Re: Sheppard-Talyor circuit up on my website/ low Z construction technique

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

Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rob-at-pythonemproject-dot-com>

Tesla list wrote:
> 
> Original poster: "Daniel McCauley by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>
> 
> > So aside from that stage (the output stage I'll call it) the MOSFET's
> > are no doubt operated as switches, meant to pull current into the
> > 160uH inductance and "release" this current to the rest of the
> > circuitry. So the MOSFET's are switched quickly here in order to
> > minimize I^R power loss in the junction....and therefore there will
> > most likely be large voltage spikes...
> >
> > So it is here in this stage (where there is high-speed switching of
> > significant current) that I think there should be clamp diodes. Maybe
> > even isolation of the MOSFET body diodes (with a series shottky) if
> > it comes down to it...I've heard the body diodes can create damaging
> > reverse recovery transients if driven into forward conduction too
> > hard (from free-wheeling current).
> >
> 
> Yeah, you definitely should bypass those internal MOSFET body diodes with
> some external diodes.
> I've built two solid state tesla coils already and currently use the MUR1660
> type schottkey diodes (in a TO-220 package) to
> bypass the MOSFETs internal body diodes.
> 
> Also, what kind of gate drive circuits are you using?  If you are operating
> above 200kHz, you have to be able to switch those FETs on and off very fast
> and a simple gate drive circuit isn't going to do it.
> 
> Dan

You generally only need to bypass the body diodes in hard switching
half/full bridge circuits.  This is a soft switching supply.  What
happens in half bridges is that the body diode is turning off in one FET
while the other FET starts to turn on, i.e. short circuit.  This doesn't
happen in the Sheppard-Taylor topology.

I am using a 6A driver running at 20V through a transformer, so each fet
gate sees 10v during turn on.  If I lower the duty cycle, then the
voltage increases substantially on the positive side.  Rob.

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