Re: It works! Solid state driver

From: 	Harri Suomalainen[SMTP:haba-at-cc.hut.fi]
Sent: 	Monday, December 08, 1997 3:59 PM
To: 	Tesla List
Subject: 	Re: It works! Solid state driver

On Sun, 7 Dec 1997, Tesla List wrote:
> >systems might be around 100ns or so. That is usually quick enough to
> >protect the switching devices.
> You make it sound so EZ...

See datails for eg. UC3825 or UC3524 or something to see how easy it
actually is!

> >As the primary discharge occurs the secundary goes from normal load to
> >very high impedance. Then the arc will not short-circuit anything.
> What? My thinking is, when a secondary-arc or streamer happens, it will load

Nope. It acts a lot like a series RLC circuit would. The current will
increase gradually (and power gets stored in resonating elements). If you
remove all the sored energy at some point you will be back in square one:
current is not large, but it keeps increasing.

> in FET current, possible destroying them. Are you saying the secondary will
> go to high-Z when an arc/streamer ignites?

All the details would be nice to know. Perhaps I some day will dig more
closely into all the details. I wish I had a storage scope. Without one
all the transient condisions remain more or less uncertain. Still, it is
a commonly known fact that the coil jumpes to high-impedance state. 
Anyway, practical prototypes have proved that you definately need a good
current limiting without needle electrodes. By limiter I mean some system
that will end the cycle prematurely if current gets too high. A current
limiter is always needed for push-pull configuration anyway.

> >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

Yap, fairly high. That means unfortunately that I need a higher voltage
to drive it. I think I'll some day wind a low-impedance coil just taylored
for the job to be able to drive it with low voltage (meaning a smaller

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

Nope. I was driving a maxinum duty cycle half-bridge. The transformer was
nowmal SMPS transformer having tight coupling and no gapping to help with
anything. It was no special taylored system. It was just a normal SMPS
with high-voltage output transformer end-feeding a coil.

> >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...

No overloading. The same happened with quite a small ball electrode too.
Surge impedance with that system was not very much altered. The fets used
were IRF740 capable of taking 10A cont. and 40A pulsed. The power range
enough for destroying fets was around 1-2A of current in operation.
Certainly high enough marigin for some surge impedance changes, uh? :)

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

Me too!
See "Malin Tunic: Chemistry of structure - function references in cheese" for
further details!
Harri.Suomalainen-at-hut.fi - PGP key available by fingering haba-at-alpha.hut.fi