by Min Zhang, the EMC Consultant
X type capacitors are heavily used for differential mode filtering, they are often seen between the mains AC lines or across the high voltage DC link.
Generally, the X capacitors perform the following functions:
- Attenuate the conducted noise directly from any switching event on the lines (for instance, a Triac switching the mains AC line).
- Together with an inductor, it forms a low pass filter for differential mode noise appearing on the lines.
- It helps limit thee peak voltage stress on the lines if there is a surge event.
You probably already know all of these, but do you know, the capacitance value of an X type capacitor could reduce by a lot over the years? This is particularly true if the capacitor is used in humid environment. I have seen cases where in a year or two, an X capacitor’s capacitance value dropped to only a few percentage of its rated value, so one ended up with a system without X cap effectively!
So what has been happening?
Damp air can leak into the capacitor up the wires and between the box and the epoxy potting compound. The aluminium metallisation can then be oxidised. Aluminium oxide is a good electrical insulator, so the capacitance reduces. That’s one problem that all film caps can have. The problem I was talking about was the film thickness. The reputable cap brands like TDK, Vishay etc use a thicker film and a subsequently larger cap than other brands. The thinner film makes the cap less robust to overload (voltage, current or temperature). Less likely to self heal too.
If the X capacitor is not permanently connected to the supply, then you have less concern. For instance, for a product that has a hard switch between the mains and the capacitor, size probably will be more important than life time, you can then choose a thinner capacitor.
If the capacitor is permanently connected to the supply, then it has to be highly reliable. Oxidation of capacitors is not inevitable. if the epoxy material of the capacitor is well chosen and the temperature extremes aren’t too much, the capacitor value degradation should be minimal.