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Thread: Design challenge - switching regulator
face BY : Roman Black email (remove spam text)

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)

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:
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See also: www.piclist.com/techref/power/priswitch.htm?key=switching
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