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Re: Terry's DRSSTC - Odd IGBT damage??
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- Subject: Re: Terry's DRSSTC - Odd IGBT damage??
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- Date: Thu, 03 Feb 2005 08:08:18 -0700
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Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>
These days, the voltage waveforms (blue) from my DRSSTC look like this:
The used to look like this:
Ever since I tried running with the deadtimes removed on the IGBTs, the
corners of the voltage waveforms have gotten very noisy and the ~400nS flat
portions of the waveform have gone away. I put the dead time back in but
"things have changed". I have checked and rechecked all the drive
electronics and the dead times still seem to be there even if one really
looks close even at the output waveform (the current blips due to lack of
current drive). So the corners are noisy and the turnoff is slow and messy
now. I am also getting spikes that go well over the rail voltages (like
100V!) which should not be possible. The IGBTs seem to have lost their
"warp"... I double checked all the test equipment and probes too.
I am thinking the anti-parallel diodes have gone away. Strange since one
would think the IGBTs themselves would get beat up from tiny dead times and
fast switching rather than the separate diodes....
So I am going to switch out the set of IGBTs and see if it all returns to
normal. If so, I will have the die checked to find what blew up.
Interesting that the thing still runs fine from an overall perspective. If
the diodes are gone, it is just pulling down the voltage with the opposite
IGBT now instead of doing it the nice way.
So for the smaller high speed co-pack IGBTs, running with very low dead
time my not be a good idea. I also have just 5.1 ohm gate resistors in
there but that does not seem to be a problem according to the
literature.... I think I just have to get the IGBTs out and start testing
them specifically to pinpoint what is going on.
So I "think" I blew something up!! But it still "runs" fine :o)) Hard to
say but I think the noise my be getting a little worse with time which is
probably to be expected...
I just played with the coil at low power last night and there are all kinds
of mysteries there to be played with ;-)) The very first burst seems to
really "make the sparks", Tuning and all that mode stuff have a pretty big
effect too... I wonder about our tuning of the primary to a lower
frequency in regular coils. Perhaps is not so much a streamer loading
compensation but rather forcing the coil into the lower mode where things
seem to run better... It does seem to help the DRSSTC, but those effects
should be the same in any coil!
Much to ponder...