Simplified simulationof voltage transients at the IGBTduring the zero crossing
of thephase current
To illustrate the phenomenon of inadvertent tripping of the U
shows a simplified simulationof the switching actions for this type of event, which is also
known as passive turning on.
rovides another example.
monitoringwith capacitive loads
If power electronics circuits are operated on a capacitive load, there are high charge
reversal currents which (depending of their time constants and the characteristics of the
power electronics switch) can cause desaturation of the IGBTs. This will be explained
belowusing cable capacitanceas an exampleof the problem.
A known cause of U
tripping is desaturation of an IGBT resulting from the charge
reversal of a sufficiently large capacitance. This occurs when the semiconductor is
turned on, for example onto a screenedmotor cable. Depending on the IGBT, the turn-
on current can be many times the semiconductor rated current. The longer the cable,
the greater the cable capacitance. Screened cables have a higher cable capacitance
than unscreened cables. Particularly in IGBTs with low rated currents, desaturation is
possible in screened cables of only a fewmetres in length. If this is not covered by a
long enough blanking time t
in the U
monitoring, a fault trip occurs inadvertently. A
filter (mainly an inductor) directly at the output of the inverter solves the problem