You have no capacitor from opto collector to ground, -there should be.
-You have no compensation components from cathode to reference of the tl431
-You have an output LC filter which has too high inductance...also, the most of the output capacitance should be upstream of the output inductor...also, the LC resonant frequency of the output filter is about 700Hz, and that's too close to what is likely the crossover frequency.
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Your 180k, 1w supply resistor is not of sufficient power rating
-You have no capacitor on the vref pin, eg 100n to 1uf
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-You need a ceramic cap say 100n to 1u on the vcc pin of uc3843
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Are you now saying you only need the 5v output, and not the 15v output?
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Stacked winding will be troublesome with so much current flowing in the 5V secondary, it means you will get cross-regulation problems, but it may work to a degree.