Peter L. Peres wrote:
>
> On Sun, 29 Sep 2002, Roman Black wrote:
>
> *>There's not a lot I can do with the buck diode, the one
> *>I have now is 0.36v @ 3A x 60% which is 0.648W and

> Of course you know that you can use a bipolar as buck 'diode' too. Try it.
> You need another PNP with E at GND and negative base drive during flyback.
> I'd put it in parallel with the diode for a first test.
>
> I don't think that you can improve efficiency very much without adding
> lots of parts.


Hi Peter, yes you are right about the efficiency.
As the simple circuit stands it is very good when
tuned for a set input voltage. Most improvements
come from making the efficient range wider and
flatter, which comes only after adding circuit
complexity.

Re the MOSFET diode issue, tests showed me that it
only saves maybe 0.15v saturation over the good and
cheap schottky diode, total efficiency change is
around half a percent which is just not worth it
for all the extra complexity.

It is what it is, for two *cheap* transistors and
a decent input range it can't be beat. Once you
start adding better comparators and better timers
then it's worth it just to buy a dedicated
switcher chip. :o)
-Roman

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