We have got a few boards with failed power. Some boards went alive after replacing a bad capacitor (swollen can). Some boards went alive after replacing a zener diode (somehow shorted to ground I think. My colleague threw it away so I do not know what condition it was at). The said capacitor, C101, and zener diode, ZD1, are located in the lower left corner in the SMPS circuit shown below:
These two are neighbours so naturally I think they might fail due to the same source. However, I have no clue why they failed. Are there any design flaws in the circuit? These are of small percentage of the boards out in the field, but I think it must be some design problem that triggers the problem intermittently. Please also note that the capacitor that died in the field is rated 50V.
Here are some measurements at Vtrans, Vdiode, and Vzener. Vtrans (with load at Panel_12V):
Vdiode (with load at Panel_12V):
Vzener (with load at Panel_12V):
Vtrans (without load at Panel_12V):
Vdiode (without load at Panel_12V):
Vzener (without load at Panel_12V):
The waveform before D120 looks like this during a typical load at Panel_12V (It looks terrible but I don't know if it's normal or not...):
Zoomed shot of the waveform before D120 looks like this during a typical load at Panel_12V:
The board also emits a hissing sound (rather loud, compared to other boards with switching power circuit I have seen). I don't know if it's relevant to the cause of the bad capacitor and zener diode. It is also a problem we want to solve though.
Please find the datasheet of the SMPS IC, FSDM0465RB, here: https://www.fairchildsemi.com/datasheets/FS/FSDM0465RB.pdf