IGBT Modules - Technologies, Driver and Application (Second Edition) - page 458

Theoretically, it is possible to determine the junction temperature in inhomogeneous
but this requires more extensive calculationmethods that also have to take
thermal capacities into account.
To minimise the online calculation work, the expected operating scenarios and the
junction temperature resulting from them, and the baseplate temperature and heatsink
temperature, are calculated offline. The maximum permissible baseplate or heatsink
temperature is derived from this calculation andmust not be exceeded during operation.
Thismakes it unnecessary to calculate the junction temperature while the application is
operating, which can be a lengthy process. At the same time, the system is still
protected fromoverheating.
A direct option for measuring the junction temperature during operation is made
possible by using IGBTs that have an integrated temperature sensor
. The
expected precision is usually around 10%. The modules currently available have one
temperature sensor for the IGBTs but not for the diodes. Thismeans that measurement
of the diode temperature, for example during regenerative operation of a motor, is
limited and has to be achieved indirectly. Furthermore, the read-out electronics of the
on-chip sensors should have an isolation sector, as the on-chip sensors only have
functional isolation against the load circuit and there needs to be the same number of
read-out electronics as the number of IGBT chips. The high costs involved often do not
justify the use of on-chip sensors and are substituted by cheaper alternatives, such as
NTC resistors.
Infrared cameras provide another method of measuring the temperature directly for
experimental operation during the development phase. These camerasmake it possible
to observe both static and dynamic operation but have the disadvantage of taking a lot
of time to set up. The silicone of IGBT module has to be removed, which would
otherwise prevent a clear image of the infrared distribution. Afterwards a uniform colour,
usually black, has to be applied to the area observed so that the emission rate is
identical at all locations and does not cause faulty measurements. At critical field
strengths, there may be electric discharge or the module may even be destroyed,
because of the lack of silicone. It may be necessary to operate the motor at reduced
DC-bus voltage.
The emission rate is a normalised value that states the relationship between a body and
a black body in terms of its ability to emit infrared radiation. For correct temperature
measurement, the infrared cameramust be set to the emission rate of the object being
studied. Manuals for infrared cameras usually contain instructions for determining and
setting the emission rate. Theemission rates of selectedmaterials are given i
For example, inhomogeneous load is operation of amotor at 0Hz but full torque, inwhich, usually, only three of
the six IGBTs operate ina full-bridge.
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