贮存/工作环境温度与试验温度之间的关系

—AmbientTemperature(Ta).Ambienttemperatureistheroom
temperatureoroutsideairtemperaturewhentheunitisoperatingunder
normalconditions3.GR-63-CORE,NEBSTMRequirements:Physical
Protection,specifiesaroomambientairtemperatureof23°C(73°F)
measured1.5m(59in)abovethefloorand400mm(15.8in)infrontofthe
equipment.A25°Cambienttemperaturemaybeusedastheairintaketothe
equipmentcontainingtheunit.Foroutdoornaturalconvectioncooled
equipment,ambienttemperatureisdefinedastheoutsideairtemperature
(includingtheeffectsofsolarloading)outsidetheunitbasedonthespecific
application.
—DeviceOperatingTemperature.Operatingtemperatureforadeviceisthe
temperatureinsidetheunitinwhichthedeviceresides.Thistemperatureis
definedasthetemperature0.5inchabovethesurfaceofthedevice.When
temperaturevariationsacrosstheboardarerelativelyconstant(asinforced
aircooledsystems),theaverageboardtemperaturemaybeused.Board
temperaturemaybedeterminedbytemperaturesensors,thermocouple
measurements,computersimulations,orInfrared(IR)imaging.When
temperaturesacrosstheboardvarysignificantly(asinconvectioncooled
equipment),devicecasetemperaturesmaybeusedforthemajorhighheat
dissipatingdevicesandtheaverageboardtemperatureforallotherdevices.
•UsageExample:Thenormalizeddeviceoperatingtemperatureof40°C
assumesa25°Cambienttemperatureplusa15°Caverageboard
temperatureriseduetolocalheatgeneration.
—CaseTemperatures(Tc).Casetemperatureistheexternaldevice
temperaturemeasuredoutsidethedevice’scase.Thecasetemperatureof
majorheatdissipatingdevicesmaybeusedtodeterminetheoperating
temperatureforthesedevices.
•UsageExample:Thermalmappingofapowertransistorshowsanaverage
casetemperatureof70°C,withanaverageboardtemperatureof50°Cfor
allotherdevices.
—JunctionTemperatures(Tj).JunctionTemperaturescannotbeused
directlywithTable9-1todetermineaTemperatureFactor,sinceTable9-1is
calibratedtodeviceoperatingtemperatureorcasetemperatureasdefined
above.Tjcanonlybeusedtodeterminefailureratewhenadevice
manufacturerprovidesaprovenJunctionTemperature-FailureRateModel.

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