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APPLICATIONINFORMATION
PowerDissipation
V
Sn
I
Sn
(1)
(
V
Ln
I
Ln
)
(2)
T
J
T
一个
P
D
JA
(3)
T
J
T
一个
P
D
ja(s)
(4)
ja(s)
JC
CA
JB
BA
JC
CA
JB
BA
(5)
40
60
80
100
120
140
0 100 200 300 400 500
热的 阻抗 − c/w
d, low−k
DW, low−k
DW, high−k
d, high−k
tb5r1,tb5r2
SLLS588B–NOVEMBER2003–REVISEDMAY2004
Notethat
θ
JA
ishighlydependentonthePCBon
whichthedeviceismountedandontheairflowover
thepowerdissipationrating,oftenlistedasthe
thedeviceandpcb.电子元件工业联合会/eiahasdefined
packagedissipationrating,isafunctionoftheambi-
standardizedtestconditionsformeasuring
θ
JA
.二
enttemperature,t
一个
,andtheairflowaroundthe
commonlyusedconditionsarethelow-kandthe
设备.thisratingcorrelateswiththedevice'smaxi-
高-kboards,coveredbyeia/jesd51-3and
mumjunctiontemperature,sometimeslistedinthe
eia/jesd51-7respectively.figure10showsthe
absolutemaximumratingstables.themaximum
低-kandhigh-kvaluesof
θ
JA
versusairflowforthis
junctiontemperatureaccountsfortheprocessesand
deviceanditspackageoptions.
materialsusedtofabricateandpackagethedevice,
inadditiontothedesiredlifeexpectancy.
Thestandardized
θ
JA
valuesmaynotaccurately
representtheconditionsunderwhichthedeviceis
Therearetwocommonapproachestoestimatingthe
使用.thiscanbeduetoadjacentdevicesactingas
internaldiejunctiontemperature,t
J
.inbothofthese
heatsourcesorheatsinks,tononuniformairflow,或者
方法,thedeviceinternalpowerdissipationp
D
tothesystempcbhavingsignificantlydifferentther-
needstobecalculatedThisisdonebytotalingthe
malcharacteristicsthanthestandardizedtestpcbs.
supplypower(s)toarriveatthesystempower
Thesecondmethodofsystemthermalanalysisis
dissispation:
moreaccurate.thiscalculationusesthepower
dissipationandambienttemperature,alongwithtwo
deviceandtwosystem-levelparameters:
andthensubtractingthetotalpowerdissipationofthe
•θ
JC
,thejunction-至-casethermalresistance,在
externalload(s):
degreesCelsiusperwatt
•θ
JB
,thejunction-至-boardthermalresistance,在
degreesCelsiusperwatt
ThefirstT
J
calculationusesthepowerdissipation
•θ
CA
,thecase-至-ambientthermalresistance,在
andambienttemperature,alongwithoneparameter:
degreesCelsiusperwatt
θ
JA
,thejunction-至-ambientthermalresistance,在
•θ
BA
,theboard-至-ambientthermalresistance,在
degreescelsiusperwatt.
degreescelsiusperwatt.
TheproductofP
D
和
θ
JA
isthejunctiontemperature
inthisanalysis,therearetwoparallelpaths,一个
riseabovetheambienttemperature.因此:
throughthecase(包装)totheambient,和
anotherthroughthedevicetothepcbtotheambi-
ent.thesystem-leveljunction-至-ambientthermalim-
pedance,
θ
ja(s)
,istheequivalentparallelimpedance
ofthetwoparallelpaths:
在哪里
Thedeviceparameters
θ
JC
和
θ
JB
accountforthe
internalstructureofthedevice.thesystem-水平的
参数
θ
CA
和
θ
BA
takeintoaccountdetailsof
thepcbconstruction,adjacentelectricalandmech-
anicalcomponents,andtheenvironmentalconditions
includingairflow.finiteelement(fe),finitedifference
(fd),orcomputationalfluiddynamics(cfd)pro-
gramscandetermine
θ
CA
和
θ
BA
.detailsonusing
theseprogramsarebeyondthescopeofthisdata
薄板,butareavailablefromthesoftwaremanufac-
figure10.thermalimpedancevsairflow
turers.
8
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