IGBT Modules - Technologies, Driver and Application (Second Edition) - page 279

is to be observed, however. An IGBT in a cold state switches with a higher
. If only
small currents are switched, the
increases yet again and the intervention of the
DVRC increases, because the higher
increases the resulting current through C
and therefore the voltage generated acrossR
Fig. 6.61
Comparison of turn-off losses of a 2.4kA/1.7kV IGBT module operated with
andwithout DVRC as a function of the appliedDC-bus voltage DynamicActiveClamping
Under certain circumstances, Active Clamping may act too late, i.e. a retroactive effect
on the gate does not occur until the gate voltage has already fallen below the value of
the threshold voltage U
. This phenomenon is particularly apparent when high
power modules are used and can lead to the destruction of themodule if the intention is
to turn off high currents and the high power over-voltages associated with them.
shows a fault of this type.
The cause of the fault is that the MOS-channel of the IGBT is already closed, although
there is still a considerable amount of load current flowing through the IGBT. From the
time that theMOS-channel prevents any further flow of electrons, the load can only flow
as a hole current. The idea behind Dynamic Active Clamping is simply to keep the
MOS-channel open until the load current decreases in amplitude, thereby permitting an
electronandhole current
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