Flyback converter feedback isolation with opto-coupler

[blah1]

Hi,

I'm building a voltage feedback flyback converter to turn a 12v input into a 24v output at 1Amp. I've successfully simulated and built this circuit on breadboard without isolation. Now I'm adding isolation. A tranformer in the forward path and an opto-coupler in the feedback path, but I'm having major headaches with the opto-coupler.

I've never used optos before and hence I'm a little lost. I'm just trying to put an opto in feedback path for isolation, keeping my compensation network on the primary side. So far I got whats in the attached image. This works great but obviously there is no isolation in the feedback network. I tried putting an opto-coupler in many configurations but it never transmits the voltage level properly causing the SMPS output to rise to about 70v instead of settling at 24v.

Can somebody please help me out with figuring out how to correctly use this optocoupler.

Thanks,
-Igs

 

[FvM]

There are tons of applications circuits with isolated voltage feedback. Most use a TL431 as voltage reference on the secondary.

 

[peggy]

sorry..what flyback transformer used in this converter?

 

[Orson Cart]

Possibly a linear opto coupler circuit (IL200, IL300 series) to give you a re-creation of the o/p volts on the pri saide that you can use for regulation.
Regards, Orson Cart.

 

[grittinjames]

817 Opto will do?

 

[Orson Cart]

Nope - not if you want the regulation circuitry on the primary side.