Use offline PWM controller for non-isolated DCDC?

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cupoftea

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Yes a suitably slowed op-amp could be used - as could a cheap opto

running the opto into a de-generated xtor would give the same " pull down only " drive to the FB pin ...
 
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..Thanks yes ill have to look into that...i've often tried to slow up an NPN so i could "make an opto", (but without it being an opto)......so i put resistance in the emitter of the NPN, but then the voltage across the degen resistor goes up more than would be wanted.
 

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Thanks, i presume that the NCP1239 , cannot have an opamp output directly connected to its FB pin?, and that a diode would need to interpose.....ie, anode to FB pin, cathode to opamp output.....(the external opamp, of course, being the error amplifier)
 

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it can - just add a resistor to limit current into the pin ....

also use a zener clamp on the f/b loop to limit the Vmax out of the op-amp
 
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