[Big Bad Bob]

On 02/13/11 20:39, flipper so witilly quipped:
But with SS mosfet devices either
ON or OFF, effciency gets high and it seems not so difficult to make a
square wave oscillator which might run silently at 25kHz, maybe no
need to fancy-smancy add on chips and crap one sees in schematics for
most SMPS.

The "fancy-smancy add on chips and crap" is there for a reason.

maybe. easiest way to make a switcher is to use an external oscillator,
then a simple voltage comparator driving 'and' gates on the inputs of
the mosfets driving the transformer (usually through an optoisolator).
When voltage is below the desired level, allow signal in. When voltage
crosses the threshold, no signal on the mosfet gates. A similar circuit
could be designed using pentodes and triodes, and for an optoisolator, a
simple neon bulb taped to a photocell with black electrical tape. FYI
Fender did this for their tremolo circuits. Anyway, it's an exercise in
"can it be done" more than "should it be done".