Error amp circuit for opto isolated SMPS wont do AC analysis in LTspice..why?

[treez]

Hello,
I am trying to do an AC Analysis on the Error amplifier of an opto isolated Flyback SMPS.
However, the gain and phase plots are garbage, do you know what I am doing wrong?
Please find attached the Error amplifier AC analysis schematic, the flyback schematic, and the AC Analysis simulation in LTspice.

 

[dick_freebird]

Are you sure that the entire chain is operating at a point
where it's linear (ish) or at least not railed at some stage?
There could be an awful lot of gain in such a lineup. If it
was working.

But I notice your feedback seems to be going to COMP,
not FB as would be usual, in the flyback schematic. Why?

 

[FvM]

Usually a DC feedback is applied in simulation to put the circuit into the intended bias point. For circuits with moderate gain, it may be sufficient to vary the input DC voltage manually until the bias point is reached.

The circuit skips the LT1243 error amplifier and feeds the OP output directly to the comparator section. That's possible in principle, but I'm not sure if it's implemented in a reasonable way.

 

[treez]

Thanks, the flyback LTspice simulation, which incorporates this error amplifier is attached, and it does work well
https://www.linear.com/product/LT1243

 

[mtwieg]

I can't test the circuit since I don't have your tl431 subcircuit, but I'm betting it's due to improper DC biasing.

 

[treez]

In case you may wish to see them, Please find attached TL431A.sub and TL431.asy....as you know these get dropped into the relevant folder in the LTC folder