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Paralleling caps to meet ripple current requirements?

Discussion in 'Electronic Basics' started by John, Jun 21, 2007.

  1. John

    John Guest

    A DC-DC converter I'd like to use requires an input cap with a 10A
    ripple current rating (at the coverter's rated output). I'm assuming
    it's a 100KHz ripple rating, the data sheet doesn't say.

    I can easily find 4A or so rated caps at the capacity/voltage/ESR I'm
    looking for and was wondering if I can just parallel three of those
    caps to achieve the 10A ripple current requirement? Assume that all
    PCB traces are low impedance/inductance and that the caps are mounted
    close together and close to the input of the converter.

    Thanks!
    John
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  2. Eeyore

    Eeyore Guest

    What value cap are you looking for ?

    Paralleling caps for increased ripple current is quite normal btw.

    Graham
     
  3. Yes, you can. But since ripple current is what heats the
    caps and determines their life, 4 might be better. I would
    like to leave a fat margin (perhaps 50%) between the ratings
    and the actual current.

    If you want to keep almost all of that ripple current
    isolated between those caps and the converter, add a little
    inductance between the upstream supply and the capacitors.
    This will pass almost all the ripple current through the
    caps, instead of sharing it between the source and the caps,
    but that is where it belongs.
     
  4. John

    John Guest

    What value cap are you looking for ?

    It's a 12V-to-0.9V converter.
    Need 270uF for the input and 2000uF (or so) for the output. Looking
    at the Vishay OS-CON right now...nice ESR specs. They're pricey but
    this is just a one-off converter I'll be using "in-house".

    It seemed that idea would work just fine. Just wasn't sure if there
    was a "gotcha" somewhere. Thanks!

    John
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  5. John

    John Guest

    Yes, you can. But since ripple current is what heats the
    Good tip, thanks.

    Interesting...the test-rig schematic for the converter (an Artesyn
    SIL40C) showed a 270uF cap, 1.5uH series inductor, and then another
    270uF cap at the Vin to the converter. Wasn't quite sure of the
    inductor's function...now I know. :)

    Do I still need the full 10A ripple requirement for both sets of caps,
    before the inductor and after the inductor?

    John
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  6. Eeyore

    Eeyore Guest

    Really !

    If you're worried about lifetime just use higher temperature parts.

    To the OP. Electrolytic caps are usually rated as having x thousand hours life
    based on operation at their max rating (ripple current) in a specified ambient
    temperature, the 'wear out' mechanism basically being heat which slowly
    evaporates the electrolyte).

    Lifetime can be increased by reducing the ambient temp OR using a cap with a
    higher temperature rating. Lifetime doubles for every 10C reduction in temp
    typically.

    For example a cap rated for 2000 hours at 85C would last 8000 hours at an
    ambient temp of 65C and a '2000 hour 105C' cap would last 32000 hours under the
    same conditions.

    Graham
     
  7. Eeyore

    Eeyore Guest

    Ummm, it starts getting more complicated when you add inductance. The inductor
    actually reduces the ripple current in the caps since it stores some energy too.

    You won't ever go wrong by being generous with ripple current specs though.
    There's also no reason you can't simply increase the capacitance of the input
    cap (rather than fit several smaller caps - provided you still meet any required
    ESR specs) and get extra ripple current capability that way.

    Graham
     
  8. I seriously doubt it, if the design does not involve any
    resonances. The cap between inductors would normally see
    very little ripple current. I also doubt that having two
    equal capacitors is optimal. The one between inductors is
    essentially there just to capacitively divide the RF current
    that jumps through the inductor via its stray capacitance.
    I good, low equivalent series inductance (i.e. stacked film
    or ceramic multilayer) might well do better than the 270 uF
    even if only a fraction of its capacitance. But you may
    need a parallel larger capacitor to suppress any resonances
    between those inductors and the small capacitor that the
    converter generates.

    After you get past the ripple current requirement of the
    final cap, you have to deal with where that current goes.
    The loop area this current surrounds needs to be minimized
    to get the most use from those capacitors and minimize
    magnetic fields caused by the ripple.
     
  9. Similar sized parts from the same maker with different
    temperature ratings usually have shorted life ratings for
    the higher temperature units, and lower ripple current
    ratings. In other words, the main difference between the
    lower and higher temperature rated units is a derating of
    the ripple current rating and a shortened life prediction.
    I'm sure they tweak other things but this is the man difference.

    For this reason, if you derate the ripple current rating, as
    long as you don't exceed the ambient temperature rating, the
    lifetime can safely be assumed to improve.
     
  10. John

    John Guest

    I seriously doubt it, if the design does not involve any
    The test rig schematic only had one inductor.

    +-------+
    1.5uH | |
    12V---+------- ___ -------+-------+Vin |
    | -UUU- | | |
    --- --- +-------+
    ---270uF 270uF--- Converter
    | |
    GND GND

    Draws about 4.4A at 12V and delivers 40A at 0.9V.


    Good tip. I was going to try to minimize it as much as possible
    thinking that I really didn't want to sling whatever EMI was coming
    off of that 10A of ripple. :)

    Thanks!
    John
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  11. John

    John Guest

    For example a cap rated for 2000 hours at 85C would last 8000 hours at an
    Very good point. I'm not sure how many options I have with the low
    ESR I'm looking for but I guess it can be a tradeoff between ESR and
    temp rating...especially if the price difference is large. :)

    John
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  12. I don't know the physical shape of the converter and whether
    it is a package or a bunch of components, but since you are
    paralleling capacitors for the final storage, you may be
    able to make two groups and route their ground sides to the
    common point on the converter by two equal and opposite
    loops (from a current standpoint) so that the bulk of the
    external field is canceled.
     
  13. John

    John Guest

    Ummm, it starts getting more complicated when you add inductance. The inductor
    I noticed that the ripple rating went up as the capacitance went up.
    Since my load for the converter is pretty constant (other than
    ON/OFF), does that ease the requirement for ultra-low ESR caps?

    The data sheet and app notes for the converter emphasize ESR but I'm
    thinking that if I have only a slowly varying load (if varying at all)
    that low ESR caps aren't nearly as important for voltage regulation
    but might only needed to satisfy any input requirements for the
    converter?

    I guess the lower ESR caps would make it easier to keep the caps cool
    though, taking the 10A ripple current into account.

    John
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  14. John

    John Guest

    I don't know the physical shape of the converter and whether
    Great idea!

    It's a vertically mounted SIP with 24 pins (single row) spread out
    over 2.4".

    There are three Vin pins in the center with the Vout and GND pins
    alternating for pins 15-25 and scattered GND in pins 1-8 to go with
    the POWER GOOD, TRIM, and CURRENT SHARE signal pins (pins 1-8).

    I guess I have either side of that row of pins to play with.

    John
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  15. Eeyore

    Eeyore Guest

    You can end up juggling these things forever.

    Graham
     
  16. John

    John Guest

    You can end up juggling these things forever.

    LOL, you read my mind on that one! I guess I just gotta' dive in and
    start buying caps. It's only a one-off device. OK, maybe 2 or 3 if
    it works well. :)

    John
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  17. That is just the equivalent of paralleling several smaller
    caps, inside the can.
    Not at all. The ripple current is related to the value of
    the output current, not how fast it changes. It is the load
    current being drawn from the input capacitors, but only
    during part of the converter cycle, and not at the other
    part of the cycle. Remove the average from that pulse train
    and you get an alternating direction current of about half
    the DC output current.
    It lowers the actual watts dumped into the caps, but also
    lowers the peak spike voltage that appears across them and
    that propagates upstream through the filter inductor.
     
  18. That is what I was picturing.
     
  19. In that case, please include a few 1uF, 50 or 63 volt
    stacked film caps (Panasonic type V) to experiment with as
    high frequency bypass for the electrolytics, if you can get
    them. These would go as close to the SIP pins as possible,
    to keep the highest frequencies out of the larger loops.
     
  20. John

    John Guest

    Not at all. The ripple current is related to the value of
    So the ESR times the ripple current = the voltage spike?
    I can see how reducing the ESR would be valuable here.

    Thanks!
    John
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