I have constructed a 12V@10A power supply using TOP257YN device (schematic attached). The input voltage to this converter is 300Vdc from PFC stage.
The entire design was done with PI Expert software (V8).
I observed a problem, during testing in the following sequence:
1. Upon first power ON, 12V comes up normally and remains in regulation.
2. When the power supply is switched OFF, after about 5 seconds, the 12V drops to 2.8V (This is due to the input hold-up capacitors - not a problem).
3. When the power supplyis switched ON again, the output is not coming (no 12V). However, 300Vdc is present and it is stable and regulating (from PFC stage).
4. After switching OFF the power and waiting for more than 4 minutes, until the input voltage dropped below 100V (300V input does not have any bleeder), I turned ON the power again.
At this stage, sometimes 12V appeared and most of the time it did not appear.
Following observations were done:
1. Checked for drain node switching for auto-restart - NO auto-restart
2. Disabled the OVP sensing circuit by disconnecting the resistor (R3=5.1k) to V-pin. Still the problem existed.
3. Shorted the V-pin to source (disabled UV/OV feature). 12V comes up every time properly. However, during turn OFF, after 12V turns OFF, it momentarily turns ON two or three times, while the input drops from 300V to zero.
Schematic and layout snapshots are attached.
Request you to please provide suggestions on what could be the problem.
Thanks and Regards