Re: Hvguy-dot-com Feedback SSTC and New Stuff!!!

["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 Justin,

and congrats again! :)

Ken C Herrick wrote:
 >  > And another thing: be wary of having both FETs in each half of the H
 >  > being on at the same time!  You might want a bit of crossover-
 >  > control in the circuit to avoid that.
 >
 > We keep this in the back of our mind, for sure! A commercial design
 > would never use a single gate transformer to directly control four
 > (or two) MOSFET's in a bridge, and we realize that.

Well, actually... ;)
Main criteria for commercial xfmr mass manufacture is the usual one which
discerns "a good, working design" from a not so good one, i.e. total cost.

All windings on one toroid is fine, and has some minor benefits in this
SSTC case, but the more challenging work to build these (properly, anyway
:) isn't necessarily worth it.


 > But, at the same
 > time, it works! I imagine there is some degree of shoot-through
 > current (we should measure it) but it just isn't hurting anything.

As the FBSSTC has this symmetric gate drive (-x V .. +x V), and drive
windings are well coupled, and as there's just one gate resistor, there
shouldn't be any shoot through. That's because the mosfet threshold
voltage together with the far-negative bias adds plenty of dead
time, and switching hasn't any chance to overlap.
  => it isn't a problem
  => adding dead time is not required, way simpler circuit :)

At least, my experiments give around ~50mA current draw from mains when
switching at no-load. That was for a 300 kHz 350VDC IRF840 fullbridge.
50mA can be explained by all the mosfet output capacitances being
switched.

 > BTW: Sparklength with our feedback SSTC is up to 12" now, with
 > semi-filtered (130uF) 1/2 wave rectified DC input. We're working on
 > making the circuit run with raw (no filter cap) 1/2 wave input, so it
 > can arc an estimated 20" or more. Although 8" crackling flames are
 > awesome too ; )

Already tried adding a heavy duty dimmer circuit aka phase controller in
front of it?

That'd give steep rising edges on the SSTC feeding waveform envelope, and
sparks appear a bit more impressive.

Comes bundled with the option to crank up the peak power a bit... 8-)


have fun!

cheers,

  - Jan

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  high voltage at http://www.hut.fi/~jwagner/tesla
  Jan OH2GHR