Re: OLTC Thoughts

["Tesla list" <tesla-at-pupman-dot-com>]

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

Hi Richard,

At 11:16 AM 8/11/2002 -0400, you wrote:
>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?  

900 volts.  They make higher but these are really cheap.  I ran a bunch of
models studying what might happen to the voltage and where they run at and
they should be very safe.  I am also putting in some big MOVs to catch any
over voltage swings at 750V.  here is a pic of a transient and how the
coils handles it:

http://hot-streamer-dot-com/temp/OLTC-08-07-2.gif

>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. 

The IGBTs have big powerful anti-parallel diodes in them already ;-))

>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.

They are fairly well outside it now.  I have to watch out for induced
currents from the primary on control wiring.

Cheers,

	Terry

>
>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
>
>
>