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Re: Solid-state coil (gate xfrmr question)



Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Carl,
          To answer your query about the drain-source voltages: 
flyback transformers are typically designed to store energy in the 
core/airgap during the switch "on" period, and release it during the 
"off" or flyback period. A supply is configured to operate as a 
flyback by the inclusion of a steering rectifier at its output. It is 
otherwise a common transformer. A true flyback topology reflects no 
load current directly back to the switch but uses the core as an 
energy transfer intermediary. In fact, the core can store very little 
energy before it saturates so all flybacks incoporate an airgap which 
does most of the storage (you can't saturate air).

     What you are seeing is some core energy returning to the primary 
winding and to a small capacitive load across it. It doesn't take 
much energy to cause a very considerable voltage hike. This is 
normally dealt with by snubbing circuitry (a lossy approach which 
goes against the grain with me) or by energy recovery back to the 
power supply which is either accomplished by an energy recovery 
winding and steering diode or a rather more specialized topology 
which requires a bridge network of switches and ultra-fast 
rectifiers. In both the energy recovery cases, winding turns ratios 
are important as they define both output voltage and drain-source 
flyback voltage since all windings are tied by common coupling. In 
fact, case #2 ensures that the drain-source voltage never exceeds the 
supply rail. That of course implies clamping of the output voltage to 
the supply voltage multiplied by Ns/Np if k approaches 1.

Regards,
malcolm

On 6 Nov 2001, at 14:31, Tesla list wrote:

> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<cwillis-at-guilford.edu>
> 
> 
> Hi Eddie, Ed, Malcolm,
> 
> Thanks for your most informative responses.  I just sent out another post
> to the list with the same questions because I thought my e-mail was down
> and not because your answers did not suffice.  I also put photos and some
> more description of the current state of this SSTC setup on the web at
> www.angelfire-dot-com/electronic/cwillis/sstc.html.  I have some photos of the
> oscillosope while the coil was running as well, so you can comment on how
> good / bad the setup is doing from an FET survival standpoint.  The biggest
> question still outstanding to me is why I get such a high drain voltage
> (about twice the DC supply voltage) on the pulses.
> 
> Eddie's suggestion that my ferrites were too small, wrong type and wound
> with too few turns sounds exactly like the case.  Although I don't know
> what types of ferrite these all were, at least one was a ~0.6" OD,  0.1 -
> 10 MHz tiny little thing and I only had ten turns on it.  I'll follow your
> advice and sack a few old switching supplies for their toroids.  Malcolm
> corroborated what you said also.
> 
> In regards to Malcolm's question about power supply, I am using rectified
> and filtered AC line power (and, until I'm really sure of myself, a
> variac.)  And finally, Ed, the setup is a conventional tesla coil and not
> an ignition coil.  I admire the GMHEICSLR (?) efforts very much but I just
> don't have the nerve to try to get 6"+ out of one of those things! 
> 
> Thanks, y'all!
> 
> Carl
> 
> 
>