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Recently I've seen notes in datasheets from capacitor manufacturers Kemet and AVX that recommend designers not use X7R capacitors for applications in which they will be placed across mains or used for mains filtering. From AVX:

Capacitors with X7R dielectrics are not intended for applications across AC supply mains or AC line filtering with polarity reversal.

Q1) Why is this recommendation made? The datasheets don't go into detail, and I have not seen an app notes or white papers that adequately explain the issue.

Q2) Under what AC conditions is it ok to use X7R capacitors across AC signals? Is their suitability based on simply having a current-limited/high-impedance source?

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    \$\begingroup\$ It could be because X7R dielectric tends to have a fairly high dissipation factor (low Q) compared with other dielectric materials which would be lossy under AC conditions. Reference \$\endgroup\$
    – Adam Lawrence
    Apr 4, 2018 at 18:11
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    \$\begingroup\$ @AdamLawrence Depending on the dielectric, it's commonly much more than 50%. It gets worse the closer you get to the limit of volumetric density, so a 10 uF 0603 experiences a worse derating than a 10 uF 1206. I've seen many high-value ceramic caps lose 90% of their rated capacitance when you get up to rated voltage. \$\endgroup\$
    – Synchrondyne
    Apr 4, 2018 at 18:16
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    \$\begingroup\$ Possibly because for X7R has a very high ESR at 100 to 120 Hz, even though it has very low ESR at higher frequencies. I asked a related question recently. \$\endgroup\$
    – user57037
    Apr 4, 2018 at 19:55
  • \$\begingroup\$ The ESR at 120 Hz may be 1000x higher than the ESR at 1 MHz. Here is my related question: electronics.stackexchange.com/questions/358536/… \$\endgroup\$
    – user57037
    Apr 4, 2018 at 19:59

2 Answers 2

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Q1) X7R capacitors are Class 2. Class 2 ceramic capacitors are ferroelectric and contain dipoles. The dipoles group together into domains where they point in the same direction. Application of an electric field (AC or DC Voltage) will force the dipoles in the domains to try and align with the field. This movement of the dipoles in the domain cause a release of energy in the form of heat along the walls of each domain. Full polarity reversal of the field (AC Voltage) will generate the most heat because the dipoles have to completely flip their direction each time. Continually applying this condition will heat up the component until it eventually fails. It is very similar to hysteresis losses in a ferromagnetic core.

Q2) The conditions where this will not lead to failure are difficult to define, which is why the manufacturers completely avoid the situation and put the statement you mentioned in their datasheets. It is a function of frequency, voltage, footprint size, and the physical construction of the capacitor. It is particularly bad below 1kHz because this gives the dipoles time to flip with each polarity reversal and build up heat. At higher frequencies, it is more difficult for the dipoles to flip quickly enough and less heat is generated as a result. Again, it is difficult for them to clearly define it in a way that they can guarantee no failures. So, they just won't recommend it.

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  • \$\begingroup\$ Thanks for this. The lower-frequency overheating is an interesting, counter-intuitive point. I would've thought that higher frequencies meant more heat. \$\endgroup\$
    – Bort
    Nov 22, 2019 at 15:35
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It has to do with safety and being able to pass UL or other safety standards. See here:

X and Y Capacitors

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    \$\begingroup\$ Vishay has a line of X-rated X7R surface mount capacitors, so I don't think this is correct. I don't believe failsafe is a function of the dielectric material but moreso the overall construction of the part. \$\endgroup\$
    – Adam Lawrence
    Apr 4, 2018 at 18:10
  • \$\begingroup\$ @AdamLawrence You've proven your point, I think. It can't only be for that reason. Though I think most X/Y rated caps will use SL, Y5P, Y5U and Y5V dielectrics. So there may be still some correlation, too. But interesting to see X7R as an X-rated cap. \$\endgroup\$
    – jonk
    Apr 4, 2018 at 18:38
  • \$\begingroup\$ Any idea why polarity reversal is mentioned as a problem condition? I can understand voltage and current max ratings, but SMT capacitors are largely used to manipulate small signal AC, which would quite often have zero-cross/polarity change events. Are modern caps designed for unipolar signal chains only? \$\endgroup\$
    – Bort
    Apr 4, 2018 at 18:51

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