电子设备热设计(第一章)Thermal Design of Electronic Equipment

HibbelerR.C.“Force-SystemResultantsandEquilibrium”ThermalDesignofElectronicEquipment.Ed.RalphRemsburgBocaRaton:CRCPressLLC,2001©2001byCRCPRESSLLC1IntroductiontoThermalDesignofElectronicEquipment1.1INTRODUCTIONTOTHEMODESOFHEATTRANSFERINELECTRONICEQUIPMENTElectronicdevicesproduceheatasaby-productofnormaloperation.Whenelectricalcurrentflowsthroughasemiconductororapassivedevice,aportionofthepowerisdissipatedasheatenergy.Besidesthedamagethatexcessheatcancause,italsoincreasesthemovementoffreeelectronsinasemiconductor,whichcancauseanincreaseinsignalnoise.Theprimaryfocusofthisbookistoexaminevariouswaystoreducethetemperatureofasemiconductor,orgroupofsemiconductors.Ifwedonotallowtheheattodissipate,thedevicejunctiontemperaturewillexceedthemaximumsafeoperatingtemperaturespecifiedbythemanufacturer.Whenadeviceexceedsthespecifiedtemperature,semiconductorperformance,life,andreliabilityaretremendouslyreduced,asshowninFigure1.1.Thebasicobjective,then,istoholdthejunctiontemperaturebelowthemaximumtemperaturespecifiedbythesemiconductormanufacturer.Naturetransfersheatinthreeways,convection,conduction,andradiation.Wewillexploretheseingreaterdetailinsubsequentchapters,butasimpledefinitionofeachisappropriateatthisstage.1.1.1CONVECTIONConvectionisacombinationofthebulktransportationandmixingofmacroscopicpartsofhotandcoldfluidelements,heatconductionwithinthecoolantmedia,andenergystorage.Convectioncanbeduetotheexpansionofthecoolantmediaincontactwiththedevice.Thisiscalledfreeconvection,ornaturalconvection.Con-vectioncanalsobeduetootherforces,suchasafanorpumpforcingthecoolantmediaintomotion.Thebasicrelationshipofconvectionfromahotobjecttoafluidcoolantpresumesalineardependenceonthetemperaturerisealongthesurfaceofthesolid,knownasNewtoniancooling.Therefore:qchcAsTsTm()0082-01Page1Wednesday,August23,20009:51AM©2001byCRCPRESSLLCwhere:qcconvectiveheatflowratefromthesurface(W)Assurfaceareaforheattransfer(m2)Tssurfacetemperature(°C)Tmcoolantmediatemperature(°C)hccoefficientofconvectiveheattransfer(W/m2)ThisequationisoftenrearrangedtosolveforT,bywhich:1.1.2CONDUCTIONConductionisthetransferofheatfromanareaofhighenergy(temperature)toanareaoflowerrelativeenergy.Conductionoccursbytheenergyofmotionbetweenadjacentmoleculesand,tovaryingdegrees,bythemovementoffreeelectronsandthevibrationoftheatomiclatticestructure.Intheconductivemodeofheattransferwehavenoappreciabledisplacementofthemolecules.Inmanyapplications,weuseconductiontodrawheatawayfromadevicesothatconvectioncancooltheconductivesurface,suchasinanair-cooledheatsink.Foraone-dimensionalsystem,FIGURE1.1ComponentfailurerateswithtemperatureforProgrammableArrayLogic(PAL),256KDynamicRandomAccessMemory(DRAM),andMicroprocessors.DatafromMIL-HDBK-217.0.01.02.03.04.05.06.02030405060708090100110120JunctionTemperature,oCFailureRateper106hPALMicroprocessorDRAMTqchcAs----------0082-01Page2Wednesday,August23,20009:51AM©2001byCRCPRESSLLCthefollowingrelationgovernsconductiveheattransfer:where:qheatflowrate(W)kthermalconductivityofthematerial(W/mK)Accross-sectionalareaforheattransfer(m2)Ttemperaturedifferential(°C)Llengthofheattransfer(m)Sinceheattransferbyconductionisdirectlyproportionaltoamaterial’sthermalconductivity,temperaturegradient,andcross-sectionalarea,wecanfindthetem-peratureriseinanapplicationby:1.1.3RADIATIONRadiationistheonlymodeofheattransferthatcanoccurthroughavacuumandisdependentonthetemperatureoftheradiatingsurface.Althoughresearchersdonotyetunderstandallofthephysicalmechanismsofradiativeheattransfer,itappearstobetheresultofelectromagneticwavesandphotonicmotion.Thequantityofheattrans-ferredbyradiationbetweentwobodieshavingtemperaturesofT1andT2isfoundbywhere:qramountofheattransferredbyradiation(W)emissivityoftheradiatingsurface(highlyreflective0,highlyabsorptive1.0)Stefan-Boltzmannconstant(5.67108W/m2K4)F1,2shapefactorbetweensurfaceareaofbody1andbody2(1.0)Asurfaceareaofradiation(m2)T1surfacetemperatureofbody1(K)T2surfacetemperatureofbody2(K)Unlessthetemperatureofthedeviceisextremelyhigh,orthedifferenceintemperaturesisextreme(suchasbetweenthesunandaspacecraft),radiationisusuallydisregardedasasignificantsourceofheattransfer.Todecidetheimportanceofradiationtotheoverallrateofheattransfer,wecandefinetheradiativeheatqkAcTL--------TqLkAc--------qrF1,2AT14T24()0082-01Page3Wednesday,August23,20009:51AM©2001byCRCPRESSLLCtransferasaradiativeheattransfercoefficient,hr:1.1.4PRACTICALTHERMALRESISTANCESThesemiconductorjunctiontemperaturedependsonthesumofthethermalresis-tancesbetweenthedevicejunctionandtheambientenvironment,whichistheultimateheatsink.Figure1.2showsasimplifiedviewoftheprimarythermalresistances:where: tottotalthermalresistance(K/W) jcjunctiontocasethermalresistance(K/W) cscasetoheatsinkthermalresistance(K/W) saheatsinktoambientthermalresistance(K/W)FIGURE1.2Primarythermalresistancesinachip/heatsinkassembly. jcisresistancefromthediejunctiontothedevicecase. csisresistancefromthedevicecasetotheheatsink. saisresistancefromtheheatsinktotheambientair.(AdaptedfromKraus,A.D.andBar-Co