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

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

Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

Hi Rob and All,

> I am finding that the FETs dissipate little wattage, but the FREDs 
> get extremely hot.

D26 and D25 are definately in the current path, they will see the
same amount current the MOSFET's are. I suppose D24 is getting pretty
hot? It will take the full current flowing in tranformer TX4's
primary.

Ummm...IRG4PH50U is a 24A 1200V IGBT, good up to no more than 40kHz
(http://www.irf-dot-com/product-info/igbt/navigbt.pdf). No way this will
work at 240kHz but maybe you're only using those part #'s on the
schematic for simulation.

So as I look closer, it seems that both transistors are simply in
parallel, being used in a common buck converter topology...the output
of which goes straight into the primary of the step-up transformer.
This differs from the normal buck topology - where the output is
rectified and filtered.

But is this really the way the circuit works? Again, I can't see
waveforms and all, and would like to have confirmation that that is
how the circuit generally works (or should work). Otherwise I might
be misinforming...

So if this really is the way things work, then both gate signals
would be in phase, and the transistors would turn on and off
simultaneously. This leaves the option of paralleling the transistors
directly (drains go to 160uH L opposite 170VDC power,,,sources go to
GND)? This would get rid of (expensive, hot) diodes in the main
current path?!

A freewheeling diode COULD NOT be used after this modification,
because it would short out the negative voltage (coming from L16)
that is supposed to go into TX4's primary. But then again it
shouldn't be needed...?

In the current schematic, if both gate signals were 180 degrees out
of phase, the transistors would give a DC short through the 160uH
inductor.....which would only let up at very small intervals where
there is dead time. That's why I figure they're in phase and both
devices turn on simultaneously.

The gate voltage to these paralleled devices (or a single device)
could be provided by only one secondary winding on the gate isolation
transformer...that would be really nice too. No worrying about one
transistor turning on before the other.

As mentioned in a different post (Dan's I think), you will need a low
impedance driver to nail the high-capacitance gates to 10 or 12V.
With these large gate capacitances, rise/falltimes will be a
significant total of the switching waveform. Best to keep things as
fast as possible without ringing L's and C's.

But anyway Rob, you're making me want to try this!

Also real quick here, if L16's core saturates it will draw gigaamps.
I don't know if you have yet, but I'd put little current-sense
resistors EVERYWHERE and have a scope on as many of them as I could
at one point in time. (watch grounding of course...)

Take care,

Justin Hays
KC5PNP
Email: justin-at-hvguy-dot-com
Website: www.hvguy-dot-com