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TL431 and PNP for SMPS feedback.

eem2am

Aug 3, 2009
429
Joined
Aug 3, 2009
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429
Hello,

I am doing an isolated, offline flyback SMPS.

I must have low standby power, and also, very little current (less than 30uA) in the opto-diode during heavy load.

so i must use this circuit
http://i54.tinypic.com/295r18x.jpg


.....but do you know of problems with this?....will the feedback loop gain be too high due to the PNP amplfier?




The PWM controller IC on the primary side has a feedback pin which is connected to the opto collector...........just 90uA COMPLETELY stops switching.

i cannot use this circuit, as the parallel resistor invokes current in the opto-diode http://i53.tinypic.com/300qt8l.jpg

...due to the volt drop across the opto diode
 
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