[farkanoid]

Critical information for those that aren't aware: MLCC capacitance decreases with applied DC voltage, like, a whole fucking lot. [1]

Those 10uF/100V/X7R/1210 capacitors you love for your space constrained designs might only be 1uF at 48V. And it gets worse when choosing smaller package sizes.

This caught me completely off-guard. I've always thought an MLCC with a reasonable Dielectric at a given Capacitance would perform at least as well as an Electrolytic or Tantalum (minus fire hazards).

[1] (PDF) https://www.digikey.com/Site/Global/Layouts/DownloadPdf.ashx...

[LiamPowell]

Notably as the capacitance goes up in a given package size this effect increases dramatically. Every good manufacturer provides a plot of this for each product.

[LeifCarrotson]

Also, pin the selection waterfall from your capacitor manufacturer to the wall - or at least bookmark it in your browser. Here's one from Kemet: [1]

Screenshot: https://i.imgur.com/sMaXBpN.png

If you really need to be on the lowest column (highest capacitance) for a given voltage rating, you'll either pay for it in voltage derating, temperature performance, tolerance accuracy, package height, or just pay for it in literal cash.

You cannot go below the lowest column, they have not figured out how to build a 10uF/25V/X7R/0603 MLCC, that is just not a thing you can buy.

With a given dielectric, material properties science only go so far. You're leaving performance on the table if you select a given package size with less capacitance and a lower voltage rating than what's available. (Assuming decoupling, not analog stuff where you need exactly 438.6 pF for a particular resonant frequency or something). Each package size has basically a constant inductance, and usually, capacitor height isn't that critical - you don't want to be oversquare, but they don't sell many of those. Each manufacturer publishes a waterfall diagram, but all manufacturers are working with the same physics.

Conversely, if you've selected an X7R dielectric and an 0603 package for a decoupling capacitor, there's not a great reason to go with a 0.1uF value, or to restrict yourself to 6.3V rating - eg [2]. They make a 0.47 uF 25V capacitor that's otherwise identical! [3] And because designers are lazy and default to 100nF, the part with 1/5th the performance is literally 6% more expensive!

Note that for 0402 packages, the 100nF capacitor is typically the right part to select! You can't get a 120nF/X7R/0402 at any voltage rating above 6.3V, the 220nF and 470nF are exotic parts that sacrifice stability and accuracy for maximum capacitance in a volume, but a 100nF/16V/X7R/0402 is a pretty good default.

[1] https://content.kemet.com/datasheets/KEM_C1002_X7R_SMD.pdf

[2] https://www.digikey.com/en/products/detail/kemet/C0603C104K9...

[3] https://www.digikey.com/en/products/detail/kemet/C0603C474K4...

[daedrdev]

Thats because of the MLCCs you are considering, you can get ones with different voltage coefficents if you want. Like ones that keep 95% at 100v or even better. They cost more and have different materials.

Extreme parts cost a whole lot more

[SequoiaHope]

I was once caught by this issue too. The problem was that I didn’t know about the basic phenomenon (MLCC losing capacitance with voltage) so I didn’t know that I was expecting extreme parts.

[rcxdude]

They're generally bigger as well. That's more or less the tradeoff: the denser the energy storage, the worse all other attributes get.