Re: Ready for SSTC but need help

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Hi Scot,

Good to hear you will finally put those IGBTs to use ;-).  I assume
you are speaking of those Toshiba 200A (continuous) 1200V
half-bridges?

Just a few pointers:

Keep the operating frequency low... very low.  I run my coil at 45khz,
but my IGBTs are slightly faster at switching than the ones you have
(or i think you have).  If you can, wind a secondary that will
resonate at 35-45khz with a big toroid on top.  Yes, that means using
rather fine wire for a coil that size, but its better to waste a few W
in the secondary winding than it is to have your IGBTs latch up and
explode (yes they do explode) from hard switching too much current.
Hard switching can also cause voltage transients, which also kill
IGBTs in nS time.  So... the higher your operating frequency, the more
that switching time becomes a limiting factor.  I think those IGBTs
you have spec a .5uS switch time, so if you draw yourself a sine wave
(to represent the current waveform in the primary circuit) and then
look at the current .5uS past the zero crossing, that is the current
you switch.  Now you can see why lower frequency is better ;-).

As to power supplies... for 1200V IGBTs you should not got any higher
than about 900VDC on them, and that is assuming you have the proper
protection: Transient voltage suppressors (TVS) to clamp at about
950V, decoupling capacitors across the power supply rails, and
snubbers couldnt hurt, but watch out, some snubber designs burn up
power constantly, not just when they are catching voltage spikes, so
they might not be the best option here.  You must keep the H-bridge
layout very low inductance.  Mount large DC filtering capacitors (i
use about 4000-6000uF) so that the inductance between them and the
IGBTs is as small as possible.  The idea here is that if the IGBT has
to switch off with current still flowing through it, you get a voltage
spike.  This spike is directed towards the supply rails, through the
IGBT's internal diode.  You want a capacitor *right* there to catch
that potential spike, and keep it to a low level.  Over-volting an
IGBT *will* destroy it... these devices can be killed by a little
static... so they are just a bit sensitive to too much voltage!

If you havent already realized this, you are going to learn a lot
about power electronics if you persue this project :-).  You said you
wanted a learning experience, well this is a pretty good one!  Dont
expect to just find "plans" for what you want... you will be writting
your own.  I suggest you look at as many DRSSTC related material as
you can (Dan's book sounds like a great source), ask plenty of
questions, and maybe have some people review your schematics before
you build.  Then when its built, take some pictures and we can review
it some more, and hopefully keep you in the right direction.  Feel
free to email me with any questions you might have, i usually enjoy
answering technical questions, keeps the ideas in the front of my mind
;-).  Also see my website:

http://www.stevehv.4hv.org/

Good luck,

Steve Ward

On 7/29/05, Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: BunnyKiller <bunikllr@xxxxxxx>
>
> Hey All...
>
> I have been a strong proponent of Spark Gap Technology for years and
> I am ready to tackle the SSTC scene....  I have at my disposal some
> very healthy IGBT's ( 1200V 400A units  8 of them being perfectly
> matched in specs)  along with a very HEALTHY heat sink, snubbers,
> resistor sets etc..
>
> who would you suggest that I go to to read about the most detailed
> description on building the electronics for the system...  especially
> the gate drivers,
> the power supply I have can supply...  AC up to 293VAC   DC up to 343
> VDC....  as is without voltage doublers....  any help would be appreciated.
>
> seeing the BIGPIG go SS would be very kewl    ( plus I need a new
> learning experience) no spark gap  is even better if I
> understand  SSTC correctly ( seems like SSTC is the way to go to
> better usage of power) ..
>
> any help is accepted and appreciated....
>
>
> Scot D
>
>
>
>
>