Glad to see you got a result from that idea! I know the
on/off regulation is a bit rough but what the hey. :o)

Just a quick note, I would get rid of the chip, you
don't need it. Run a one-transistor self excited osc
for the primary, and keep your zener/transistor to
kill it for regulation. In many of the new VCRs I see,
the osc is a high gain tran or fet, in a typical
self-ex configuration, and the feedback opto affects
its turn on period to give regulation. Very simple.

For the self-ex primary, try a high gain tran like
a cheap darlington, with base to + through 56k resistor.
This will give a fairly guaranteed turn on at most
voltages. It's emitter to gnd (obviously) and its
collector to + through the main primary. Then use
a feedback winding that turns the thing off. ie;
start resistor turns it on, feedback winding turns
it off, it oscillates. Now all you need to do is put
the other transistor in the feedback circuit somehow
and you can get anything from on/off to a reasonable
linear regulation.

Shouldn't be that hard, and takes two transistors, NO
chips and a few discretes. You can probably even get
rid of the zener and use the second transistor's
base voltage and a resistor divider, you will get
a few % voltage variance between units but this is
probably no big deal.

Sometimes simple is best?? :o)
-Roman

PS. One potential problem with self-ex primaries is
the worst case deal where the feedback winding can't
quite turn the transistor off. Can you say "exploded
fuse"?? Many circuits include a driver transistor
that switches the main chopper on, and if it doesn't
oscillate the driver tran can't keep the main tran
biased on. So it takes 3 transistors total. Such
is life! ;o)



Russell McMahon wrote:
>
> >Russell, what did you think of my self-excited oscillator
> >idea? This can be made to startup at 2v and will work
> >fine to 130v with a simple on/off voltage regulation.

> Well, I'm afraid that that seems to work quite well! :-)
> I was not too keen to go to a discrete design as parts can tend to
> proliferate and with the large input voltage range I was quite keen to have
> a closed cycle by cycle current loop as well as the voltage control loop.
> You can of course do that with discretes but again parts count rises. I
> think an LM339 quad comparator (which has the virtue of being about the
> cheapest IC out) would do everything and have 1 or 2 sections left over.
>
> However, I decided to try a "cheap and nasty" design using a 40106 hex
> Schmitt trigger.
> One gate oscillates and is followed by 2 more as buffers with a resistor
> from gate two out to gate 3 in.
> If you ground gate 3 input  (with a transistor or whatever) the oscillation
> output from gate 3 stops (this is a wired OR style AND gate) .
>
> Drive a FET with gate 3 using a 2 transistor push sort-of-pull driver.
>
> Two coils on core. One in FET drain and other is ringing choke output.
> Rectify, smooth.
> Connect zener from this output via a resistor to a common emitter npn
> transistor.
> When  Vout exceeds Vzener + Vbe (very roughly) the transistor turns on.
> Connect transistor collector to gate 3 input.
> Voila - voltage regulation.
> Very crude.
>
> At present I am driving 40106 from a fixed supply but it will self power off
> own derived supply easily.
> Should start with under 3 volts or so on Vcc and self boot itself to full
> output very quickly.
>
> With a 50% max duty cycle (oscillator was 50:50 square wave) Vout van not
> exceed Vin (but can be less).
> Adjust oscillator to 66:33 square wave as seen at   FET gate
>
> Output will now be "more" than input before regulation cuts in.
> In practice this makes Vout about 1.5x Vin which is not quite enough in very
> very very worst case but a little more playing (design?) will see it right.
>
> Works extremely well considering.
> Loop gain?, Phase margin?, .... ??? - you've got to be kidding :-) :-(.
> Now I've got to have a good look at this approach and see how it compares to
> the other contenders.
>
> 1 x 40106
> 3 x BC337 or equivalent.
> 1 x zener
> 1 x osc cap
> smoothing & decoupling caps
> diodes
> resistors
>
> Maybe I need to look at your entirely self oscillating design too.
> Back to the good old days of the self resonant ringing choke converter :-).
>
> regards
>
>       Russell McMahon

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