piclist 2001\01\20\084009a >
Thread: Design challenge - switching regulator
www.piclist.com/techref/power/priswitch.htm?key=switching
picon face BY : Russell McMahon email (remove spam text)



>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.
>
>I see so many self-ex switchmode primaries lately when
>fixing VCRs. They use one transistor/fet, one optocoupler
>and a few passives for the whole primary! You don't need
>the expense of a controller chip, unless you need really
>tight output voltage regulation.
>-Roman


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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