Re: Gap Dwell Times (formerly: Beating Solved)

To: tesla-at-pupman-dot-com
Subject: Re: Gap Dwell Times (formerly: Beating Solved)
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
Date: Mon, 30 Sep 1996 08:35:58 +1200

Jim wrote in reply...

> >I started from k approx dF/F, and ringup time = 1/2dF so you can see 
> >how I derived that. The problem when cutting the gap off (if you 
> >could) when Ip is maximum is that with k<1, most of the primary 
> >energy is coupled to the primary. I tried doing exactly this with the 
> >MOSFET gap and the spikes hit the roof. Virtually none of that energy 
> >was coupled to the secondary. The spikes in a real gap would have re-
> >ignited it anyway.
> Malcolm,
>     That artifact is called "leakage inductance" and it behaves
> exactly like one had put an inductor in series with the transformer.
> To make matters worse, it follows V= Ldi/dt, so the faster you quench
> your gap, the higher V will be!

I think this is another good argument for making the tank wiring as 
short as possible too. Not only does the energy in the wiring outside
the primary coil not get coupled to the secondary, but it is left free
to re-impress itself on the stray primary capacitances with major high
frequency rings the result - transformer dealing-to stuff. This is the
reason why wiring lengths around the transformer in a switchmode 
power supply are kept as close to zero as possible where winding
inductances in the tens of microHenries are the norm.

Thanks Jim,
Malcolm