Thanks, i see it is a P controller indeed, and i noted that on the schem the C21 and R18 are not fitted.
I think it would be creating unecessary difficulty for the OP to try and stabilise the circuit using just an error amp configured as a simple inverting amplifier, since already the OP is expressing difficulty.
If there was a capacitance at C21 then at least the OP would have a pole to play with, with which to help get stability.
This type of compensation, as you know, compared to integrator type feedback, has low gain at DC, and therefore exhibits relatively poor load regulation.
Its nice to have a pole in that error amp, so that you can roll the (open) loop gain off and get your crossover frequency down to a point where you can get a nice gain and phase margin.
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As is known, in some power converters, there are right half plane zeros, and for these, we need the crossover frequency to be less than one fifth of the RHPZ. In these cases, its very useful to have an error amplifier with a pole there so you can roll your gain off well short of the dreaded RHPZ. The OP may struggle to do that with their chosen compensation method.
Also, there is very little literature available for SMPS’s compensated with anything other than type 1 , type 2, or type 3 compensators…so the OP, will be struggling and without the comfort of some tutorials.
I would think the OP would want to move away from the shown method of feedback compensation.