Re: Solid State coils.


Ive had fun, and smoked a lot of FETs
driving my primary coil. But I think I
need another approach. 

My present circuit is essentially a big
flyback transformer. Set up a current in the
primary, switch it off and in theory the energy
has no where to go but into the secondary.
In practice, very high voltages appear in the
primary, with unfortunate effects on the
electronics!!! It runs better clamped but
it looses a lot of voltage. 

I may in time try a full bridge but I havent
tried driving high side FETs.

Im going to go to bottom feeding.
I have ordered a transformer (ETD 49).
Im planning to drive it in push pull, I
think I know how to do this.

Could an electronics wizard tell me
if Ive got all this right:

1. Abandon the strip board for a double
sided PCB. One side devoted to ground
(ground plane). Power connections at
least 1cm wide, hot shoe the FETs.
Short path to gates, snubbers as close to
FETs as possible. Ill get some Ferric
Chloride and a paint brush and some
gloss paint. (Ill get into photoresist
some day). I remember in the physics
faculty etching PCBs in seconds using
a concoction of hydrogen peroxide and
conc hydrochloric acid - wicked!

2. 2 IRF740s on each side, mounted
on a 2 c/w heatsink have handled
2A -at- 150v, 200kHz continuous.Ill
use 4 to allow more power later.

3. Transformer design.
Never done this before.

I have the formula, giving the minimum
number of primary turns, in terms of
voltage*period, and B max flux density.
B max  is 320mT but this reduces for
push pull and further reduces because
of core loss to about 32mT.
I guess that I want at least a 10:1 step
Build carefully lots of insulation between
I was considering lacquering each layer
as well. Any tips?

4. New secondary.

I have a polythene bucket, diameter
11.5 height 14. I plan to wind the full
height. And mount a 12 by 2 toriod
about 4 above it.

In theory using r*r*n*n/(9h+10r).
this will have about 6 times
the induction of my 4 x 24 coil if wound
with the same 24 gauge wire, but the
induction nearly doubles again if I
use 30 gauge - so I think Ill go for
30 gauge. That should sink the frequency
well under 150kHz.

Comments tips suggestions anyone?

Alan Sharp (UK)