Electronic – Ceramic caps and applied voltage


I'm using a 22u 6.3V X5R 0805 ceramic capacitor to filter out a 3.3V supply (for a buck converter.) I've heard that applied voltage has an effect on the capacitance of certain ceramic caps; a higher voltage causes a reduction in capacitance. How true is this? Would it decrease too much as to not work as a suitable filter cap? I think 10-15u is probably the minimum for it to remain stable, although the ripple will probably increase at lower capacitance. Any ideas?

Best Answer

This page has the following table:
alt text

So, if the 22uF is a 20% tolerance part such that is may actually be 17.6uF (-20%), and it is really 15uF (-15%) due to heating, it may be reduced to ~10.5uF (-30%) near its tolerance or ~14uF (-5%) near its operating point. To reduce the effect, use a higher breakdown value capacitor.

Although this webpage tests much higher breakdown voltage ceramics, the results are illustrative: alt text

This part, the 08056D226MAT2A by AVX, states that it meets its tolerance tested at 0.5 VRMS, not close to its breakdown:

For Cap > 10 μF, 0.5Vrms @ 120Hz

It also mentions that capacitance may change up to 12.5% over its load life, 7.5% due to soldering thermal shock, and 12% due to flexure fissures (cracks).

Info tidbit: The term X5R means it is composed of a class 2 dielectric (ie: ceramic) that will maintain its capacitance to within 15% (18.7uF - 25.3uF) over a temperature range -55oC to 85oC.
Something that may interest you, though it has more to do with the final application than the question -- the great 'pedia also mentions:

Due to its piezoelectric properties, they are subject to microphonics.

... And the previously linked page:

High-K ceramic capacitors can show significant piezoelectric effects; if you tap them they will produce a voltage spike. This is caused by the barium titanate, the main material in high K ceramics. The higher the K, the stronger this affect.