Re: SSTC-Will This Work?

["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!

> 	I threw my SSTC drive circuit together without any real knowledge of
> the way it SHOULD work.  The schematic is:
> www.hot-streamer-dot-com/temp/SSTC_circuit_1.jpg
> 	Will this work?  I've omitted the schottky diode, and the transistors
> shown are supposed to be MOSFETs, not bi-polars.

Looks quite ok, but you can't connect signal 1 with the same wire to both
mosfets. One is up at 340V, the other at 0V. You could use two IR2110
driver chips. Or if you don't plan to do "excessive" PWMming ;o) then two
gate drive transformers with two outputs
 http://www.hut.fi/~jwagner/tesla/SSTC/general-sstc-notes-gatedrv.htm
will work very well too.
Two IR2110 => ground current loops can be a bit of a problem. There was
a schottky diode trick to this, however.

> Signal 1 is simply a
> pulse, variable in both width and frequency, and signal 2 is the same
> pulse shifted 180 degrees.

If you plan to use an interruptor (halts the gate drive for a few ten
milliseconds, i.e. low-frequency-pulses the SSTC) you should add a series
capacitor to the primary. Must be FKP or MKP type pulse cap. Maybe two or
three in parallel. >1uF total.
This is only necessary if two mosfets remain conducting while the SSTC is
halted. Which would place DC accross the primary, and blow the fets.

While at it, place a 470nF or larger FKP/MKP pulse cap in the center of
the bridge, between Vdd and ground. As in
 http://www.hut.fi/~jwagner/tesla/SSTC/pics-fullbr/mk2-view3.jpg
For local decoupling. Very important at higher power levels.

(a 500W..1kW bridge can really be fist-sized, by the way! :o) keep it
small)

> I'd like to run my coil on rectified 240
> VAC.  The MOSFETs I want to use are 600V, 12A, and are in a T0-3
> package.
Pretty beefy! :)

> PS-I finished my PWM circuit yesterday.  It works perfectly :-)).  I'm
> putting the main signal into a pulse differentiator with a variable
> resistor (just an RC circuit, really), and feeding its output into a
> schmitt trigger.  I made this thing up as I went, so I ran into some
> problems.  One big one was controlling two independent signals (the
> outputs of a flip-flop) with one control.  I ended up using two
> transistors, biased by one variable resistor, as the resistors in the
> PWM circuit.  To my surprise, it worked :-)).  As the capacitor charging
> curve levels off, the schmitt trigger output goes high for a longer
> time, and vice versa.
Sounds complicated, but nice to hear it works! :o))

cheers,
 - Jan

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