Re: It works! Solid state driver

From: 	Harri Suomalainen[SMTP:haba-at-cc.hut.fi]
Sent: 	Saturday, December 06, 1997 8:39 AM
To: 	Tesla List
Subject: 	Re: It works! Solid state driver

On Thu, 4 Dec 1997, Tesla List wrote:
There was wonderes about tuning. As the tesla coil is a high-Q system
the tuning will need to be quite sharp. As I pointed out you will see
the system difference (capasitive vs inductive loading) at very small
frequency change around the resonant point. To avoid problems you should
not operate below resonant point becouse capasitive load will mean a lot
of dissipation in fets. However, the system might work very satisfactory
with "broad" frequency range. (Say a few kHz or so with a 200kHz coil).

>   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. 

I'd stick with E-cores. As soon as you head for larger power you'll find
out that power is not dissipated from the well insulated toroidal core
that well. Electrical insulation tends to be thermal insulation as well.
Still, it may be ok. For a brand new system I'd definately take an E-core.
Winding quite a lot of turns is easier with those too!

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

High primary half coupling is a must. Then, there should not be huge
voltage spikes at the fets. Bifilar winding is a must. Small snubbers
will help a lot too.

>   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

MOV's tend to be somewhat resistive. Eg. a 250Vdc operation specified
MOV will be at low resistance region only at somewhat higher voltage.
Zeners (transient suppressors) are better. Snubbers will allow you to
operate without suppressors and they work reliably. Try them 1st!

Transistor blow-ups may also be due to saturation of core resulting in
huge current. Make sure your pulse-by-pulse current limiting is working.
Without one you'll certainly be in problems.

> > 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
> limited.  

EWB is almost no use for this situation.

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