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Electrolytic caps

Discussion in 'Electronic Design' started by JSF, Feb 15, 2006.

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  1. JSF

    JSF Guest

    When using electrolytic caps in a power supply is it any advantage in
    using a cap that is twice or three times the output voltage, like a 35 volt
    cap on a 13 volt supply.
    Also any ideas on electrolytic caps behavior when the caps are tested
    at -55 C, any changes?
     
  2. They'll tend to last longer. I don't believe there's much advantage
    over about double, though.
    Assuming you're talking about aluminum electrolytics, capacitance
    drops and ESR increases by a fairly large factor. Limits may not be
    specified as low as -55°C (maybe at -40° or -25°C), even if they are
    rated for operation that low. Check the data sheets carefully if
    that's a real consideration for you.


    Best regards,
    Spehro Pefhany
     
  3. I have heard that capacitors may become leaky if not used reasonably close
    to their rated voltage, because the voltage helps maintain the aluminum
    oxide dielectric. I have used 35 VDC capacitors for 5 VDC regulator outputs
    without problem, but usually I specify 10 or 16 volt. I don't think it is as
    much a problem with tantalum.

    Paul E. Schoen
    www.pstech-inc.com
     
  4. Terry Given

    Terry Given Guest

    Hi Spehro,

    did you read Michael Gaspari's paper in thenov/dec 2005 IEEE industry
    apps? very good.

    Cheers
    Terry
     
  5. Terry Given

    Terry Given Guest

    thats the professional comic; I meant the journal :)

    The guts of his model is this:

    Michael Gaspari wrote a great paper in IEE trans. Industry applications,
    vol.41 no.6 nov/dec 2005, pp1430-1435.

    his cap model is:

    ---[Ro]---[R1]---+----[R2]----+----[C1]----
    | |
    +----[C2]----+

    R0 = resistance of foil, tabs & terminals
    R1 = resistance of electrolyte
    R2 = dielectric loss resistance
    C1 = terminal capacitance
    C2 = dielectric loss capacitance


    R2 and C2 give a large variation in ESR with frequency. typically the
    effect of R2,C2 peters out above 10kHz, so you can take the ESR at
    100kHz as the combined value of R0 and R1. this can be seen from the
    ripple current multiplier tables a decent cap data sheet has.

    R0+R1 = ESR @ 100kHz

    R2 = ESR @ 100Hz - (Ro + R1)

    and pick C2 to get the right values for ESRs in the 100H - 10kHz range

    the reason the ripple current varies with temperature is the loss in the
    cap is kept constant (for a given lifetime) so lower ESR means more
    current. you can thus translate a ripple-current multiplier table (eg
    see LXZ cap datasheet) into an ESR multiplier.

    ESR_multiplier = 1/(ripple_multiplier)2

    the LXZ table for 220uF - 560uF caps is:

    120Hz 1kHz 10kHz 100kHz
    0.5 0.85 0.94 1.00

    so the ESR multiplier is:

    4 1.4 1.13 1


    the ESR of these caps have at 100Hz is 4x th 100kHz value....

    ESR variation with temperature is due to the increased conductivity of
    the electrolyte.

    R1(T) = R1o*exp[(To-Tcore)/E]

    R1o = ESR at temperature To

    E = temperature sensitivity factor

    R1 is usually 5x Ro, and you can calculate E from the two ESR
    measurements (-10C, 20C) for a given cap family.



    Cheers
    Terry
     
  6. On Sun, 19 Feb 2006 23:12:18 +1300, the renowned Terry Given

    <deliberately TOP posted so that Terry's synopsis gets repeated below
    for archiving>

    Excellent summary, Terry. I'll save this, it will likely come in
    handy.

    There are some huge differences (5:1) between different capacitors in
    their very-low-temperature ESR behavior. Eg. 3* to 15* room
    temperature ESR.

    Thanks!

    Best regards,
    Spehro Pefhany
     
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