Re: It works! Solid state driver

From: 	Scott Stephens[SMTP:stephens-at-enteract-dot-com]
Sent: 	Sunday, December 07, 1997 6:14 PM
To: 	Tesla List
Subject: 	Re: It works! Solid state driver

At 06:50 PM 12/4/97 -0600, you wrote:
>From: 	Harri Suomalainen[SMTP:haba-at-cc.hut.fi]
>Sent: 	Thursday, December 04, 1997 12:32 PM
>To: 	Tesla List
>Subject: 	Re: It works! Solid state driver

>> 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.

>Go for K=1. Higher K will mean the energy is transferred to the secundary
>side much better.

And load reflected back to the primary, resulting in more I^2 R loss in the
FETs too. I'll increase coupling when: 1) I see its safe to slowly increase;
and/or 2) I have some type of over-current/voltage shutdown.

>You still need to make your hard-ware able to take even
>full short circuits (like saturation of core during transients etc). That
>is usually done in almost every SMPSU too. Usual reaction times for such
>systems might be around 100ns or so. That is usually quick enough to
>protect the switching devices.

You make it sound so EZ...

>> 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.
>As the primary discharge occurs the secundary goes from normal load to
>very high impedance. Then the arc will not short-circuit anything.
>However, the sudden huge change in load (removal of it) will easily cause
>other problems, mainly saturation of transformer.

What? My thinking is, when a secondary-arc or streamer happens, it will load
the secondary, which will reflect back through the primary (or ferrite
step-up transformer, depending on the configuration) and cause an increase
in FET current, possible destroying them. Are you saying the secondary will
go to high-Z when an arc/streamer ignites?

>This effect is not noticed with some discharge electrodes. I've put around
>1kW through a prototype having a needle-like electrode (coil upper end
>wire just pointing up).

Your secondary Z is high, (L/C)^.5

>No problems with K=1 and no current limiting

You designed your solid-state driver and transformer for this environment...

>The same prototype wouldn't have a chance of survival with
>toroid discharge electrode. With toroid it will die out at the first
>spark due to load-change problem. Gapping to core might help a lot.

Big torroid, Big C, Low secondary-Z (L/C)^.5, transformer saturates cause
it's overloaded, now FET's get hot as they're overloaded...

>> Continous, 8A, but pulsed, up to 40!. Of course, with 1 ohm Rds, efficeincy
>> will stink.

>Go for maxinum duty cycle. Reducing duty cycle in a cw-device is not
>helping you with anything. The coil will be storing the energy pumped in
>anyway untill it has enough to strice arc.

The arc strikes, secondary gets loaded by the now low-Z arc, and stays lit
at a low voltage.

This is my thinking from working with flourescent/HPS tubes. By pulsing, the
arc dies, and voltage can go up until the next one. I would like to hear
from someone who has observed pulse vs CW for similar power levels, and know
which is more spectacular.

>> And using transient suppressors (zeners) across Drain-Source.
>Not needed in a properly made bridge topology system.
>> 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
>> 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!
>Oh, c'mon. I used no shield nor any ground planes to protect from emi.
>At 1kW the coil being a couple of meters away made no problems with
>anything except perhaps some very high-impedance circuits with it did not
>have. Scope worked of too.

Glad to hear it. I've killed enough semi's to be paranoid :(