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Re: Mosfets survive

>Date: 22 Mar 96 20:26:07 EST
>From: Alan Sharp <100624.504-at-compuserve-dot-com>
>To: "INTERNET:tesla-at-grendel.objinc-dot-com" <tesla-at-grendel.objinc-dot-com>
>Subject: Re: Mosfets survive
>
>Greetings All,
>
>I make progress.

GREAT!

>Am I correct in saying that my coils should be closely coupled. And that
>I should construct a primary which has a reduced width, more hieght
>(I dont anticipate sparks bigger than 6" to 10"), but increase the number
>of turns to keep the induction about the same. (too low an induction
>would give fast rising currents overwhelming the current proctection).

My first guess would be YES. With a "standard" transformer, loose
coupling translates to a leakage inductance that causes voltage spikes
that your snubbers have to take care of. You can think of the leakage
inductance as an inductor in series with your "perfect" transformer.
This inductor causes voltage spikes that are not coupled into the
secondary of the transformer. This wastes power and causes a voltage
spike that will blow your driver.

>
>Snubbers still seem too hot, I'm using 2   5W, 3.3K resistors in // plus a
>1000pf 2kV cap (in series) . time constant about 1uS, I figured that 
>worst case if the full voltage was across them all the time then at 75v,
>P =V*V/R about 2W each. But they still seem very hot, and wouldn't
>cope at 150V. I'm planning to reduced the capacitor there to  200pF 
>leaving  a little spike protection at the FETs and put a big snubber
>across the coil, say 600ohm, 50w plus 10,000pf. - does this seem
>reasonable?

2 thoughts here: 1. because of the loose coupling between your primary
and secondary, the leakage inductance of the primary is causing large
voltage spikes. Winding your primary closer to your secondary will
improve this.

2. Don't forget that while your primary impresses a voltage on the
secondary that is based on the turns ratio of the primary:secondary.
The secondary then acts like a (tesla coil) series resonant circuit
and multiplies this by the Q of the secondary. ( 300 - 400 in Malcolm
Watts measurements!) This now Q multiplied voltage in reflected back
to the primary divided only by the turns ratio of the transformer.
(note to theorists: I've left out the coupling factor for simplicity)

So, the voltage on the subbers may be Vcc times the secondary Q. I've
not tested this you simulated it. It is just a WAG for now.

Alan, please feel free to email me directly with questions. I've not
built any large switching (tesla) power supplies yet, but due plan to
in the future.

cheers,

	jim