Capacitance variation with applied voltage

Question

I have found that the capacitance of some capacitors like ceramic capacitors can actually change when a DC bias voltage is applied across them. The voltage need only be a few volts to make significant difference to the capacitance. Capacitors are used in filters and also decoupling capacitors, this means that it can cause change the cut-off frequency and the performance of noise filtering on the power supply rails.

Does this not mean that we should stop using ceramic capacitors of this type, or is there a way to mitigate this problem?

Answer 1

Note that the severity of the described DC bias issue depends the class of ceramic dielectric used. Class 1 material like NP0 are very stable. Class 2 materials like X7R have moderate performance variations. Class 3 materials and some class 2 like Y5V have severe DC bias and other variations. A valid question is why are class 2 and 3 made if 1 is so much better? The reason is for a given package size and voltage rating, these classes offer greater capacitance.

If target application is a filter with a specific frequency response characteristic, then NP0 or similar are generally used. All performance characteristic are far better. Note that for class 2 & 3, not only will frequency response change with applied amplitude, but also this will slightly distort the signal. So applications like high performance audio or radio frequency circuits should avoid class 2 and 3 in the signal path.

For supply rail decoupling the exact capacitance is generally not an issue. So the DC bias issue is not too problematic. If necessary, specify a larger capacitor so when biased the desired capacitance is achieved. Values like 10uF can be found in 0603 size. Yes this might only be 3uF when biased on a rail, but in NP0, 10n might be the largest option. Class 2 materials are often the best choice for rail bypassing. Given values like 100uF can be reached they are taking over from electrolytic in switched mode supplies, smaller and much lower ESR and ESL.

I have concerns about class 2 materials starting in Y. The performance variations seem so wild they seem to have questionable merit. They may have valid applications but I suspect the range will be more limited.

Answer 2

A good designer will understand the characteristics of the parts she or he specifies and will choose parts that are optimal, all things considered.

The billions of ceramic capacitors used are testament to the broad range of applications for which they are appropriate.

Voltage coefficient, temperature coefficient, aging, and initial tolerance should always be considered. In some applications they are more important than others. For bypassing, within reason, we usually care mostly about the minimum capacitance over all conditions and over the expected life.

There are ceramic dielectrics eg. COG, NP0 with more ideal characteristics than, say, X7R, and some that are much worse.

In filter applications you may also be concerned with dielectric absorption.

In any case, it’s best to deal with suppliers who can provide detailed performance characteristics if your application is sensitive to variations, or at least get voltage and temperature coefficient curves even for bypassing.

Answer 3

Just compensate for it in the design, same as how you spec an inductor to operate under mild saturation. Definitely don't just stop using MLCCs; they're tiny and cheap and very convenient!

If you need to filter a 5 V power rail with at least 100 nF of capacitance, find a capacitor that's at least 100 nF at 5 V applied voltage. If it needs to be an accurate 100 nF for whatever reason, it might be harder, but it still should be possible. Maybe a 220 nF capacitor rated for 6.3 V might have -50% capacitance at 5 V, for instance. Capacitor manufacturers often show typical curves for C vs bias (Murata's "SimSurfing" web app (no affiliation) is particularly nice for this).

If you need the capacitor to be an accurate 100 nF over a broad range of applied voltages, though, that is when you have to use something other than a type II dielectric. Either go for a C0G or U2J ceramic capacitor (type I dielectrics that don't exhibit this behaviour), or get a film or electrolytic capacitor.

Answer 4

Does this not mean that we should stop using cermaic capacitors of this type?

Four parameters that make ceramic capacitors desirable:

1. Low ESR
2. Low ESL
3. Small size, especially surface mount.
4. Very inexpensive

Or is there a way to mitigate this problem?

So if, by design, the capacitor volage is constant, then there is no reason not to use a ceramic. If 10\$\mu\$F is desired , but at the operating voltage the capacitor has 50% of its value, then use a 20 \$\mu\$F value or two 10\$\mu\$F in parallel.

Edit:

The variation of capacitance with voltages is more severe with smaller volumes. Example 0402 exhibits extreme dependance while 1206 is mild. If the board real estate allows, then the larger volume capacitor may be preferred.