Re: Modeling a flyback transformer Tesla coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "Brian Howden by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <bhowden-at-shaw.ca>
> 
> I am attempting to use ORCAD to model a transistor driving a flyback
> transformer for a small solid state tesla coil.  What would be a reasonable
> approximation of the inductance I am driving (10 turns #14 wire added to the
> core)? 

A guess: 50 uH.

> I am attempting to model a
> snubber network to minimize damage to my driver transistor.  I am assuming
> the idea is to absorb enough of the energy released by the collapse of the
> field to protect the driver but that any energy I absorb is taken from my
> spark output so the trick is to absorb enough to bring the voltage spike
> down below what the transistor can withstand and no more.  Have I got the
> basic idea correct?

Yes, but you can also add only a capacitor across the primary, and make 
the circuit operate in a way similar to a Tesla coil, with the energy
that goes to the capacitor being transferred to the output after a few
oscillations. Make a model with a transformer with quite high coupling
coefficient (try k=0.756, the first "magic value" for this
kind of circuit), a capacitive load (the primary capacitor of the Tesla
coil) and the snubber capacitor across the primary of the flyback.
Experiment with the snubber capacitor value until you maximize the peak
output voltage, without destroying your driver transistor. With some
luck
you will find a capacitor that maximizes the output without causing 
excessive input voltage. You will see that the primary and secondary
circuits are not tuned to the same frequency, as would happen in a
conventional capacitor-discharge Tesla coil. With k=0.756, the primary
capacitor charges up to a certain voltage, but discharges back to zero
at the instant when the output voltage reaches the maximum. At this same
instant, the currents at both sides of the transformer cross zero.
 
>  ORCAD seems like an amazing thing.  I get many of the same frustrations I
> get with real circuits (when they don't do what I expect!) without any of
> the smoke.  I can't get over wishing I had had something like this when I
> was a teenager trying to learn all this stuff in the first place.

Just take care with careful modeling, as the simulator doesn't have
much compromise with reality. "Garbage in = garbage out" ;-)

Antonio Carlos M. de Queiroz