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:

Circuit protection

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?

  • \$\begingroup\$ Usually in the SMPS 90-240VAC there is a 275V MOV. \$\endgroup\$ Commented Apr 9, 2017 at 7:13
  • \$\begingroup\$ In my 24V-automotive-aimed low power (<50W) designs (which can operate between 18 and 36V), I use 47V 1210 case SMD VDRs. In offline systems, I use 275V VDRs. For fuse selection, I recommend this article. \$\endgroup\$ Commented Apr 9, 2017 at 7:21
  • 1
    \$\begingroup\$ Can you show us the circuit that the capacitor is in please? ... If the problem is anything other than "bad caps", or "dirty power," we have no way of knowing/answering your question without seeing the circuit that the caps are failing in. \$\endgroup\$ Commented Apr 9, 2017 at 8:39
  • \$\begingroup\$ You would normally have a MOV in your supply when you design it. If you didn't add one then you should. It is common to use a fusible resistor to both act as a fuse and limit inrush current. \$\endgroup\$
    – Majenko
    Commented Apr 9, 2017 at 13:24

1 Answer 1


If you design an SMPS, make it a standard practice to include a fuse and MOV. They ensure that your supply does not blow up in the field due to erratic input supply.

For the MOV, you may use 330V or higher rated part, since you mentioned your input supply in the range 180-280V. That should provide enough headroom. I design supplies for the range 180-265 and use 330V MOV. Works well for me.

For choosing the fuse, give this a read: http://www.powerelectronics.com/passive-components/selecting-fuses-simple-procedures-get-right-overcurrent-protection-dc-dc-converte

  • \$\begingroup\$ Thanks a lot, Sachin. I'll follow your advise. If you can leave your email, I'd love to connect with you and discuss more on designing power supplies. Thanks again. \$\endgroup\$ Commented Jun 9, 2017 at 12:17
  • \$\begingroup\$ @Whiskeyjack: You can mail me on [email protected] Cheers!! \$\endgroup\$
    – Sachin
    Commented Jun 10, 2017 at 9:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.