Specifications

40
Dec. 2012
Mitsubishi IPM-series Application Note
(3) tw2>1s
In a similar way to (2), temperature change of heatsink should be taken into consideration as well. It is necessary to know
the transient heat impedance of the heatsink. (Fig.10.9)
Fig.10.9
Similarly, the temperature difference between junction and ambient can be calculated by using the following formula.
ٌT(j-a)㧩R
th(j-a)
P
AV
㧙Z
th(j-a)(tw2)
P
AV
㧗Z
th(j-a)(tw2)
Pav㧙Z
th(j-a)(tw1)
Pav㧗Z
th(j-a)(tw1)
P1
㧩R
th(j-a)
P
AV
㧗(P
av
㧙P
AV
)Z
th(j-c)(tw2)
㧗(P1㧙Pav)Z
th(j-c)(tw1)
ѕTj㧩Ta㧗ٌT(j-a) (Ta is measured by a thermometer.)
c. Heatsink Selection
Fig.10.10 shows the thermal equivalent circuit when two
or more modules are mounted on one heat sink.
According to this equivalent circuit, the temperature of
the heat sink is
Tf㧩Ta㧗(P
T(AV)
㧗P
D(AV)
)NxR
th(f-a)
Ta㧦Ambient temperature
P
T(AV)
:Average power loss of IGBT
P
D(AV)
:Average power loss of FWDi
N:Arm number
R
th(f-a)
:The heatsink to ambient thermal resistance
The case temperature Tc is,
Tc㧩Tf㧗(P
T(AV)
㧗P
D(AV)
)R
th(c-f)
Rth(c-f)㧦The case to heatsink thermal resistance
Tc(max) can be calculated by using the below formula.
ѕT
c(max)
㧩Ta㧗(P
T(AV)
㧗P
D(AV)
)NxR
th(f-a)
㧗(P
T(AV)
㧗
P
D(AV)
)R
th(c-f)
Therefore, the heatsink to ambient thermal resistance
can be computed as
N)PP(
R)PP(TT
R
)AV(D)AV(T
)fc(th)AV(D)AV(Ta(max)C
)af(th
×+
×+−−
=
−
−
Moreover, power loss of FWDi should be considered
as well. In thermal design, the allowable case
temperature Tc(max) is up to the smaller one of IGBT
power loss and FWDi part.
Fig.10.10 Thermal Calculation Model
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Power Loss and Junction Temperature