Re: which MOSFETS for SSTC?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

The other posters already gave you quite good tips. Anyways I'll add 
my $0.02 too...

> I eventually want to build an SSTC, so I'm going to search for
> suitable MOSFET's at hamfests.  Can someone tell me the
> best ones to use? 
 
http://ec.irf-dot-com/ec/adirect/ir?cmd=navigate&category=37&categoryName=%2F%2F
HEXFET+Power+MOSFETs%2F%2FDiscrete+HEXFET+Power+MOSFET%2F
  IRF740, IRF640, IRFPE50, IRFP460, etc
Also other manufacturers mosfets are just nice, as long as they're not
very old "models" usually sold at surplus places.

But... not just _any_ mosfet will do...

You have to consider first how much amps you like (ok, plenty of course
;o), and then decide on how your SSTC primary side will be built. 

The pri side topology sets the minimum voltage rating of the mosfets, and,
in contrast to current, you will want to keep the pri voltages low.
Reason being that unlike bipolar transistors, mosfets with >500V start
getting darn expensive and don't stand much current anyways, so they're
kinda useless for SSTCs.

You can go push-pull topology (no extra diodes and floating gate drives
required) to start simple and at low powers:

  http://www.hills2.u-net-dot-com/tesla/ssmag.htm
  (use direct TC primary drive, not the ferrite cored xfmr setup Alan
  Sharp used there - saves a lot of grief)

For 110Vac mains the mosfets will have to stand at least 2*sqrt(2)*110V =
311VDC, so use 400V mosfets like IRF740, or better yet 500V mosfets like
IRFP460 (push-pull at non-50%-duty gives you huge voltage spikes on the
primary, so 200V safety margin is pretty mandatory if you don't use
precise 50%-duty-always-style-driver like from a CD4046 VCO, and your
interruptor system shuts down gatedrives instead of the mains power
supply).

Also, for a single mosfet setup i.e. forward converter style, IRFP460
or one with even higher voltage (IRF840?) is necessary for 110Vac 
recitifed mains.

cheers,

 - Jan

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