Optocoupler feedback in flyback converters

[mrinalmani]

Hi!
Most flyback controller ICs have a feed-back pin that needs to be connected to the output of an opto coupler. What is the mechanism behind this?
I mean what is required by the IC at the feedback pin? Some sort of voltage? current? I understand that it needs an optocoupler, but what is the optocoupler doing?
I have tried to find the working behind this but I can only find example circuits.

My real purpose is to use this IC NCP4371 which is a "quick charge 3.0" controller IC for flyback topology. But I need to use it in buck topology, not flyback. I cannot find any controller IC for quick charge 3.0 that is made for buck topology.
I want to tweak this IC for buck operation. An on board MCU will control the buck controller.


Please help...
Thanks

 

[KlausST]

Hi,

Function:
* when output voltage is high enoguh, then the optocoupler_LED becomes alight.
* optocoupler sensor detects the light
* and reduces or stops SMPS operation.

Klaus