Re: It works! Solid state driver (fwd)

---------- Forwarded message ----------
Date: Tue, 16 Dec 1997 00:38:53 +0200 (EET)
From: Harri Suomalainen <haba-at-cc.hut.fi>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: It works! Solid state driver

On Sun, 14 Dec 1997, Tesla List wrote:
>    I will probably change pwm chips as well. I'm in the process of
> looking at some parts made by Telcom Semiconductors Inc. They are
> available from Digi-Key. As well as that Hexfred I was told about. 

Almost any chip should be able to do the current limiting pulse-by-pulse.
Using external diodes should only be needed *below the exact resonant
frequency*. Operating at that region would not be wise for other reasons.
Zero voltage switching is got above the resonant freq. That will reduce
switching losses to zero.

> > Gapping the core to make it able to handle enough dc component as well
> > is probably the best solution to try out with push-pull.
>  What about U cores used in TV flyback transformers? All you would have
> to do is slide a piece of paper between the two halfs.

OK if leakage reactance does not matter. I've (unfortunately) been
unable to get any U-cores except the ones from used flybacks. It should
work as well. It gives you more room for the winding too! If winding
fits I prefer the double-E cores.

> > Bridge topologies on the other hand are easier in this problem: just put
> > a dc-blocking cap in series with the transformer and the problem is
> > solved.

[question about dc blocking cap elsewhere]

I use polypropylene caps (readily available, used in other PSU's too!).
Polyester caps are certainly out but there are lots of polypropy caps
available fortunately. They have low esr/esl and are therefore capable
of low-loss caps for the frequency range I've used (around 200kHz). I
think (and have read in the literature) that up to 1-2MHz operation would
be feasible too but that has not been tested nor calculated by me

>     Has anyone tried series zener diodes across the drain/source
> terminals to limit flyback voltages? Digi-key has some transient voltage

Sure, it should work at least in theory. However, fast suppressor average
power capabilities are usually low.

> suppressor zeners rated at 1500watts. I know this is only for a few ms
> but they also have a steady state power rating as well. Maybe 4 or five
> in series in addition to another clamp that would take the brunt of the
> power? These zeners are supposed to have a clamp time of 1 pico second!

They are fast. Series use is ok and a working idea. However, in bridge
topologies you have no voltage spikes which the parasitic/external
diodes would not suppress already.
Harri.Suomalainen-at-hut.fi - PGP key available by fingering haba-at-alpha.hut.fi