Update: The most common large ceramic caps I use are some 10µF 0805 16V X5R ceramics from AVX. No mention whatsoever of capacitance loss over voltage in the 3 page datasheet, and you have to dig down to page 83 of the generic information for their entire family to find a single chart on this (shame on AVX) for such significant information and they only discuss AC, without going into any detail over package sizes and with DC voltage, etc. … though perhaps I just missed something? Seems like a good experiment to pull some caps out and check the numbers myself!
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Wow! If capacitance can vary so much over bias voltage, how linear are these things when you run AC with voltage swings that are a significant proportion of their voltage ratings? And what frequencies are most affected if we have to take internal swelling of the ‘plates’ into account?
@KA10S, it depends on what frequency the AC is. At low frequencies you will actually see a rise in capacitance. The change is due to the dipoles in ceramic aligning with the electric field. At low frequencies, this alignment actually causes a slight rise in K.
The temperature and voltage coefficients are one of the reasons KEMET offers a simulation tool. You can apply temperature and DC bias to ceramics to see how their capacitance will change. Our tool is based on actual measurements, not just calculations.