OLTC Thoughts

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Original poster: "Wall Richard Wayne by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rwall-at-ix-dot-netcom-dot-com>

Terry,

Here are some other OLTC design thoughts.

Your TC primary, 47 mF cap(s) and IGBT switch(s) form a large LC tank. 
Once fired this thank will have huge oscillatory currents.  What are the
reverse stand off voltages of the IGBTs?  Hopefully they have a built in
protection diode.  But, I would not depend on this small diode, if it
exists, for protection.  Which brings up another point.  Richard Hull's H2
thyratron magnifier, by necessity, quenched at the first zero crossing of
the wave form.  So only the positive half of the energy cycle was passed to
the primary.  While a very interesting design, it is not a spectacular
performer, if one measures performance by spark length.  It's a phenomenal
research device though.  You may consider adding free running back diodes
across your IGBTs for protection and for a full one cycle energy transfer. 
Have you calculated the resonate oscillatory frequency of the primary tank
without the switches?  Also consider moving the switch(s) outside the LC
tank altogether to allow the tank to oscillate freely and transfer even
more energy before you electronically quench it.  I'll leave these design
thoughts up to your fertile electronics imagination.

RWW

> Hi All,
> 
> To update the IGBT current sharing in the OLTC...
> 
> We have say 2000 amps and ten 200 amp IGBTs to switch it.  There is a
> problem in how to be sure the IGBTs share that current equally.  If one
> IGBT takes like 400 amps, it is a "bad" thing!  The circuit looks like
this:
> 
> http://hot-streamer-dot-com/temp/OLTC8-10-01.gif
> 
> Basically, you slam all those IGBTs on with very low inductance, impedance
> balanced "buss work".  Done well, it is simple and easy, once some nasty
> details are worked out.  I have put a lot of thought about how I would do
> this for the OLTC.  Lots of worry over the 10nH wire bonds inside the
IGBTs
> and stuff...  However, I guess that was all wasted now since the solution
> is...:
> 
> http://hot-streamer-dot-com/temp/OLTC8-10-02.gif
> 
> Yep!  We have ten independent primary caps too.  If we hook them up like
> this, the cap's impedance naturally hard balances the current to the
IGBTs.
>  It is just a done deal, "painfully" simple... nothing can go wrong.  It's
> gotta work.  Even if just one IGBT turns on I "think" it self protects...
> If one stays out, it self protects...  (the frequency is wrong, but who
> cares...)  
> 
> This solution to current sharing of the OLTC's IGBTs is so beautiful, it
> makes me wanna cry...
> 
> Cheers,
> 
> 	Terry
> 
> 



--- Richard Wayne Wall
--- rwall-at-ix-dot-netcom-dot-com