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1、0082-05.fmPage279Wednesday,August23,200010:09AM5RadiationHeatTransferinElectronicEquipment5.1INTRODUCTIONRadiationcoolingofelectroniccomponentsandboxesisnotusuallyaconcerntothermalengineers.Thethreefactorsthataffectcoolingbyradiationarethetemper-aturedifferencebetweena
2、nobjectanditssurroundings,thesurfacecharacteristicsoftheobjectanditssurroundings,andtheviewthattheobjecthasofitssurroundings.Typically,thetemperaturedifferenceisthedeltabetweenthedevicecaseandanouterchassis,oranouterchassisandthewallsofaroom.Sinceradiationheattransferi
3、sbasedonthistemperaturedifference,whenthedeltabetweenacomponentanditssurroundingsbecomeshighenoughforradiationcoolingtomatter,thedeviceismostlikelyalreadyaboveitsmaximumjunctiontemperature.Atroomtempera-ture,thetotalemissivepowerofaperfectemitterisabout450W/m2.Thisisle
4、ssthan10%oftheheatthatconvectioninaircantransfer.Thecharacteristicsofthesurfacethatcoversthedeviceorthechassisinquestionisanotherimportantvariable.Materialsusedinelectronicsarenormallyopaquetoradia-tion.Therefore,thesurfacecharacteristicsareimportanttoadepthofonlyabout
5、2.5106mformetalsandabout0.5103mfornonmetals.Intherangeoftemperaturesusedforelectronics,thecolorofthesurfacedoesnotaffecttheradiationemittance.Thethirdvariableistheviewfactor.Thisisthefractionoftheradiationthatleavesonesurfaceandisinterceptedbyanothersurface.Thiscan
6、beashighas1.0forthecaseofasphereinsidealargersphere,orquitelow,suchastwoplatesatanangleapproaching180°.Algebraicequationscancalculatetheviewfactor,andthisisusuallydonewiththeaidofacomputer.Althoughradiationisnotusuallyaconcernwhentryingtocoolanelectronicpackage,thepack
7、agemayabsorbradiationheatbybeingnearahigh-temperaturesource.Suchacasemayoccurintheenginecompartmentofacar,whereelectronicmodulesareexposedtotheradiativeheatofhotenginecomponentsandexhaustmanifolds.Althoughthecolorofanobjectisnotimportantinradiativecooling,colorisimport
8、antwhentheobjectmayabsorbheatenergyfromawide-bandradiationsource.Thiscasemayoccurwhenweexposeanelectronicpackagetothe
