Re: It works! Solid state driver

From: 	Scott Stephens[SMTP:stephens-at-enteract-dot-com]
Sent: 	Thursday, December 04, 1997 4:59 AM
To: 	Tesla List
Subject: 	Re: It works! Solid state driver

At 12:17 AM 12/3/97 -0600, you wrote:
>From: 	RODERICK MAXWELL[SMTP:tank-at-mail.magnolia-dot-net]
>Reply To: 	tank-at-mail.magnolia-dot-net
>Sent: 	Tuesday, December 02, 1997 9:31 PM
>To: 	tesla-at-pupman-dot-com
>Subject: 	It works! Solid state driver
>I'm here to report a successful test! I have two IRF440 mosfets
>running at about 30% duty cycle on each leg for a total of four. They
>barely get warm when I crank the variac up to line voltage (no fans
>nessasary)! I think this is because of the new drivers. They turn the
>transistors on and off so quickly they don't get a chance to stay in
>transistion very long.

I use to think the MOS input meant no power. How wrong I was! At 100KHz, the
effective input impedance drops to 100's of ohms! I'm considering a 5 watt
bipolar comp-sym drive, maybe with transformer coupling to facilitate
isolated monitoring of gate drive, and phase splitting. International
Rectifier and National Semi have good Web sites with MOSFET app notes.

>  Now comes the real drudgery, finishing the secondary wind wind

My secondary is done. I'm working on modeling what my coil will do to the
drivers, when a plasma ignites. If I use end-feeding with a K of maybe .6,
and a hot spark ignites, and I use a ferrite transformer with a K of near 1,
then I'm effectively shorting the FET's. They will die before I can pull the

So I'm interested in using a ferrite block between primary and secondary
windings, to give a more modest coupling for leakage reactance ballast, and
pulsing the Fets with a low duty - cycle high current impulse, in the hopes
no high current streamers will stay lit long, and keep the secondary Z high.
Continous, 8A, but pulsed, up to 40!. Of course, with 1 ohm Rds, efficeincy
will stink.

I wish I had a model of secondary plasma discharge. I'll find out.

Or building the transformer E-core with the pri. on one core, and the sec.
on the other core. Thus allowing me to vary coupling as change air gap and I
observe the effect of the secondary discharge on the Drain/Source transients.

And designing current protection for the fets.

And using transient suppressors (zeners) across Drain-Source.

And maybe even external (Hexfred?) diodes to ease the dissipation of the
MOSFET's internal diodes. The worse the primary tuning (too little C), and
the more power this diode will heat during turn off.

This is why I'd like to simulate my FET's and transformer before I end up
with some expensive, dead, semi's.

I plan to use a 600W, double-filtered triac dimmer, because I don't have a

And of course, my logic control will be isolated from my FET's & drivers,
through 15 feet of steel conduit, with isolated grounds on the driver's
coax, and shielded boxes, to protect it from what must be some VERY ugly EMI!

One neat trick which may be possible, is to use a sense antenna to pick up
the secondary voltage, and use it to drive the coil. Basicaly the feedback
tap in the oscillator. So now, when objects are brought near the coil, and
its C increases/frequency decreases, the coil can stay tuned to it
resonance, lowering power reflected into the primary.

Good Luck! (I'll need it too!)