IGBTgate and thiswill arriveat values higher than the nominal gate voltage provided by
the driver unit. Due to the transconductance of the IGBT the rise of the gate voltage is
automatically followed by an increase of the collector current (as long as this is not
limited by external measures). This dependency is described in the datasheet as the
transfer characteristic. This is usually only depicted up to a valueof twice I
Example of a datasheet excerpt detailing the transfer characteristics I
of a 450A IGBTmodule
To limit the short circuit current then, it is important to limit the maximum IGBT gate
voltage. To do this, thereare variousways of approach:
The use of a low gate resistor R
, so that the current I
can only build up to a
low voltage across this resistor which superimposes the nominal gate voltage.
The use of an additional external gate-emitter capacitor. The current I
then to charge a larger total gate capacitance. This benefit is limited by
possibly existing internal gate resistors of the IGBT module which will partly
decouple theexternal capacitor from the internal capacitance.
Clamping of the gate voltage to emitter potential with fast Z-diodes. Given the
tolerances and the temperature drift of the diodes used, values of approx. 2V
to 3V above the nominal gate voltage canbe realised.
Clamping of the gate voltage to the positive supply voltage of the driver with
fast pn-diodes or Schottky diodes. Depending on the tolerance of the power
supply and the internal voltage drop across the driver unit, values of approx.
0.3V to 3V above the nominal gate voltage can be realised. When using
Schottky diodes, care should be taken that in conditions of high ambient
temperatures diodes with a low leakage current are selected. Otherwise, at
high junction temperatures, high leakage currents will occur and these will
intensely strain the driver supply.
Reduce the nominal gate voltage to a value of around 14V.With one or several
of the measures mentioned above, lower maximum gate voltages can be
achieved in case of a short circuit. On the downside, rising conduction losses
of the IGBTare to be considered.
Turning the IGBT on via the emitter whilst the gate is held at the ground
potential of the driver (chapter