Re: It works! Solid state driver

From: 	Harri Suomalainen[SMTP:haba-at-cc.hut.fi]
Sent: 	Monday, December 08, 1997 3:43 PM
To: 	Tesla List
Subject: 	Re: It works! Solid state driver

On Sun, 7 Dec 1997, Tesla List wrote:
> >huge current. Make sure your pulse-by-pulse current limiting is working.
> How do you implement this? My thinking would be to turn off the FET when a
> comparator, monitoring Drain-Source voltage triggers.

Not a working idea.

> Using a current-sense winding off the transformer would fail if the core
> saturates?

Use a separate current transformer to measure current (if transformer
coupling is wanted). Make sure CT won't saturate before it is capable of
tripping the over-current system.

The other method (good for push-pull etc) is to include a (non-inductive!)
resistor in series with the drain (usually from drain to gnd). Monitor
the voltage across this resistor. Do include some sort of low-pass filter
like an R-C circuit to filter out the leading edge spike. Lots of examples
of this methods are available in most pulse-width controller chip data

> I see this feed-back connection, for pulse-width current regulation, putting
> some interesting poles in what would otherwise be a simple, clean transfer
> function. Yuk. Good thing I've got a simulator. This may also mean using
> clocked logic (flip-flops) to gate rather than simple gates, so higher
> frequency oscillations don't occur, resulting in terrible switching losses.
> OK, I need a switching regulator chip. Recommendations?

Oh, forget the poles etc. The coil driven at (almost) resonance will
appear as pure resistive load. After it has stored enough energy it will
give a spark (perhaps after a few cycles or so). Then it will go into
*high-impedance* state. Current protenction comes here if the transformer
is not gapped enough to take the transient. Then, pure resistive load
again. By resistive I mean it is like a resistor but draws sinuoidal
current because of the series LC-network.

You can use transmission line theory to calculate the power it takes
(the value of the "resistor"). If theory is not wanted just try it out.
Make 1st eg. 1:2 transformer turns ratio. Add more turns for higher
voltage and higher power. Test it every now and then. Finally, it takes
just the amount of power you want. Easy, uh? No simulation required.
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