Re: It works! Solid state driver

From: 	RODERICK MAXWELL[SMTP:tank-at-mail.magnolia-dot-net]
Reply To: 	tank-at-mail.magnolia-dot-net
Sent: 	Thursday, December 04, 1997 6:37 PM
To: 	Tesla List
Subject: 	Re: It works! Solid state driver

You fellas are really advanced! This is only my second attempt at a
solid state driver, and I ain't no enginneer! My first attempt was
miserable failure.

> 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.

 My diver is a HV400IP you can see the tech info at Harris
Semiconductors site. I'm using it as a low side switch. 

> 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
> plug.

  The transformer I'm using is a iron large powder toroid made of #26
material part no. T400A-26. It is wound in a push-pull cofiguration. 
                                                 Primary: 40 turns #12
center tapped                                                    
Secondary: 140 turns #26  


> 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.

  In my limited experiance the zener fried at low power and took out my
transistor and driver! I have tried many methods to suppress transients
but the only thing that seems to protect the transistor at turnoff is a
capacitor snubber across the drain/source terminals. I have also tried
Mov,s with the same result(the smoke and flame were much more
spectacular though)!
  My secondary is wound directly over the primary to achieve maximum
coupling. There is a layer of high voltage tape between them rated at

> 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.

The only simulator I have is Electronics Workbench, but is somewhat

> I plan to use a 600W, double-filtered triac dimmer, because I don't have a
> variac.
> 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!)

   I,ll have to leave the more involved stuff for later! I'm just
starting to get my feet wet!

                           Frankensteins Helper