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

["Tesla list" <tesla@xxxxxxxxxx>]

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

Right.  Terry earlier pointed out my illogic :-)  The
crux of the idea, though, was that the SIDACs might be
replaced with an air gap and the drive to the IGBTs
might be consolidated so as to simplify construction
of the beast for high voltages.  Something like Dan's
"IGBT stack" (referenced earlier in this thread) with
drive provided by a cap charged when the gap fires.
Hence, an "IGBT-assist" spark gap.

I still wonder about Terry's concern about the gap
being open too wide and permitting you to fry your
IGBTs.  Certain that would suck, but seems like more
of a design margin thing to me, i.e., give yourself a
few "bonus" IGBTs in your stack, and don't set your
gap too wide (or else...)

That, or just pick up one of those 6500V bricks (a
steal at $1000+! ;-)) and build a mega-tough one-stage
SISG.  Would the present drive circuit be enough for
one of those beasts?  Might have to use the SISG as
presently designed just to drive the brick ;-))  That
would be amusing...

Regards,
Aaron, N7OE


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

> Original poster: "Gerry  Reynolds"
> <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Aaron,
>
> I'll through out some thoughts.  If the SG has
> fired, and the gate
> drive is at the voltage generated by the TVS,  there
> may be a problem
> with Vce drop across each of the IGBT's.  Dont know
> what the
> saturation voltage is for each one, but the bottom
> IGBT would get the
> most gate drive and the top IGBT would see the
> smallest gate drive
> since its emitter voltage would be the accumlative
> sum of Vce from
> all the IGBT's below it.
>
> If the SG is OFF,  each emitter of the IGBT would be
> at a different
> voltage whereas the gate voltage would be at the
> emitter voltage of
> the bottom IGBT. All the other IGBT's above it will
> see a gate
> voltage negative relative to its emitter voltage and
> the top IGBT
> will see a negative gate voltage as large as the SG
> breakdown
> voltage.  I dont think these parts will survive
> this.
>
> Gerry R.
>
> >Original poster: "J. Aaron Holmes"
> <jaholmes@xxxxxxxxxxxxxxxxxx>
> >
> >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
> >
>
>
>
>
>