X7R dielectric is not recommended for AC line filtering or pulse applications

I have seen this note in several datasheets from Kemet AVX and others with no explanation.

Does anyone know the reason for this?

piezo effects causing mechanical cracking? failure mode is a short circuits? not self healing? .

thanks

Reply to
mook johnson
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Your answer - the X7Rs in question are not X or Y rated:

CAPACITORS FOR RFI SUPPRESSION OF THE AC LINE: BASIC FACTS Fourth Edition

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Quote: "... Symmetric interference can be envisioned as a source connected between the main and neutral wires. A line-to-line capacitor (designated as type X) properly applied is effective for this type. Asymmetric interference is represented by a source between either main or neutral and chassis ground. A line-to-ground capacitor (type Y) provides filtering in this case. ... A European study by Unipede (a consortium of power producers) revealed the types of transients found on the line: ...

80% of all transients have a duration between 1 and 10?S and amplitudes up to 1.2kV. These occur more than 10 times per day. Assuming a lifetime of 10 years the RFI capacitors must withstand over 30,000 such surges, as well as lesser numbers with amplitudes up to 6000 volts. The Unipede data agrees with IEEE Std. 587 which recommends equipment be tested with a 6kV damped sinusoid. IEEE 587 is discussed on page 7. ... Class X2 ... There are three types of RFI capacitors commonly in use on the AC line. These are impregnated metallized paper, metallized film and ceramic. The first two are categorized as ?self healing? or ?clearing? while ceramic is not. The property of self healing, properly designed into a capacitor, can extend its life while maintaining a small size and safe operation ..."

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See page 2 note 4 and 6.

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Quote: "... Capacitors with X7R Dielectrics are not intended for AC line filtering applications.

Contact plant for recommendations. Capacitors may require protective surface coating to prevent external arcing. ..."

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Quote: "... Note: Capacitors with X7R dielectrics are not intended for applications across AC supply mains or AC line filtering with polarity reversal. Contact plant for recommendations. ..."

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$file/KEM_C1037_X7R_KPS_HV_AUTO_SMD.pdf Quote: "... Extremely low ESR and ESL ... X7R dielectric is not recommended for AC line filtering or pulse applications. ..."

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Quote: "... X7R dielectrics are not designed for AC line filtering applications. An insulating coating may be required to prevent surface arcing. ..."

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/Glenn

Reply to
Glenn

I think it's the shorted failure mode. If the Y caps fail then ground leakage goes thru the roof. If the X cap fails then the AC line is dumped into the X cap and poof.

A failure mode that does not pose a fire hazard or User hazard (ground leakage) is perfered for obvious reasons.

Cheers

Reply to
Martin Riddle

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