I´m repairing a treadmill board with SMPS integrated. It uses TOP244YN. The power supply is not switching on. I can see the 320Vdc at drain pin but not switching. I have checked the external components involved with TOP244YN and I don´t see anything wrong. I have replaced the the TOP244 by a TOP245 and same behavieur. The coils of transformer seems to be all good, so I don´t know why it don´t try to switch on as start up bias.
I attach a very similar schematic extracted from TOP244YN datasheet.
I have found SMD D2 damaged, after replacing is still not working, only when I switch off AC power I see how it tries to switch on, I think may be also U2 and TL431 can be involved with the problem. I don´t have time to continue with this until next week, I´ll keep you informed about what I have found. Thanks.
Any failure of D2/U2 and associated components would result in lack of feedback to pin C, so the SMPS output voltage would rise uncontrollably and do mischief.