Re: [TCML] Non-DR SSTC primary - interesting facts

["DC Cox" <resonance@xxxxxxxxxxxx>]

You might also consider some late model, more robust MOSFETs.

Fuji just came out with some small monsters with an incredible low "on"
resistance of only 0.11 Ohms!

They are p/n 2SK3680-01 in a TO-247 package and rated 500 V, 52 Amps
continuous, and 208 Amps pulsed.

They have just been released and I'm anxiously awaiting my samples which I
intend to use in an 8" x 40" SSTC system.
Pri is 6.5 turns tapped for resonance at 5.2 turns.

Dr. Resonance

On Tue, Aug 26, 2008 at 4:32 AM, TheBypasser <bypasser@xxxxxxx> wrote:

> Well, upon an attempt of further civilizing my flower-pot primary I removed
> a turn (getting 6 turns instead of 7) and blew two fets (was a pretty stupid
> idea - even the primary itself lost ~25% of it's inductance, first blow on
> the finished construct btw) - and the interesting part is - the spark almost
> didn't grow (was up to 60cm - remained the same). Of course I put the turn
> back and replaced the fets (working fine again, same spark - and the heating
> of the bridge is very subtle btw, even with my really bad cooling), but it
> led to some interesting thoughts, also based on my older research.
> The idea is - you have a fixed-height primary with a significant number of
> turns and start reducing that number one by one, keeping the height
> constant. At first you get a low transformation coefficient and high
> inductance, thus the power is too low. As you remove the turns, the
> transformation coefficient gets higher and therefore the power transfer into
> the secondary increases. That's good, but - you slowly increase the
> reflected power by doing that, therefore the SWR gets worse and worse,
> making LESS power supplied into the secondary. The question is - is the
> resulting power supplied into the resonator rising all the time, or there's
> a peak after which you start making the coil be able to transfer less power
> again? By the way (didn't test that on my new coil, actually as far as it's
> working well and never blows I'm not going to change much - 60cm on a bps up
> to around 30 with a pretty high pulse length feels like a proper result for
> a bridge of 30a fets ;) - and the secondary heats up pretty much anyway) the
> tests on the previous version of this coil (there was no feedback at the
> point, and the secondary was much smaller and ran at ~300kHz instead of the
> present ~150) showed that the primary height (meaning not how high the top
> turn is, but the distance between the top and the bottom turns - eg the top
> turn remained at ~1/4th of the secondary, but I tried both spreading the
> primary to the bottom of the secondary and winding it tight) doesn't affect
> much as long as the winding remains close to the secondary..
> So, is there a point of an "optimal" primary, at which further turn
> reduction has only negative effects?
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