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Any criteria on bypass capacitor selection?

Discussion in 'Electronic Design' started by Myauk, Mar 16, 2009.

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  1. Myauk

    Myauk Guest

    Dear all,

    I am now checking on a PCBA on which the power rail for digital
    circuit (microcontroller and its peripherals) uses many 0.1uF, Y5V,
    0805 capactiors in parallel between VCC and Gnd.

    I wonder why 0.1uF capacitor was used there?

    I could also see 4.7 uF used together with 0.1uF capacitors.

    I would also like to know the criteria to choose voltage ratings on
    bypass capacitors.

    Is it safe to use 0.1uF, 50V bypass capacitor for 24V supply rails
    which drive inductive loads?

    Regards.
     
  2. You are after the lowest HF impedance between VCC and Gnd. Each
    capacitor has an inductance. Parallel capacitors increases capacitance
    and reduces inductance.

    As a matter of principle I don't use Y5V, it may be OK in this
    application but they have high voltage coefficients and temperature
    coefficients of capacitiance. So the capacitance is always changing
    dramatically. I would treat a 0.1uF Y5V as a 10nF.

    I would fit at least a 100V caps on the 24V rails unless I had reason
    to know a lower voltage would be OK.
     
  3. Myauk

    Myauk Guest

    As a matter of principle I don't use Y5V, it may be OK in this
    Please let me know what type of dielectric material would be the best
    for this application.

    The reason that I am looking into the use of this capacitor is because
    one of the 0.1uF, 50V, 0805, at 24V rails is found severely burnt.

    The average current drawn in the circuit is around 3 to 4 Amperes.

    Although 0.1uF and 4.7uF capacitors (50V rating) are placed in
    parallel, only one particular 0.1uF is burnt, the rest of the
    capacitors are still OK.

    Please help me to give some suggestions on this issue.

    As I am a newbie in this issue, I am totally lost. At least I would
    like to request some help for the start. Any article discussing on
    such kind of issue will be of great help for me.


    Thanks and Best Regards
     
  4. J.A. Legris

    J.A. Legris Guest

    Nice article with several worked out examples:

    http://www-micrel.deis.unibo.it/~augusto/bypass1.pdf
     
  5. Eeyore

    Eeyore Guest

    One with less tempco. NEVER use use those cheap brown plate looking things.
    They seem to be designed as fire starters.

    Not uncommon. It would appear to be an inrush current dominated effect.
    I've seen this happen myself in a production environment where we had to do
    a formal change.

    Best result was to use 'box polyester' type capacitors instead. They seem
    to be far more durable. I've even seen dipped wound mylar types burn up. I
    bet it happened at power-on.

    A smaller value cap may be more appropriate as well.

    Graham
     
  6. Myauk

    Myauk Guest

    Not uncommon. It would appear to be an inrush current dominated effect.

    But I wonder why only the 0.1uF was burnt but the other 4.7 uF
    capacitor in parallel with it was not burnt under the same condition.

    Reagards
     
  7. Myauk

    Myauk Guest

    One more thing...

    When I tried to capture the 24V power rail flucactions due to
    different load turn on/off in PCBA, I don't see any spike large enough
    to kill the capacitor.

    I could only see a momentary voltage dip less than 1V on the 24V
    supply rails.

    Any suggestion for this?

    Was my approach a wrong approach?

    Regards
     
  8. Nobody

    Nobody Guest

    Just a guess: smaller capacitor = less thermal mass.
    What's the impedance of the power rails? A 1V drop could indicate a lot of
    current, and it's the current spike which will kill the capacitor.
     
  9. Eeyore

    Eeyore Guest

    No surprise.

    You were looking at the wrong thing. Look at the charge current at switch-on. Sadly
    very few manufacturers specify what the cap is capable of. If you connect a modest
    value electrolytic cap across the supply as well, it may help limit the supply
    dV/dt and hence the dI/dt into the decoupling caps. That would be one approach
    that's nice and simple.

    But the scenario you've described is one of those little dirty secrets the cap
    manufacturers like not to talk about.

    I would personally fit no larger than 47nF these days too.

    Graham
     
  10. Eeyore

    Eeyore Guest

    Exactly. A relatively little known phenomenon.

    Graham
     
  11. Myauk

    Myauk Guest

    Now I am testing the same PCBA in the same working system with 0.33uF,
    X7R, 16V rated voltage capacitor, while the supply is 24V. I turn the
    3A load on and off for a few times, it has been more than an hour and
    the capacitor has been alive until now!

    Any suggestions to repeat the burnt capacitor failure?


    One article from KEMET also says that it's FLEX CRACK, a mechanical
    crack on the capacitor will ignite it occasionally.


    Regards..
     
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