piclist 2002\11\04\043522a >
Thread: Son of simple step-up SMPS challenge
face BY : Russell McMahon email (remove spam text)

> > Here's a simple RC oscillator that MAY show promise for driving very low
> > voltage boost inverters.

> I'm guessing from looking at the circuit that
> it wouldn't saturate well ie not a great square
> wave in the inductor??

Not with the 2 transistor design, as you would expect. I only tried that at
the end of the session just to see how it would go. A 2 transistor version
could be optimised but I don't see that as its target. When used as a driver
for a 3rd transistor base you can get emough drive for saturated operation.

> I think it is really going to need the 3rd transistor
> to be a viable SMPS power circuit, and still don't
> see any real advantages over my (or Dave Tweed's)
> 2-tran circuit. :o)

As I noted, it's a two edge sword. As I noted, it frees up the oscillation
frequency and control from the inductor characteristics or how you drive the
inductor. This means it's hard to get every last skerrick * of performance
from the inductor BUT it would probably make it easier to drive it well
without the drive tailing off at the ends. (This is essentially a Schmitt
trigger oscillator which should allow.reasonably good transitions due to
regenerative action).

If you want absolute minimum component count the 2 transistor design is
probably a better bet.



* skerrick n.
(usu. with neg.) esp. Austral. colloq. the smallest bit (not a skerrick
left). [N. Engl. dial.; orig. uncert.]

The Oxford English Reference Dictionary, ) Oxford University Press 1996

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