Use offline PWM controller for non-isolated DCDC?

[cupoftea]

Hi,
We want to use the FAN7544 for a non-isolated SEPIC converter controller....60Vin to 15v 2A output.....no opto needed...so can we just have an external error amplifier, courtesy of a MCP6411 Opamp, and connect its output to the FB pin of the FAN7554?

FAN7554

https://www.onsemi.com/pdf/datasheet/fan7554-d.pdf

MCP6411

MCP6411 Datasheet by Microchip Technology | Digi-Key Electronics

View datasheets for MCP6411 by Microchip Technology and other related components here.

www.digikey.com

 

[Easy peasy]

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 ...

 

[cupoftea]

..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.

 

[Easy peasy]

resistive divider to base, e.g. 15k : 1k to linearise base drive ...

 

[cupoftea]

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)

 

[Easy peasy]

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