RE: OL-DRSSTC 7 - It's Alive!

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Jason,

At 05:45 PM 10/9/2005, you wrote:
> Hi Terry, thanks for sharing your latest work.
>
> Do you think the gate switch glitches will become a problem at higher
> power levels ?

No!  I think higher power will switch them far better!!!  Right now, 
the gate drive signal is "weak" because the primary current is 
weak...  It is sort of cool in that as the current goes up and gets 
more critical, so follows the gate drive currents...  Of course, 
right now I am "playing" with weak signals because I really have no 
idea what to expect and a big blowup will set me back a week...  Just 
trying to get it to work "at all" without doing any damage.  LOTs of 
safety backups are limiting thing right now ;-))

But the IGBT gate drive current follows the primary loop current 
"period"!!  "Nothing can go wrong" :o))  The CTs also are rather band 
limited to "reject glitches".  A whole "odd" idea right there...

>  I was seeing the same thing when I was trying to run my
> DRSSTC with primary feedback. At higher power levels the glitches seemed
> to be turning the IGBTs back off for about 100ns. The coil worked fine
> up to about 2' sparks, but then the IGBTs started to fail. I had thought
> the glitches were the problem, but it now seems that late switching was
> making the IGBTs swich at more than their 205amp (IRG4PG50WD) rating and
> causing latch up.

Were you using gate drive resistors and all??  The IRG4PG50WD can 
take 750 amps!!  But if the gate voltage is being driven high by 
"shoot through" (Miller capacitance from drain to gate is driving the 
gate voltage too high and blowing out that gate capacitor), they will 
blow real fast...  Maybe clamp the gates and slow the dV/dT...

I don't know for sure, but your problem sounds like it needs TVS 
right across the gates and gate drive resistors to slow the switching 
down a little...

Cheers,

        Terry



> Jason.