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

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

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?  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?

--- 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