SC peak current by means of clamping the gate voltage to values below 15.3 V has to be guaranteed.
Measures to limit the gate voltage and SC current respectively are:
1) Clamping of the gate voltage to a fixed potential If the gate driver can supply a stable, internal 15V, the gate voltage should be clamped to this potential by a feedback diode with a low voltage
drop (Schottky diode). The lower the inductive connection between the 15V supply and gate terminals, the more effective this measure will be.
2) Clamping of the gate voltage by gate-emitter Zener diodes
The closer the diodes are mounted to the auxiliary terminals of the module, the more effective this measure. Diodes with low temperature drift and tolerance (e.g. transient voltage suppressor diodes) should be used.
In accordance with the above explanations, the following condition has to be fulfilled at room temperature:
At a chosen gate voltage of 14.0V the turn-on resistor Rgon can be reduced to 2.7 Ohm. This can keep the switching losses at the value given in the data sheet for 15V. The reduction of the gate voltage has a negligible influence on the turn-off losses.
3) Introduction of a negative emitter feedback
For this measure the emitter of the gate-driver must not be – as is customary – connected to the auxiliary emitter, but to the main emitter terminals. The induced voltage drop inside the module is superimposed to the externally applied gate voltage. Due to the typically high short circuit di/dts, the negative feedback leads to an effective reduction of the gate voltage. In addition, an overshoot of the gate voltage can be avoided. The impact of this measure is shown in the following picture: