SMPS with TOP244

Hi guys,

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.

Check R4 2M ohm to pin L, If this is open circuit or gone too high then it wont start. It kick starts the SMPS.

Thanks, I already checked that, there are two resistors in series of 1Mohm, they are good.

I think U1 is dead but first if D1 is shorted the supply will not start up.

What voltage is at pin X?

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.

OK, after changing the U2, VR2 (TL431), D2 and U1, I see how it tries to switch on but pulsing... so some more problem on the board.