1. We are trying reproducing the exact same design which was refereed in TI design PR798 (400VIN 12OUT 5W AUX BIAS SUPPLY). We have used the exact same transformer and all other resistor and capacitor as per PR798 design.
2. While we are in the process of bringing up the board, we have applied the supply voltage gradually increase from 0Volts to 400Vdc, during that time the primary out was turning off at 240Vdc.
3. Based on our observations we found out that 7th pin (OVP) of The UCC28600 is very sensitive (Also mentioned in datasheet)
4. As per the datasheet (based on the calculation) and PR798 design we have used Rovp1-143.3K and Rovp2=40.2K, We have observed that OVP pin of UCC28600 going to shutting down mode at 240VDC.
5. We also observed that the sudden turn on the module at 200Vdc also it is going to shutting down.
6. We also observed the drain to source Waveforms, Nbias waveform are satisfying as the waveforms referred in datasheet, and the OVP pin voltage waveform ringing voltage is 2.8Volts only, while we are increasing the input supply voltage the ringing amplitude is not changing. Even though at 240vdc it is going to shutting down.
7. We also tried replacing Rovp1 and Rovp2 resistors with different combination ratios to achieve the rated over voltage cut-off (over voltage cut off we need at 400vdc), but we are unable to make it work.
8. We also looked for unwanted noise on the OVP pin, based on our observations we did not find any noise at OVP pin waveform.
9. We also observed without mounting Rovp1 resistor and having Rovp2=40.2K, we can turn module up to 340Vdc input supply with load and without output load.
10. Any Assistance in resolving this issue is highly appreciated.