I have designed an SMPS circuit in which i have seen bulk capacitor getting damaged in a few cases (2-3 cases out of 100, which is still huge if I am aiming for a bulk production). The circuit has been running stable for over 6 months at some places and I have seen the capacitor failure within days at others. As such I feel that it could be an edge case between capacitor quality and power line stability. It could be that capacitor is just on the verge of failing and if there are minor surges or power line fluctuations, the capacitor gets damaged. Am I correct in thinking so or should I investigate something else in the circuit?
To mitigate this, I am planning to make two changes - (a) Use a branded capacitor like panasonic or so and (b) Use some sort of surge protection circuit as shown below:
This is an inexpensive circuit that I could come up with - a fuse in series and MOV in parallel to absorb spikes.
1) What current rating should I choose for the fuse? Taking care of circuit inefficiency and a little headroom, I guess 50 mA is a safe number. Shall I choose next higher rating available? Is there a better way to do this - like using a PTC?
2) How do I select MOV for my use case? On digikey, there are fields like Varistor voltage - Min, typ, Max, energy and Max AC volts. What values will be suitable for my purpose?