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.
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