piclist 2002\09\21\022959a >
Thread: [EE]: 2-tran SMPS 5v reg now 90% efficicient!
face BY : Russell McMahon email (remove spam text)

> (Posted on [PIC] and [EE] as this is a 5v regulator design.)

Note that only ONE tag is effective (don't know which - probably the first).

> I think i've found the "holy grail" of 2-transistor 5v
> SMPS regulators!

That performance is utterly superb.
Haven't got time to play with it at present but I will definitely be
investigating it further. The porr regulation of the prior design made it
less suitable for many purposes than my 3 transistor version. The new
circuit's regulation make a lot of difference.

Re efficiency. Don't know what the Schottky drop is here but at 0.5v it
would add about 10% inefficiency and at 0.3V about 6% losses. This is of
course only during the "flyback" portion of the cycle. At 100 mA in inductor
losses in 0.7r are about Iout^2 x Rind = say 30 mW. This is around another
3%. [[Hard to simply calculate this loss due to two halves of cycle and
waveform shapes]].
So the result is extremely good already for what you are using. If you want
to cheat you could try it at Vout = 12 volts, where current losses will be
smaller and voltage losses will be a smaller percentage of Vout.

Adding complexity to such a fantastically simple circuit hardly seems
worthwhile but you could consider synchronous rectification with a FET
rather than the Schottky to get even less losses here. A second winding on
the inductor (complicating the coil) would provide an easy drive source or a
feed from Q2-collector is a possibility.

Don't be scared of an extra transistor if it has significant advantage -
transistors are (as you know) a very minor expense at this power level.

A vast advantage that this circuit potentially has over many simple IC
solutions, is that it has a sensibly unlimited potential maximum input
voltage capabiltity - dependant mainly on the Vceo of th transistors. At
very high voltages Rc and R1 have problems with powwer dissipation and in
due course voltage rating but this is unlikely to be a problem in most
applications this is aimed at.

The on web documentation is excellent and the full public domain release is
appreciated. .

Roman's original is at


       Russell McMahon


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Subject (change) [EE]: 2-tran SMPS 5v reg now 90% efficicient!

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