Voltage ratings of capacitors: ceramic vs electrolytic

Question

Now I know when it comes to electrolytic capacitors it's usually good practice to use a voltage rating 1.5x - 2.5x the maximum rating you ever expect the capacitor to be exposed to, as the lifetime can be significantly reduced by running it near its limits.

But does the same protocol apply to ceramic capacitors? For example, assume a 25V ceramic capacitor - will running it at 24V reduce its lifespan significantly?

I'm currently designing a PSU which needs to be reliable, so at the moment I'm using 50V caps for a maximum 20V input (35V caps aren't available), but I'd like to switch to 25V caps if possible.

Answer 1

You do not have the same lifespan issues with ceramic caps. You will lose capacitance as the voltage increases. The amount you lose is dependant on the dielectric.

When choosing the electrolytics you also need to look at the ESR, temperature rating and the maximum operating temperature. Since the lifetime is predicted using the Arrhenius activation energy law running using a 105°C rated cap will give you a significant increase over running a 85°C cap (assuming all other conditions are the same). There are some 5000 hour rated electrolytics.

Answer 2

ceramic caps are really durable, MLCC's are often tested at rated to 200% rated voltage for ~1000 hours before failure.

That being said, I often find that higher voltage MLCC's can actually be cheaper than the lower voltage for a given capacitance due to higher industry volume for the higher voltage ratings. For instance finding 16v 0.1uf MLCC is never a problem, but something like 100pF may be cheaper at 50V than it is at 16V

Answer 3

You should actually not be comparing ceramics to electrolytics in terms of lifespan at all, they are actually intended for totally different purposes. Most beginners (I did untill recently) think they are just higher values, but they are as different as inductors are to resistors. Mainly because electrolytic values are more approximate, and their value much more temperature dependant than ceramic, polycarbonates or tantalum. Note: Tantalum are very voltage sensitive.

You answer is that I would also use a 50V cap for a 20V supply, in fact if you glance at the charge curve for electrolytics you will notice that they work better (more farads) at about 1/3 of their rating anyway. As for being for different purpose, you will get heating in an electrolytic if you allow it to completely charge and discharge on a cycle, for instance using a 220uF to service a 10 amp load on a rectifier will tax the cap and cause heating in the leads, and as the other answers alread indicate, heating equals less farads.

I'm just an amateur, so no clever math, sorry.