How can I lower the chance of burning out capacitors?

Question

I have an AC/DC power supply circuit fitted to many machines, which are all running well within the rated power capability of the supplies. However these keep failing, every one burning out a (large) electrolytic capacitor (after say 1 years' service). Oddly these boards vary slightly, some have one cap, some 2, but invariably this is the part(s) that fails.

The boards with 1 cap are rated at 22µF 450v, and with 2 rated at 47µF 450v.

So, what can i do to take the stress off this component? run several in parallel?

UPDATE: Having had a good look over the PSU's they are indeed weird! the motors in the machines (which are cardboard shredders) are rated at 220v DC! so the main "chunky" part of the PSU board is just a rectifier/filter cap (which is the bit burning out) with no transformer, and the rest is a smaller, separate step-down DC supply for things like lights, switching, protection etc.

Answer 1

The typical universal input range switching power supply uses a circuit topology much like shown here:

The larger valued high voltage capacitor (highlighted in yellow) has the job of smoothing the rectified DC voltage from the AC power line.

There are two main failure modes for this capacitor. One is high voltage spikes at the input of the supply that make it in through the common mode choke. Spikes in excess of the capacitor voltage rating can cause damage to the insulating dielectric layer of the capacitor leading to internal shorts. High voltage problems should best be solved by finding the source of such spikes in the power system and taking steps to clamp spikes where they are generated. It can also help to improve the input filter of the power supply, add transient absorbtion devices such as transzorbs or MOVs to the input section and increase the voltage rating of the large capacitor.

Another failure mode is the internal heating that can occur when current changes in the capacitor reacting with the series resistance (ESR) of the capacitor. This generates heat that can dry out the internal electrolytic materials in the capacitor which causes a decrease in the capacitance. It can also increase the series resistance thus causing additional heating to occur. In the off-line type power supply this capacitor is working at twice the line frequency and the current pulses in the capacitor (known as ripple current) occur as the capacitor is charged on each half cycle and discharged as the rectified AC voltage goes to zero whilst the capacitor is asked to continue to supply current to the output sections of the supply. Several factors can be considered to increase the reliability of this capacitor. First and foremost is to calculate or measure the ripple current during supply operation and make sure it is well within the ripple current range for the selected capacitor. Another parameter to consider is the temperature rating of the capacitor to ensure that it higher than the operating temperature of the power supply. Finally it can be useful to to find capacitors that have a lower ESR value which will lower internal heating for a given amplitude of ripple current.

Answer 2

Using two capacitors in parallel will be equivalent to using two resistors in series, the capacitance will add up, and it may adversely effect the operation of the circuit (it may not, but thinking worst case here.)

A better way to relieve stress from these components, without any knowledge of the circuit itself would be to use a capacitor with a higher voltage rating. This will translate to a larger, more expensive component but it will stress less. Another way would be to get a "quality" capacitor, from a brand name supplier. There are audiophile capacitors which are known for their quality, but I am not familiar with the exact brand names.

Answer 3

If it'd be possible (given the size constrains that you have), I'd de-rate your capacitor (use a higher voltage rating than required) and also put a smaller ceramic capacitor in parallel. These are more tolerant to short high-voltage spikes and will help reduce the stress on the electrolytic.

Sparkfun recently ran a video related to this, I'd recommend you to check it out.

Answer 4

IF they are burning up quickly, there is surely something wrong with the circuit. caps do die eventually, but they should last years if everything is designed right. That said, a higher voltage High quality cap will be less likely to blow. depending on the purpose, you could even put one with higher capacitance to avoid it discharging and recharging fully quite so much. That is only a good idea with caps meant to smooth out the power supply, or something else where the actual capacitance is not very important.