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Re: Hybrid SG/SISG idea? "IGBT-Assist Spark Gap"?



Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Aaron,

At 02:26 PM 6/5/2006, you wrote:
Thanks, Terry!  Makes sense.  The illustration was
really only intended to suggest that the IGBTs would
be stacked with (effectively) a single gate.  I should
have realized the potential difference thing, though.
Curiously, has anybody done any IGBT "stacking" like
this for, e.g., a triggered gap?

See Dan's new post on this!

I'd be interested to
know what's required to avoid, e.g., overvolting IGBTs
if one "fires" before the others.  In the SISG, what
prevents this from happening?  Or is the turn-on time
considered "sufficiently-identical" (provided you're
using the same parts for all modules) to avoid this?

It is a very messy thing... The sections have significant capacitance across them that keeps the voltage from instantly jumping. The SIDACS turn on in the nS time frame but they don't start to "really conduct" till far latter. The primary circuit will not deliver current either for say 20nS and then the current rise is governed the LC...

If all the sections turn on but one, the voltage is still 900V there since there is no current in the string yet to drive it higher. As the current does start up, the dV/dT is easily slow enough for a lazy SIDAC string to get the message and start up too. I think the real key is to remember that at the instant the voltage across the gaps goes to zero, the current is also "zero". The dI/dT is pretty slow compared to what the SIDACs can react to.

That is pretty over simplified and SIDACS are designed with "tricks" to make stringing a "happy thing". There also "really is" current in the string to charge the caps to turn on the IGBTs. That can't happen unless they are "all" on.... You can't charge one without charging all... SIDACS are real tough too and could handle the first cycle all by themselves!! The IGBTs would be in danger after 1200V, but the SIDACSs are way too fast to let that happen. The IGBTs could ride the voltage far far longer than the SIDACS could sleep...

I avoid this issue since it is such a mess to explain... But it will not blow up ;-)) It might get to be messy if sync triggering or quenching circuits come into play...

Cheers,

        Terry




--- Tesla list <tesla@xxxxxxxxxx> wrote:

> Original poster: Vardan
> <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
>
> Hi,
>
> At 01:02 PM 6/5/2006, you wrote:
> >No comments?  In case there was any trouble
> >visualizing this (I doubt it, but...), here's a
> sketch
> >done using top-of-the-line schematic-drawing
> software:
> >mspaint.exe ;-)
> >
> >http://silicon-arcana.com/SG-SiSG.jpg
> >
> >Doubtless this sketch is **WAY** oversimplified!!
> It
> >probably doesn't include a lot of necessary evils
> >required to place IGBTs in series (may need a TVS
> per
> >IGBT, divider to equalize the voltage across the
> whole
> >stack, etc.--thoughts?).
>
> The gates of the IGBTs are at about 900V potential
> difference each up
> the stack.  They cannot be hooked together.  One of
> the earlier
> problems was how to get an isolated power supply to
> each of the gates
> and how to trigger them.  Batteries and fiber optics
> would probably
> work and I did some tests on that years ago.  But it
> was all just too
> complex and messy...  The SISG fixed all that.
>
> There is a problem if the spark gap does not fire at
> just the right
> voltage.  If it fires at too high of voltage the
> IGBTs could
> breakdown and that is a bad thing.  If the sparks
> gaps were divided
> among each device to get around the above, then they
> would have to
> fire at 900V which is not easy.
>
>
> >In short, the modularity of Terry's SISG is very
> cool,
> >but I'm wondering what simplifications one could
> make
> >if he/she started out assuming that they were going
> to
> >need a LOT of IGBTs, e.g., for use with a 15kV NST
> or
> >something bigger.
>
> Boards like Mike's are 3600V each.  So you can have
> "big" modules
> too.  If it all works out real good, someone will
> probably sell the
> whole thing pre-made.
>
> >Ah...if only those 6500V IGBT
> >bricks were cheaper!! :-)
>
> :-))))  At least they do exist!!!  Not sure they
> have the reverse diode?
>
>
> >And again, the ability to adjust the firing voltage
> by
> >just varying a spark gap like in a regular SGTC
> would
> >sure be neato.
>
> I just add and remove sections by moving the wire:
>
> http://drsstc.com/~sisg/index_html_m2dc0296e.jpg
>
> SIDACSs cost like 50 cents each so there is no great
> price
> advantage.  Some people have trouble getting them in
> other countries
> so they are trying Zener, and TVSs.  Not sure how
> well that works.
>
> Of course, all this is very young, so who knows...
>
> Cheers,
>
>          Terry