IGBT Modules - Technologies, Driver and Application (Second Edition) - page 113

well as the distribution of the U
(IGBT) and/or U
(diode) values and their
temperature dependencies.
Fig. 2.42
Exampleof parallel connection of three IGBTs anddiode chips per switch
IGBTs, except for PT IGBTs, have a positive temperature coefficient even at low
currents, while diodes have a negative onewhichmay change to a positive temperature
coefficient with higher currents ("low" or "high" refers to the rated currents as per the
manufacturer's datasheet). Generally, a positive temperature coefficient makes
connection in parallel easier, as the system regulates itself. If one semiconductor
conducts a higher current than the others connected to it in parallel, it will heat upmore
than the others. Its inner resistance rises (expressed by the increase in voltage U
and/or U
). This means that the current in this semiconductor reduces again and
eventually a dynamic balance is established for the current sharing in the
semiconductors connected in parallel. For semiconductors with a negative temperature
coefficient, therefore, there is always a risk that a semiconductor within the parallel
connection will be overloaded by current mismatching if the thermal coupling between
the individual chips is not satisfactory. This thermal coupling is given in IGBT modules
because of the proximity of the relevant DCB or baseplate, so both the IGBTs and the
diodes can be connected in parallel inmoduleswithout undesirable current mismatching
The influence of the bondingmaterials on the overall behaviour at different temperature
levels of the system is negligible, i.e. the resistance values and therefore the
temperature dependence of copper and aluminium are negligible in comparison to the
IGBT or diode resistance. Electrical bonding technology therefore has only a secondary
influence on static current sharing in IGBTmodules.
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