Re: LF: Advice (again) please

[brian hodgson <g3ykb@yahoo.co.uk>]

Hi Chris -

 

I've been dabbling with some PA circuits recently - the usual hard switching, push-pull fets and transformer output follwed by a pi (C-L-C) Lowpass filter.

 

One trick I did discover was that by increasing the inductance of the low pass filter (using the tried and tested technique of  poking a large ferrite rod inside the coil) from 120 to 185 microhenries the voltage waveform on the drains became a lot smoother - more like class E - and the overall efficiency went up to better than 90% (over 200 watts out for 220 watts input).

 

Adjustment was quite sharp but not "critical". Watching the drain waveform voltage on the scope was the best way to tweak but just tuning for minimum supply current seemed almost as good.

 

Brian

  
"OSBORN, Chris" <Chris.OSBORN@astrium.eads.net> wrote:

> I'm generating 100 watts into my 50 ohm dummy load but 
> only getting an efficiency of about 50%.
> The output transformer toroidal core is getting quite hot and I
> suspect that it's permeability is too low as the capacitance
> required
> to bring it to resonance is far higher than those of published
> designs 
> in the LF Experimenter's handbook, using the same number of turns.
> These capacitors are running very hot too (as mentioned by PA0SE in
> his LFEH article).

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