OLTC update - Primary inductor voltage spike

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Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi All,

Tonight I have been working on a current glitch problem.  Since I can
quench the IGBT array spark gap at any time.  I can pick some really "bad"
times.  In this circuit:

http://hot-streamer-dot-com/temp/OLTC08-21-01.gif

Consider the instant in time of the primary circuit ringdown when the
voltage on the cap is zero but the current is say 2000 amps through the
inductor.  If we open the switch right then, the inductive kick off the
primary will cause a voltage spike.  

http://hot-streamer-dot-com/temp/OLTC08-21-02.gif

http://hot-streamer-dot-com/temp/OLTC08-21-03.gif

No problem in the negative direction since the anti-parallel diode will
catch it (which it does perfectly).  However, in the positive direction,
there is nothing to stop the voltage spike except the IGBT breakdown and
the MOVs.  But the MOVs are too slow :-(

But I think TVS transient absorbers will work.  I stung some odd valued
ones together and they seem to hit the spike very hard.

http://hot-streamer-dot-com/temp/OLTC08-21-04.gif

http://hot-streamer-dot-com/temp/OLTC08-21-05.gif

So I'll order up the right values from DigiKey and that should take care of
it.  The energy of the spike is not very great so should work fine.

"Normally" this is not a big problem, but the coil should be able to handle
any adjustment of the gap dwell time without worry.

I ran a bunch of models and tests looking for such odd glitches and turn
on/off transients but everything else looks perfectly fine.  So the primary
circuit appears to be all done and working.  

I am going to run it off 120VAC at first with a 20 ohm power resitor in the
charging circuit.  This will limit the IGBT current to a nice safe level.
I seem to have far more power available than I know what to do with ;-)  I
worked so hard getting power into the coil...  Now I am try to find ways of
getting rid of power :o))

Cheers,

	Terry