Specifications

37
Dec. 2012
Mitsubishi IPM-series Application Note
㧵㧼㧹 㧵㧼㧹
10. Power Loss and Junction Temperature
Junction temperature can be used as an indication of IGBT module situation. This section will discuss how to calculate junction
temperature and give an example based on waveform shown in Fig.10.1. Here, only power loss of IGBT part is given. The power
loss of Diode can be obtained by using the same method as IGBT part. Moreover, junction temperature must never be outside of
the maximum allowable value. It also has impact on the power cycle life.
Fig.10.1
a㧚Power Loss
In order to estimate junction temperature for thermal design, it is necessary to compute total power loss. The first step is the
calculation of power loss per pulse.
Two most important sources of power dissipation that must be considered are conduction losses and switching losses. (Fig.10.2)
(1) Conduction Losses
The total power dissipation during conduction is
computed by multiplying the on-state saturation voltage by
the on-state current.
1w
2)sat(CE2C1)sat(CE1C
)sat( t
2
VIVI
E ×
×+×
=
(J)
Note)The above equation is a simplification of the below one
VCE(sat) VS㧚Ic characteristics at Tj=125°C is used in power
loss calculation.
Fig.10.2
(2) Switching Losses
The most accurate method of determining switching
losses is to plot the Ic and VCE waveforms during the
switching transition. Multiply the waveforms point by point
to get an instantaneous power waveform. The area under
the power waveform is the switching energy expressed in
watt-seconds/pulse or J/pulse.
n: number of partitions
(divide interval between ta and tb equally into n parts,
compute average power loss for each interval.)
Calculation of Eoff has the same method.
The total power loss of one pulse is the sum of (1) and (2).
offon)sat(1 EEEE ++=
t
a
t
b
Fig.10.3
Power Loss and Junction Temperature