2
\$\begingroup\$

What is the rule of thumb for the DC voltage rating when selecting a ceramic capacitor (MLCC).

Example: selecting a X7R decoupling capacitor, system voltage is 5V. Can I safely chose a capacitor with a DC voltage rating of 6.3V? Or should the voltage rating be at least twice or even more times as large as the system voltage?

\$\endgroup\$
6
\$\begingroup\$

It is electrically safe to use a 6.3v ceramic cap on a 5v rail.

However, ceramic capacitors have a feature that the manufacturers are a bit coy about, the capacitance is voltage dependent, and you may not meet your required capacitance.

With most X7Rs, there's only a relatively small effect. However, if you choose a Y5U for instance, you can lose more than half of your rated capacitance by the time you get to the rated voltage. If the capacitor is stabilising the output of an LDO and needs to meet a minimum value, that can be important. You can lose another half if you operate at maximum temperature.

A further warning. Designations like X7R and Y5U only describe the tempco, not the voltco, and the effects depend on package size as well (not because of something spooky about package size, but because manufacturers use the cheapest ceramic that gets them to the target capacitance in any given package). As a rule of thumb, as the rated capacitance becomes more 'impressive' for the size, the voltco and tempco get worse. You must research ceramic capacitor choice down the the specific brand and package size if the minimum value is at all critical. The data is out there from reputable manufacturers. If you can't find it, don't use that brand, but you may have to dig deep on their sites, and even contact them for specifics.

\$\endgroup\$
  • 4
    \$\begingroup\$ There's a great talk by James Lewis of Kemet called "They're JUST Capacitors" which discusses the voltage / capacitance relationship in some detail, and it gives some good advice on capacitor selection and derating. \$\endgroup\$ – Polynomial Jan 21 '17 at 10:22
  • 1
    \$\begingroup\$ @Polynomial Went to that video, clicked randomly into 9:30 (I'm impatient), and he's showing 1206 vs 0805 in X7R, with 50% cap loss at rated voltage for the 0805, none for 1206, and saying you can rarely find this data in data sheets! Good find, worth a look! \$\endgroup\$ – Neil_UK Jan 21 '17 at 10:42
  • 1
    \$\begingroup\$ Yup, it's a great talk with lots of insight into the physical construction and chemistry of various capacitor types and how it relates to rated capacitance, voltage bias, aging, self-healing, and failure. I knew little about it before and it has informed my purchases since. \$\endgroup\$ – Polynomial Jan 21 '17 at 10:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.