measurement, first the driver was connected to the IGBT gate with a twisted cable of
approx. 6cm and then in the next test with 18cm length. Thedifference in turn-on energy
for different gate lead inductances L
The faster turn-on of the IGBT causes a faster commutation of the current off the
associated freewheeling diode with the appendant increase of reverse recovery losses.
If the current commutation happens too fast it may possibly come to a snap-off of the
diode current and theassociated oscillations or even the destruction of the diode.
Oscillations and the exceedance of the diode SOA limits due to high stray
inductance in the gatepathof the IGBT turning on (1.7kV IGBTmodule)
Within the application, it is therefore advisable to minimise the gate lead inductance.
The best way to realise that is to mount the driver or at least the output stage of the
driver directly onto the module. In this way, not only will the gate lead inductances be
minimised but also all IGBTs in the application will have the same value. If the driver
stage connects via a twisted pair of wires, however, the risk is that different lead lengths
will be used (due to different distances of the driver to the gates). Another aspect of
different gate lead inductances arises when IGBTs are connected in parallel. Here
especially, a non-symmetrical construction causes significant differences in the dynamic
current sharing, which in theworst case, can lead to the overload of one IGBT and thus
the entire system.