IGBT Modules - Technologies, Driver and Application (Second Edition) - page 156

shows the influence of different levels of DC-bus voltage on the tail current
behaviour at the same temperature. The IGBT used for this is a Trench-FS. The drift
zone is correspondingly thin and at high DC-bus voltages the space-charge layer
saturates almost the entire region. Almost no carriers that could contribute to the tail
current remain through this.When theDC-bus voltage is reduced then also the extent of
the space-charge layer in the IGBT reduces. Hencemore carriers remain which lead to
amore significant build-upof the tail current.
Fig. 3.28
Dependency of the IGBT tail current I
on theDC-bus voltageU
With high DC-bus voltages and the use of FS or Trench-FS IGBT it may lead to the
space-charge layer extending into the field-stop region. The result is that virtually no
carriers remain for the tail current and the collector current decays rapidly. If this decay
happens too fast the IGBT behaves snappy, i.e. the collector current snaps off. This
sometimes causes strong oscillations which may lead to EMI problems in the
application. With applications in the low power sector this is often of less importance
since a lowor no tail current causes less turn-off losses here and this has priority above
the possible (relatively low) oscillations in the system. This is quite different in high
power applications. Oscillations here may not only cause significant interference but
also contribute considerably to the turn-off losses. For this reason various
manufacturers offer IGBTs which show so-called soft turn-off behaviour. Also it is
possible to adjust such soft turn-off behaviour by measures in the application. For
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