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Mark Barton's "Sheets of Flame" 10Kw FET driver.

A lot of you will have read the reference to this on Bill Beatty's site:

"The idea of making an offline switchmode power supply driver for a TC primary (using IRF840 FETs in a full h-bridge configuration) occurred to me a while back while I was designing a supply for a European Customer. You could put at least 500VA into the primary with such a setup and probably get a nasty RF burn, to boot, if you were careless.

This is a piece of cake with modern PWM control and CMOS power FET driver IC's. Anybody out there try this yet?? Certainly not as mad-scientist-looking as a rotary spark gap but the potential for EMI control is certainly there and would make for a much happier neighborhood."

 Geoff,

I have posted info on this system before, but will tell you it briefly. A friend and I have built a VFET H-bridge driver that operates at 10KW CW and about 40KW in burst mode (carrier bursts at about 100Hz rep rate). The carrier frequency is 80KHz. The effects are startling. CW operation yields a 4 foot SHEET of electrical flame that sprays from the discharge electrode with hisses and crackles. Burst mode gives the familiar tendrils of a classic TC along with that rrraaaaaaattt sound we all love.

Power supply: rectified and power factor corrected 240VAC yielding 400VDC. Large bank of electrolytics to support burst mode operation. I believe the FETs are IRF540 type, 4 per leg.

Mark

(If he used 540 they would have to be in series for 400v)

Mark did post some information on this:

I must confess, I did not design our 10KW system. It was designed by a guy named John Ross, a brilliant engineer. I just helped and financed the thing. It just sits dormant in my garage now because John has moved out of state.

OK, I do know some things:Use transmission line construction techniques.

Keep ALL connections to FETs SHORT (like less than 1/8 inch) to FET body.

Use RC snubbers on each FET to protect from kickback and parasitics.We had zeners as well.

Cross your fingers.

You can run the FETs at their ratings. It just must be done right. You must stop all ringing and spurs. I have never experienced kickback from the secondary as I have in sparkgap TCs.

Wind a coil on a polyethylene 55gal barrel. These are used for water and food storage, easily available and not expensive. Winding 22 awg wire on this form yields a coil that rings at 80KHz. Adding an upper terminal will drop it to 60KHz or so.

The circuit itself is quite involved and was not designed by me. The main problem was FETs blowing up like firecrackers. They require heavy output snubbing and transmission line construction techniques to keep parasitics under control. Not a project for the faint of heart (or budget). Mine was VERY expensive. I would not do it again.

The H-bridge made of 16 FETs fed the primary winding directly with no primary capacitor. This means the system had only one resonant frequency instead of two and made things much much friendlier. The coupling was tight (link type coupling) and the resonant nature of the secondary reflected back into the primary winding presenting a load that is purely resistive (mostly) at resonance. Power regulation was had by slight FMing of the carrier frequency (detuning).

Good luck Zap, Mark

I've lost other postings on this but this was a conventional coil, with the secondary very tightly coupled to the primary, lots of polythene insulation between primary and secondary. Because this was built sometime ago, is now defunct and the electronics engineer has gone - Mark may prefer not to be asked more about this. It is the biggest solid state coil that I've heard of.

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