So you are just sensing the DC from the output and sending it back to the control circuitry.Sorry Crutschow,
This basically DC DC bidirectional ,the input to the primary is 40-100VDC and the output from the secondary is 500-1000VDC ,the switching frequency is 65Khz ,i need to sense the output voltage from the secondary side, the control is in the primary side,
That is a slow loop and does not require high speed response (a few kHz should be adequate).
Instead of trying to make the opto very linear to transmit the actual output voltage, you can put the voltage error reference circuit on secondary side of the circuit.
That way the gain or accuracy of the opto is not critical since it's inside the feedback loop.
Here's an example using a TL431 as the secondary voltage error circuit.