Re: Another SSTC Question

["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>


> 	I'm trying to build a driver for my (future) MOSFETs, but I'm not sure
> about a few things.  I want to isolate the signal electronics from coil
> driving electronics using a ferrite toroidal xfmr (that's the right
> thing to do, right???).

For example, yes. You could also use optocouplers + a bit of additional
circuitry. Or driver ICs like IR2110 (without a bootstrap diode and use a
separate floating supply instead)

>  The problem is, I can't get a "good" clean
> signal out of the transformer.  Depending on the pulse width and
> frequency fed to it, I get a bumpy output that ranges from positive to
> negative (which I kind of expected, yet it isn't helpful).

Try as high permeability toroidals as you can find. Also, micro-thin coax
for the winding seems to work quite well. Primary = cable shielding

>  I know a
> noisy signal is OK, as long as there's enough power to saturate the
> MOSFETs.

Not power, but voltage. If the gate signal drops below 8V then that will
be a problem. >=10V is ok, 15V or 18V might be even better because you'll
have better margins for gate signal noise. Also the gate signal voltage
will probably drop a bit when you connect the rectified mains (Miller
capacitance, gate<=>drain) so a higher gate voltage allows for higher
"drops" without any harm done.
 
> 	Also, is it OK to run a SSTC with BiPolar transistors?  I've got a ton
> of them, and my previous futil attempts at a SSTC driver chewed up all
> my MOSFETs and spat them out :-(.

You can always try! :) Designing transformers for this is a bit easier
because you just need the base drive current, and not a clean >10V square
wave output voltage.
Bipolar transistor selection for SMPSs/SSTCs isn't as easy as with fets or
igbts, though. Large V_ce_sat and storage times can cause problems...

>  I actually toasted a 600v unit with
> only 30v in, although I only had a power diode across the transistor for
> "protection" (which probably helped little, if at all).

A diode won't help much there. A varistor (VDR or MOV), which basically
is a very big zener diode, helps better.

cheers,

 - Jan

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