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ThermalanalysisofLEDarray精
ThermochimicaActa455(2007
21–25
ThermalanalysisofLEDarraysystemwithheatpipe
LanKim,JongHwaChoi,SunHoJang,MooWhanShin∗
DepartmentofMaterialsScience&Engineering,MyongJiUniversity,38-2Nam-Dong,Yongin,Kyunggi449-728,RepublicofKorea
Availableonline3December2006
Abstract
ThispaperreportsonthermalcharacterizationofhighpowerLEDarrays.Thermaltransientmethodsareusedtomeasurethejunctiontemperatureandcalculatethethermalresistance.TheemphasisisplacedupontheinvestigationofjunctiontemperatureriseofLEDarrayforalimitedrangeofboundaryconditionswhichincludedesigneffectofheatpipe,convectioncondition,andambienttemperature.ThejunctiontemperaturesofLEDarraywithandwithoutheatpipeatthesameairvelocityof7m/swere87.6◦C,and63.3◦C,respectively.ThecorrespondingthermalresistancesofLEDarrayweremeasuredtobe1.8K/Wand2.71K/W.ItwasfoundoutthatthemeasuredjunctiontemperaturesandthermalresistanceofLEDarrayareincreasedwiththeinputpowerandambienttemperature,butdecreasedwiththeairvelocity.Ananalyticalthermalmodelanalogouswithanequivalentparallelcircuitsystemwasproposedandwasverifiedbycomparisonwithexperimentaldata.
©2006ElsevierB.V.Allrightsreserved.
Keywords:
LEDarray;Heatpipe;Junctiontemperature;Thermalresistance
1.Introduction
Lightemittingdiode(LEDisasolidstatesemiconductordevicethatconvertselectricalenergyintolight.LEDsdemon-strateanumberofbenefitscomparedtotraditionalincandescentlamps.Nowadays,highpowerLEDsarebeinginvestigatedasreplacementsforcoldcathodefluorescentlamp(CCFLintheLCDdisplaybacklightsandheadlightlampforautomobiles[1,2].WithfurtherimprovementLEDshaveagreatpotentialtobecomeanewilluminationsource.However,therealchallengeisthatthelifetimeofLEDsstillbeeasilyshortenedbyheat;notonlybytheheatfromambientbutalsobytheheatgeneratedwithintheLEDitself[3,4].Inaddition,theperformanceofunitLEDisknowntosignificantlydependonthesystemswheretheLEDpackagesarelaiddown.Inanextremecase,thelifetimeofunitLEDpackagewithanexcellentthermalperformancecanbeveryshortifthesystemaroundithasapoorthermaldesign.Therefore,effectivethermaldesignandreliablethermalcharacterizationofLEDsystemareimportantfortheunitLEDpackage.ThermalcharacterizationofLEDsinanarrayisverydifferentfromthatofsingleLEDpackage.Thejunctiontemper-atureofLEDarraywillbesignificantlyinfluencedbyambienttemperatureandsideeffectfrommultiplechips.Itisgener-allyknownthatthermalbehaviorofLEDarrayisaffectedby∗Correspondingauthor.Tel.:
+82313306465;fax:
+82313306457.
E-mailaddress:
mwshin@mju.ac.kr(M.W.Shin.morefactorsthaninthecaseofunitpackage.TherehavebeenseveralreportsonthermalcharacterizationofLEDpackageswithasinglechip[5,6].However,therehavebeennoreportsonthethermalanalysisofLEDarraysystemsofartothebestknowledgeoftheauthors.
Inthispaper,thermalbehaviorofLEDarraysystemisreported.Testchipsarewidelyusedtopredictthejunctiontem-peratureofarraysystemwithelectronicdevices(CPU,CMOS,etc.[7,8].However,extraspecialtestchipsarenotusedinourexperimentandLEDitselfisusedasatestchip.Themethodcanreduceseveralmeasuringparameterswhichcanmisleadarealjunctiontemperaturewithoutdestructingitselectricalcircuits.
Thermaltransientmeasurementwasdoneusingtheso-calledstructurefunction[9].ThermalcharacteristicsofLEDarraywithheatpipeandwithoutheatpipearecomparedunderdifferentambienttemperaturesandforcedconvectionconditions.
2.Theoreticalbackground
AtJEDECStandardNo.51-1,thermalresistanceofasinglesemiconductordeviceisdefinedas:
RJX=
TJ−TX
PH(1whereRJXisthethermalresistancebetweendevicejunctionandthespecificenvironment,TJthejunctiontemperatureofdeviceinasteadystatecondition,TXthereferencetemperatureforthe
0040-6031/$–seefrontmatter©2006ElsevierB.V.Allrightsreserved.doi:
10.1016/j.tca.2006.11.031
22L.Kimetal./ThermochimicaActa455(200721–25specificenvironment,andPHthepowerdissipationinthedevice
[10].
Theequationisforasinglechippackage.Thermalresistance
ofLEDarrayswhichhavemultipleheatsourcescanbedescribed
asthefollowingrelationusingtheaveragejunctiontemperature
ofLEDarray,Tj,avg.
θja-avg=Tj,avg−Tamb
P(2
whereθja-avgisanaveragejunctiontoambientthermalresis-tance,Pisthepowerdissipationoftheentirepackages,andTambistheambienttemperature.TheequationassumesthateachLEDmountedonthearrayexhibitsthesamethermalchar-acteristics.BecausetheLEDsusedinthisexperimentareofidenticalgeometryandpowerdissipation,theemploymentoftheaboveequationisvalidinouranalysis.Totalpowerdissipa-tioniscalculatedbythemeasuredvoltageandtheinputcurrent.TemperaturerisecanbeinterpretedbythechangeofvoltagedropinafollowingwayforLEDandtheslopeisknownasaKfactor[10];
slope=dVF
dTJ
(3
wheredVFisthedifferentialofforwardbiasvoltage,anddTJisthedifferentialofjunctiontemperature.
ExpandingthistheorytoaseriesofmultipleLEDsleadstothefollowingexpressionformodifiedslope:
slopetotal=dVFtotal
dT
=
ndVF
dT
=n·slope(4
Eq.(4indicatesthattheKfactorfortheLEDarrayiseasilydefinedfromtheslopeforaunitLEDpackage.TheslopetotalforthearraysystemisntimesoftheslopeforasingleLEDpackage,slope.ForanLEDarray,then·slopeisaconstant,sothetotalforwardvoltageoftheLEDarraycanbeusedasatemperaturesensitiveparameter(TSP.
3.Experiments
CommercialGaN-basedLEDscoatedwithyellowphosphor(LuxeonVwereusedforthefabricationofarrayinthisexperi-ment.LEDarrayswerepreparedeitherwithorwithoutheatpipe.LEDarrayiscomposedofsixhighpowerLEDsandmountedon5cm×7.5cmmetalcoreprintedcircuitboard(MCPCBwitha2.5cmpitch.Fig.1showstheschematicstructureofLEDarraymountedonMCPCBandarraysystemwithheatpipe.
Thediameteroftheheatpipeis1.27cmandthelengthis30cm.LEDsinthearrayareelectricallyconnectedinseriesandsimultaneouslydriven.Sensorcurrentof20mAwasusedtodetecttheforwardvoltageofthearray.Measurementswerecarriedoutbyathermaltransienttester(T3ster®.Thetheo-reticalframeworkoftheevaluationoftheT3sterisbasedonarepresentationofthedistributedRCnetworks.Thestructurefunctionsareobtainedbydirectmathematicaltransformationsfromthecoolingcurve.Afteracalibrationprocess,whichdeter-minestheratiobetweenthetemperatureandtheforwardvoltagedropasatemperaturesensitiveparameter(TSP,cooling
curve
Fig.1.SchematicstructureofanLEDarraymountedonMCPCB.wasobtained.ThesizeofAlchamberusedinthisexperimentis800mm×140mm×100mm.Theflowrateofcoolantinthechamberwasoptimizedsothattheambienttemperatureofthesampleswaskeptconstantduringthemeasurement.
4.Resultsanddiscussions
Fig.2istheforwardvoltageversustemperatureplotobtainedfromtheLEDarraywithoutheatpipe.ThelinearitybetweenthevoltagedropandtemperatureistheKfactor.TheKfactorofthearrayis0.01969V/◦Catthesensorcurrentof20mA.
TheKfactorofLEDarrayissixtimesofoneLED,becausetherearesixLEDsconnectedinseries.Fig.3representsthederivativeofthermalcapacitanceasafunctionofthermalresistancefortheLEDarraywithoutheatpipeunderseveralconvectionconditions.Thermalcapacitancevariesdirectlywithbothspecificheatandmass;itisthequantityofheatabsorbedbythesamplewhenitstemperaturerises1◦C.Thepeaksimplythematerialtransitionsintheheatflowpath.
Thetotalthermalresistanceofarrayisfoundtodecreasewiththevelocityofairflow.Thethermalresistanceisabout6.8◦C/Watthenaturalconvectionstateandabout2.8◦C/Watanairvelocityof7m/s.Atanairvelocityabove2m/s,thethermalresistanceofarraydropsrapidly.Itisworthwhilecon-sideringthedifferenceinthermalresistancebetweenthe
unitFig.2.Forwardvoltagevs.temperatureplotshowingaKfactorofLEDarray.
L.Kimetal./ThermochimicaActa455(200721–25
23
Fig.3.DifferentialstructurefunctionsofLEDarrayasafunctionofconvection
condition(withoutheatpipes.
LEDpackage(8◦C/W[11]andthevalueof1.8◦C/Wwhichwasmeasuredinthisexperiment.ThedeviationcanbewellexplainedbyconsideringanequivalentthermalcircuitofLEDarrayaswasdescribedinFig.4.
BecauseMCPCBisone,thusthetemperatureoftheMCPCBisassumedtobethesame.ThetemperaturedifferencebetweenthejunctionandtheambientisexpressedforeachLEDchipasTJi,a=TJi,MCPCB+TMCPCB,a
=Piθi+6
k=1
Pkθ0=Piθi+Ptotalθ0
(5
whereTJi,aisthetemperaturedifferencebetweentheithchipandambient,TJi,MCPCBthetemperaturedifferencebetweentheithchipandtheslug,TMCPCB,athetemperaturedifferencebetweentheMCPCBandtheambient,Pitheinputpoweroftheithchip,θithepartialthermalresistancebetweentheithchipandtheMCPCB,andθ0isthepartialthermalresistancebetweentheMCPCBandambient.Becausetheheatisgeneratedat
the
Fig.4.EquivalentcircuitofLEDarraysinvestigatedinthisstudy.
junctionandflowstotheenvironmentthroughtheMCPCB,PtotalisthesumofPi.DefiningTJ,avg=Tj,avg−Jamb,letusassumethatTJ,avg=TJi,a(i=1–6.ApplyingEq.(2intoEq.(5,leadstothetotalthermalresistanceasfollows:
θja-avg=
TJi,aPtotal=Piθi+Ptotalθ0
Ptotal(6ApplyingEq.(6intotheLEDarraysystemwithsixLEDpack-ages,itissimplifiedasθja-avg=
(1/6Ptotalθi+Ptotalθ0total=1