduetoan-nealingeffects.Thetimeittakesforthelightoutputtoreachthemaximumvariesdependingontheoperatingconditions.Astheoperatingtemperatureincreases,thetimeittakesfortheLEDstoreachthemaximumdecreases.Fig.4showstherelativelightoutputasafunctionoftimeforthehigh-powerwhiteLEDs.Thelinesinthisfigurearetheregressionfitsforthedatacollectedatthedifferenttempera-tures.BecausemostoftheLEDarrayscouldtakeseveralyearstoreachthe70%lightlevel,itisnecessarytouseamathemat-icalfittoextrapolatethedataandestimatelife.ForwhiteLEDsofthistype,thelightoutputdecreasefollowsanexponentialdecaycurve.Itisworthnotingherethatwithotherpackagetypes,thelightoutputdecreasemaynotfollowthesameex-ponentialdecay.Itisourexperiencethattheestimatedlifetimeusinganexponentialfitisreliablewhentheinitial1000hofFig.4.Lightoutputasafunctionoftimeforhigh-powerwhiteLEDsoperatedatvariousambienttemperatures.Thelinesaretheregressionfitsforthedatacollected.T-pointtemperaturesforeacharrayareshown.Fig.5.LifeasafunctionofT-pointtemperature.dataisomittedandaminimumof5000hofdatabeyondtheinitial1000hisused.(Wewouldliketopointoutthatwithim-provementsinproductperformance,moredatamaybeneededtoreliablyprojectLEDlife.)InFig.4,alllightoutputvaluesarenormalizedtotheirvalueat1000h,andanexponentialfittotherespectivedatapointsproducedthelightoutputdecaycurves,whichwereplottedonalogarithmicscale.Lifevalueswerees-timatedusingthesecurves.Fig.5illustratestheestimatedlifeofthewhiteLEDsasafunctionofT-pointtemperaturefortheninearrays.ThelifedecreasedastheT-pointtemperatureincreased.ThelifeasafunctionofT-pointtemperaturealsofollowedanexponentialcurve.However,onedatapoint,denotedbyanopentriangleinFig.5,didnotfallonthistrendlineanddeviatedsignificantly.Toidentifypossiblereasonsforthisdeviation,theT-pointtempera-turedataovertheperiodofthelifetestwerecarefullyanalyzed.However,nothinglookedabnormal,andtheaveragevaluewasmaintainedthroughout.OnepossibleexplanationforthedeviationcouldbethattheperformancevariationbetweensimilarLEDsislarge,duetomanufacturingissues.Itisnotpossibletoverifythisexplanationinthislifestudy,sincethesamplesizeistoosmallandthelightoutputofeachindividualLEDinthearraywasnotmonitored.AlargesamplesizewithLEDsfromdifferentmanufacturingbatcheswouldyieldtheaveragedegradationrateandthevari-ancebetweensimilarLEDsiftheirindividuallightoutputweremeasured.WearenotawareofanypublisheddatathatshowthisinformationforthesetypesofLEDs.FromFigs.4and5,itappearsthatthelifeofwhiteLEDsofthistypeisover50000hatroomtemperature,25.170IEEE/OSAJOURNALOFDISPLAYTECHNOLOGY,VOL.1,NO.1,SEPTEMBER2005TABLEIITESTINGCONDITIONSFORTHEHIGH-FLUXLEDARRAYSINEXPERIMENT2Fig.6.Relativelightoutputovertimeforseveralcommercialhigh-powerwhiteLEDsoperatedunderthesameconditions.B.Experiment2Asmentionedearlier,therearemanyhigh-powerwhiteLEDspresentlyavailableinthemarketplace.Thegoalofthesecondexperimentwastostudyhowthedifferentcommercialhigh-powerwhiteLEDsperformedwhenoperatedundersimilarcon-ditions.ThisexperimentwasverysimilartoExperiment1.Al-together,sixarraysofwhiteLEDpackageswerelife-tested;twoarraysconsistedofmulti-diepackagesandtheremainingweresingle-diepackages.AllLEDswereoperatedattheirratedcur-rent(350mA)andatanambienttemperatureof35.Theop-eratingconditionsofthearraysandthemeasuredT-pointtem-peraturesaresummarizedinTableII.Fig.6showstherelativelightoutputasafunctionoftimefortheLEDarrays.Here,thedataisnormalizedtothevalueatzerohours.AsseeninFig.6,thedifferentcommercialwhiteLEDsdegradeatdifferentrates.Fromtheseinitialdataitappearsthatforthesameoperatingconditions,thedifferentLEDshavedif-ferentT-pointtemperaturesandwouldresultinmuchdifferentlifevalues.TheseresultsindicatethatthedifferentLEDswouldrequiredifferentamountsofheatsinkwhenbeingpackagedintofixturesinordertohavesimilarlife.III.SUMMARYTheresultsofthisstudyunderscoretheimportanceofpackagingwhiteLEDsusingproperthermalmanagementtomaintainlightoutput,andtherebyextendsystemlife.Heatatthep-n-junctionisoneofthemainfactorsthataffectthelifeofwhiteLEDs.Therefore,knowingtherelationshipbetweenlifeandheatwouldbeveryusefulformanufacturerswhoareinterestedindevelopingreliable,long-lastingsystems.ResultsfromthefirstexperimentconductedundervariousambienttemperaturestounderstandtherelationshipbetweenT-pointtemperatureandlifeindicatethatlifedecreaseswithincreasingtemperatureinanexponentialmanner.ResultsfromthesecondexperimentconductedtounderstandhowdifferentcommercialwhiteLEDsperformunderidenticaloperatingconditionsshowalargevariationinlifeamongthedifferentpackages,indicatingthatthepackagesuseddifferentheatextractiontechniquesandmaterials.Aspartofongoingresearch,wehopetofurtherinvestigatehowthedifferentcommercialLEDsareaffectedbyheatandfinallydevelopafamilyofcurvesthatillustratetherelationshipbetweenlifeandT-pointtemperatureforthedifferentproducts.ACKNOWLEDGMENTTheauthorswouldliketothankR.PysaroftheLightingRe-searchCenter(LRC),RensselaerPolytechnicInstitute,Troy,NY,fordesigningandimplementingtheLEDlife-testappa-ratus.TheywouldalsoliketothankMs.J.TayloroftheLRCforhelpingtopreparethismanuscript.REFERENCES1M.S.Rea,TheIESNALightingHandbook,ninthed.NewYork:Illum.Eng.Soc.ofNorthAmerica,2000.2AllianceforSolid-StateIlluminationSystemsandTechnologies(AS-SIST).(2005)ASSISTRecommends:LEDLifeforGeneralLighting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