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

["Tesla list" <tesla@xxxxxxxxxx>]

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



> Regards,
> Aaron
>
> --- Tesla list <tesla@xxxxxxxxxx> wrote:
>
> > Original poster: "J. Aaron Holmes"
> > <jaholmes@xxxxxxxxxxxxxxxxxx>
> >
> > The main benefit of the SISG appears to be its very
> > low loss compared to a conventional spark gap.
> > However for higher voltages, it seems like the part
> > count goes up pretty quick.  Would the following be
> > possible (don't beat me if not, I'm a computer nerd,
> > not an EE ;-)) ...
> >
> > Goal #1: Common drive for all the IGBTs so that a
> > bunch of capacitors, resistors, and TVSs are
> > eliminated.
> >
> > Goal #2: Get rid of *all* the SIDACs.
> >
> > Not sure about Goal #1, but it seems doable.  It
> > just
> > means you'd have to scale some of the components
> > according to how many IGBTs you'd be driving, right?
> >
> > As for Goal #2, how about using a spark gap?
> > Replace
> > the SIDACs with a spark gap having a high series
> > impedance such that there is insufficient current to
> > cause quenching problems.  When the spark jumps, it
> > would charge the IGBT drive cap just like in the
> > current SISG design.  Then, when the IGBTs go into
> > conduction, the voltage across the gap would be
> > suddenly near-zero, further eliminating quenching
> > concerns.  For added coolness, this trigger gap
> > could
> > be adjusted to vary the firing voltage without
> > sodering, disconnecting things, etc.
> >
> > This "hybrid" of the SISG and conventional gap might
> > substantially reduce part count while retaining most
> > seemingly-critical element:  The IGBTs.
> >
> > Any thoughts?  Am I smokin', or could this work?
> >
> > Regards,
> > Aaron, N7OE
> >
> >
> >