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There are a lot of reasons for a PWM controller to not work propoerly, have you tried to check the pin to pin waveform of the controller and compare it with the datasheet parameters?
May you please share the overall spec and the line and load conditions when you see this behaviour?
Does it exibit the same behaviour at all line and load conditions?
Is the controller not starting up or entering a fault? what does the power stage look like, would you be open to share your schematic?
The Start-up threshold of UCC3843A is 8.4V typical. You will require atleast this much voltage to ensure startup. One way of chcking if the PWM has started is to check that the drive pin(OUTPUT) generating pulses.
