Hi!
We are using the UCC2897A in a design converting 90V-250V to 24V (70W).
So we use a series regulator to lower the VIN-voltage to approximately 20V to deal with the 110V limit on VIN. Our bias winding is week at low loads so we need to supply the VDD via zener in light load situations. This works fine and so does the rest of the converter but sometimes, after running several days, can the converter die. I still haven't figured out what happens but it is difficult to start it again. Wait a while and then it starts as normal. This happens on several converters.
I can't measure on the converters since they are in system tests but I have triggered the fault once in our lab.
After this I have provoked the converter wit heat and re-starts and sometimes the converter will not start. In this case has the control circuit normal voltage on VIN and VDD but the control circuit appears to be dead or sleeping, VREF = 0V etc. The series regulator becomes hot and the input current is 15mA-20mA which almost all is passed trough the UCC2897A, but it is not over heating as far as I can see with IR-camera. The cooling via the thermal pad prevents this. Or is it internally?
I have checked the start conditions p.18 and it looks okay.
The output has a large capacitor for hold up so the output is remaining high (not zero) when the input is cycled. This seems to work in our tests but there is no pre-bias information in the data sheet. Maybe we have missed something regarding this.
Any ideas what to search for in my investigations?
Best Regards / Daniel