The effect of gate emitter capacitance on oscillation in short circuit situation

[saeedmns68]

In the attached document the writer says:"the capacitor between gate and emitter of an IGBT(we put this not the internal gate emitter capacitor) reduces oscillation in the short circuit situation".how can this capacitor do it?how can we calculate the capacitance?


View attachment connecting IGBT driver to IGBT.pdf

 

[treez]

it tells you in that doc how to calculate it....what you are trying to do is just make it big enough to be a decent filter (in conjunction with the series resistor), but without making the turn-on of the igbt too slow. It depends to an extent on the switching frequency how big to make the capacitor. Of course, in addition to that RC filter you should make the connections to the igbt gate go as a close pair, and reduce internal current loop area as much as possible.

 

[saeedmns68]

it tells you in that doc how to calculate it....what you are trying to do is just make it big enough to be a decent filter (in conjunction with the series resistor), but without making the turn-on of the igbt too slow. It depends to an extent on the switching frequency how big to make the capacitor. Of course, in addition to that RC filter you should make the connections to the igbt gate go as a close pair, and reduce internal current loop area as much as possible.

Thanks treez I want to know that in the short circuit situation how does this capacitor make the oscillation low??

 

[treez]

Well the shrt cct in the igbt will not be for long otherwise it would blow, so the short circuit , when it happens, acts like a kind of step input to the intrinsic junction capacitances of the igbt, and as the diagram shows, there are trace inductances in there which can form a resonant circuit with the igbt capacitances, so its nice to put the cap in there so as to prevent really high frequency oscillation which switches the igbt gate on and off really quickly and cause too much switching loss….so you are slowing that process down basically, and reducing subsequent switching loss, and the RC filter is damping it out too.

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Its one of the niceities of igbts that they can switch off a short cct current and still survive, but as your doc tells, you still need the RC filter to damp out the violence of the sudden short circuit, and indeed the violence of the sudden switch of of the short cct by the igbt……as you know, shrt cct is pretty drastic..not surprising the onset of it can involve massive amounts of bad-boy ringing

 

[FvM]

External Cge is a usual means for high voltage IGBTs (4500 V class) to improve switching behaviour. For 1200 and 1700 V, it's not generally helpful and not usually suggested as far as I'm aware of.

For the "oscillation in short circuit situation" point, we have to look for the overcurrent shutdown behavior of the specific driver. I doubt that the suggestion in the Semikron application note is generally valid.