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Old 10-27-2011, 09:22 AM   #612 (permalink)
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Originally Posted by CollinK View Post
Another thing I found is that I might have a bit too small of a gate resistor. I'm using 3 ohm resistors on 200A IGBTs. It seems like maybe that's a little on the small side. As such my di/dt is kind of fast and that doesn't help either. I'm thinking it might be safer to go with 4-6 ohm resistors. Any suggestions there?
The only way to answer your question is to look the gate signal with scope to see what happen exactly.
If your gate resistor is too small, igbt turn OFF can be too fast and provoke spike, but also igbt turn ON can be too fast and you will have a high current spike (and then voltage spike and ringing) because of the reverse recovery time of the opposed igbt's diode.
On the other hand if you put a higher gate resistor, you can easily reach autoconduction threshold when the opposite igbt turn ON, because of the high dv/dt on the turned OFF igbt.

The only way to adjust the gate resistor is to carefully look the gate signal, to see the turn ON time, and verify that the gate voltage don't reach the commutation threshold when the opposite igbt turn ON. When igbt are not controlled with negative gate voltage the 4-5V threshold can be reached easily with high voltage bus (and thus hight dv/dt), even with pretty low gate resistor.
With high voltage Vbus it's often mandatory to have different value of turn on and turn off gate resistor with the help of a schottky diode if you don't use negative voltage for igbt turn off.
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