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Re: Solid State - air core driver??
Thanks all for the response.
Harri wrote:
>Just drive the fets with transformers and different
>source voltage is no harm. Bridge type setups are much more forgiving
>becouse they clamp the voltage to the rails.
Yes clamping the voltage to the rails is the main reason that I am no longer
blowing FET's. The circuitry is all working in a predictable manner.
I've seen schematics for transformer driven FET's but few details on
the transformers.
I'm currently using a 4421 driver chip (it can sink 9A and work with loads of
10nf) to drive 4 Fets.
First question.
I have a couple of ring cores (Philips ferro cube toroidal material 3C85,
13mm od, le 30.1, Ae 12.2mm^2 AL 1000nH.)
Can I use one of these to drive all 4 Fets. (primary and secondary what
5? 10? turns interleaved. I don't need a seperate transformer or a seperate
secondary for each fet do I? (I will put a schockly diode on each side and
10ohm resistors to the gates).
If I can get to the exact spec on the pulse transformer then I think I am
in business.
2nd question if I build a full bridge then presumably I only have to
current limit the bottom fets? If I wanted to build a half bridge then
I have another schematic showing a current transformer. Driving a full
bridge rectifier, then a voltage divider driving the current limiting input.
Again no transformer details, though the diagram shows one loop one
the current side, many more on the other side. Again can I use one of
these little cores, 10 turns to 1 turn or what?
> As a rule of thumb take around 2mH primary for 200kHz,
>2mH for 100kHz and so on. Then use the turns ratio as the required
>voltage step-up needs. (Yes 2mH with air core will be huge.
I take it thats 1mH for 100kHz?
Yup I reckon a 2mH air core would need about 175 turns -
on a 2" * 6" (r,h) former and it would have to be heavy guage wire)
10 times what I was calculating before.
And then the secondary on top, its do-able but as you say huge,
and of course it cant be
near the steel box holding the electronics or to the Tesla
secondary. (I wouldn't want to wind an air core toroidal!)
I'll try the etd49 2mH primary. Secondary - I've no scope
so its suck it and see - what 10:1 step up.
3rd Question - how do I avoid flash over inside the transformer
- a real pain with the little pot cores. (short of immersing in oil).
4th Question (only another 597 to go). If I put a transformer
inside a full bridge do I need a capacitor in series. If so
would 5 * 0.22uf 250v class y suppressor caps in // do the trick.
(at 75v and at 150v).
I'll stop before I ask the meaning of life the universe and
everything.
thanks everyone,
Alan Sharp (UK).