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X7R dielectric is not recommended for AC line filtering or pulseapplications

Discussion in 'Electronic Design' started by mook johnson, Aug 6, 2013.

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  1. mook johnson

    mook johnson Guest

    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
     
  2. Glenn

    Glenn Guest

    Your answer - the X7Rs in question are not X or Y rated:

    CAPACITORS FOR RFI
    SUPPRESSION OF THE AC LINE:
    BASIC FACTS
    Fourth Edition
    http://www.seered.co.uk/sunvic_capacitor_information.pdf
    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 10μS 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
    ...."

    -

    http://www.calramic.com/Design/Assets/PDF_files/CRT-0017.pdf
    See page 2 note 4 and 6.

    http://www.ece.ucdavis.edu/vcl/asap/asap_v1/docs/X7R_C.pdf
    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.
    ...."


    http://www.avx.com/docs/catalogs/se.pdf
    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.
    ...."

    http://www.kemet.com/kemet/web/home...D.pdf/$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.
    ...."

    http://www.ttiinc.com/object/kemet_ceramic_sm_hv
    Quote: "...
    X7R dielectrics are not designed for AC line filtering applications. An
    insulating coating may be required to prevent surface arcing.
    ...."

    ( https://www.google.com/search?q=x7R+ac-line )

    /Glenn
     
  3. 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
     
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