you do not show whole circuit so it is difficult to help you.
But your connection to comp pin is wrong, it should be as an integrator, you are just using the internal error amp as an amplifier....
Also, your vin is a pulsed voltage?
Also, you use 1n4007 in RCD clamp, this not good, you must use ultra fast diode there.
Do you know uc3844 is 50% max duty
What is your vin, vout, pout? Cannot help you without these
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Also, what simulator are you using?...some of them cannot handle smps.
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Also, the uc384x series of controllers really need a schottky diode to be placed from pin 6 (gate drive pin) to ground, cathode to pin 6, this is to stop that pin from going too negative