Re: flyback converter driving

On Sat, 20 Oct 2007 08:49:57 GMT, Jamie Morken <jmorken@xxxxxxx>
wrote:

Jamie Morken wrote:


Here is the current circuit:
http://www.nekrom.com/rocketresearch/new/flyback/flyback2.jpg

http://www.nekrom.com/rocketresearch/new/flyback/flyback2.asc
(ltspice file)

That circuit currently works and outputs 12VDC with R7 being 10k, but
when I switch it to 100k (to reduce the power dissipation) the shutdown
pin starts oscillating so that the device is being enabled/disabled, and
this doesn't allow the output voltage to ramp up. Any ideas on why this
shutdown pin is oscillating?

I think I understand what is wrong, the circuit starts up when shutdown
pin goes over the startup threshold, and then the chip starts drawing
a lot more current, which pulls the supply voltage down far enough that
the shutdown pin gets turned off before the circuit can run long enough
to charge up the supply.. so the problem is to find a way to keep the
shutdown pin above its threshold, the current zener/resistor doesn't
work. Thanks for any help.

The LT1619, with a 1V8 minimum start-up voltage, is not intended for
off-line applications. You are correct in observing that all you are
doing is modulating operation through the SS pin. In your
schematic\simulation, adjusting the D5 zener voltage or resistors in
this string regulate the chip supply, regardless of the presence of an
output voltage.

The Lin Tech version of the commodity UC3842 current mode controller
is the LT1246. You'll have better luck sticking this in your
simulations, though the compensation pin voltage is not accurate in
the currently available model.

Such a low power flyback may not justify a discrete controller or
switch. You could probably do it with a Power Integrations 8pin dip or
smaller part, by itself.

I'm thinking that simulation at this power level may be waste of time,
if not actually misleading.

RL
.

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