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After studying Multilayer Ceramic Capacitors (MLCCs), I have come to grips with the chemical phenomenon that decreases the effective capacitance of a given part.

Without getting into too much of the chemistry, higher voltage reduces the dielectric strength of such capacitors significantly, reducing the capacitance. This is called the DC bias because it is caused by a constant voltage offset applied across the capacitor.

However, I am planning to use some MLCCs as isolation capacitors, which will pass AC signals of a small amplitude regardless of the DC bias across them.

The capacitors are rated for 1 kV so they should provide reasonable isolation before breaking down. I am imagining that when isolating signals that have a greater DC offset, the dielectric will be further broken down and thus even the AC signals will not pass through with the same source/sink capability on the other end due to less capacitance.

If this is not the case, it gives me much greater design freedom. If it is, I would like some proof that accounts for both the AC signal and the DC bias.

Does the DC bias of a MLCC affect AC signals?

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  • \$\begingroup\$ Yes. But by choosing the dielectric you can control how large that effect is. Look at NPO capacitors for example. \$\endgroup\$ – user_1818839 Oct 29 '20 at 22:54
  • \$\begingroup\$ If you dig through the manufacturer's website you can usually find a plot of capacitance vs DC bias. Pick a cap that has enough capacitance at the largest bias you expect to see. Murata for example has a great search tool that lets you find parts with the effective capacitance you need at the DC bias you expect to use. \$\endgroup\$ – user1850479 Oct 30 '20 at 0:37
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Capacitance has a certain AC impedance at certain frequency.

DC bias on a MLCC causes change in capacitance.

Thus, change in capacitance changes the impedance for AC signal. If you select the capacitance so that the impedance does not change much, then it may not matter.

This is exactly the reason why even for basic voltage regulators, you need to select the MLCC capacitor capacitance and voltage rating so that you get the required effective capacitance even with the applied DC voltage bias.

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