Hi Russell, i've been following your challenge
for simple SMPS to give a small cheap 5v regulator.
This is the first chance I had to give it some
time, here is a quick attempt at a solution
using one transistor as previously mentioned.
:o)

http://centauri.ezy.net.au/~fastvid/smps01.gif

Please forgive me if there is a major error or
problem, this was done in a bit of a hurry and
I haven't built one yet. :o)

* R2 D2 C2 are a crude 5.6v regulator, which
clamps the base of Q1 to 5.6v via the low ohms
base resistor R1.

* large ouput cap on emitter of Q1 acts as an
ac decoupled voltage reference, so Q1 turn on
is greatly affected by this voltage.

* this gives simple negative feedback, ie output
volts will never get much above 5v as it would
become impossible to turn on Q1. Hence meets
the goal of simple voltage regulation.

* L1 and D1 are slightly unconventional, load
current is supplied only when Q1 is on, and when
Q1 turns off the flyback dumps excess energy back
to the supply. This should be very efficient, there
are no resistors in the main current path,
but it will give higher load voltage ripple.
It might need a RC or even LC pi filter on the
output, but that's easy for small loads like 150mA.

* Oscillation is (hopefully!) provided by the
correct choice of feedback winding and R3. Ideally
this should oscillate Q1 base around 5.6v giving
good hard Q1 turn on/off while not affecting the
average voltage at Q1 base by much. Playing with
R1 and R3 should give reliable start and oscillate.

I'm sure this circuit can be made to oscillate
fairly reliably. I'm also sure it would give crude
regulation, and with pretty good efficiency.
Some problems might be; range of input voltage,
and range of load current. Then what do you expect
from one transistor?? :o)
-Roman

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