solid state: secondary, topload?

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Original poster: "Jan Florian Wagner by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jwagner-at-cc.hut.fi>


Hi,

I've a HV switch mode power supply working, max freq 200kHz. Power should
be about 500 watts (well more is possible but I don't have enough mosfets 
yet).

The problem is, what secondary coil to use for such low frequencies and
powers? 

I already have built a 4" x 26.7" secondary wound with AWG#25 wire
(actually 0.5mm = 0.0197" diameter wire), on cardboard wrapped in
teflon sheet, gives about 1380 turns, has low DC resistance and skin
effect should not matter much, but, turns out, the self resonant frequency
is still around 300kHz. :o(

Is it possible to get a coil resonant at 150kHz and still have it decent
sized to fit on the table? 

I already considered thinner wire, but then resistance losses would get
worse. Wouldn't want to run the TC as a space heater... :o)

Also, what type of topload is best? 
It seems like VTTC coils don't have any or then a neglibile topload. On
the other hand, a large topload would store more power (but causes more
stress on the SMPS) and also makes the frequency low enough for my SMPS.
But will a SSTC with a huge topload work at so low power levels? Any
experience with this?

(I'm also considering building a SMPS driver "chip" from scratch with
discrete components (as Marco from proj. THOR pointed out, the UC3860
sucks) to get the frequency range I want. But if big toploads would work
then I could save some bucks by sticking to the old LM3524 driver...)

Many thanks!
 - Jan

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