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Re: Recent s.s.t.c work



Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

Steve (& all)-

Yes, it's a little slow; and at 100 KHz "a little" is too much. I measure about 1 us between the fall at one IGBT gate (crossing 0) and the beginning of turn-on at its opposite. The way I have the drives configured now, the gating transistor doesn't start to turn on at all until the driving wave goes above the IGBT's source-voltage level. I'm going to change that so that that turn-on starts as soon as the driving wave begins to rise from the -25 or so. That will give more drive to the gating transistor by the time the driving wave gets above the source-ref. level, & hopefully not cause overlap.

All the drivers are working OK; all the same. And I have currently only 1.1 ohms in series with the gates--& will try getting rid of that once I improve the delay.

KCH

Tesla list wrote:
Original poster: Steve Ward <mailto:steve.ward@xxxxxxxxx><steve.ward@xxxxxxxxx>

Ken, what is the impedance of your gate driver? I have never seen waveforms that slow and with that peculiar rising edge. The falling edge looks normal. Is your gate drive transistor (the one that pulls positive) falling out of saturation perhaps??? The turn on is usually faster than turn off...

The really odd thing is that your gate voltage slows down before you even hit the 0V line!!! That is *not* right. Have you checked each driver? Maybe this one driver is damaged and also causing your imbalance in primary current from hafl-cycle to half-cycle?

Good luck... in any case, i think that those large turn on delays are fix-able. My gate drivers can supply LOTS of current, as the mosfets i use in it onl have a 55m ohm Rds. I dont use any resistance on the IGBT gates, so i switch at the maximum speed possible.

Steve