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Chosing a High Ripple Current Capacitor

Discussion in 'Electronic Design' started by D from BC, Apr 22, 2007.

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  1. D from BC

    D from BC Guest

    In spice I have these waveforms across the capacitor:

    Voltage: 5Vpp 100 Khz triangle wave on 270VDC
    Current: 2App (swings -1A to +1A.) square wave at 100Khz.
    Capacitance: 1uF
    Max. Ambient temp: 30C

    Would a polypropylene film or electrolytic be better? There's lots of
    spare PCB space..I prefer picking the longer life component..

    I've read that heat ages electrolytics and one way to heat up an
    electrolytic is with lots of ripple current to make I^2R heat.

    When I looked at the polypropylene data sheets
    http://www.vishay.com/docs/28124/mmkp383.pdf
    I found AC vs frequency derating graphs.
    What is this derating based on?? Heat? Material breakdown?
    My app only has 5Vpp of AC and so a 400VDC pp film cap falls into the
    safety zone at 100Khz..
    But will it outlast an electrolytic?
    How do I figure out if this cap gets toasty from the pulse current?
    Do I look at the tangent of loss angle spec?
    D from BC
     
  2. Eeyore

    Eeyore Guest

    Polypropylene every day.

    You'd have to use back to back electrolytics or a non-polar one for an AC
    circuit and I doubt you can get a 1uF electrolytic that'll withstand that ripple
    current anyway.

    I assume it's heat that's the limiting factor btw. They do get hot in use. A mix
    of foil resistance and dielectric loss I imagine. Tangent of loss angle is just
    another way of looking at it.

    Graham
     
  3. If I understood the OP, its 5Vpp on top of 270Vdc, so a polarized cap
    would be OK.
     
  4. D from BC

    D from BC Guest

    After some googling, I found

    tan d = DF = 1/Q = ESR/Xc

    Ok... The polypropylene I picked has a "tangent of loss angle"
    =<50x10^-4 at 100Khz

    tan (50x10^-4) = ESR/(1/(2*pi*f*C))

    ESR = 0.00014 ohms... <<< That looks bogus..
    D from BC
     
  5. Eeyore

    Eeyore Guest

    Ooops ! I missed the obvious.

    An amp or so of ripple is still a lot for a 1uF electrolytic.

    Graham
     
  6. D from BC

    D from BC Guest

    I noticed that.. :)
    It's true the DC level is dominant and the voltage ripple is tiny in
    comparison. Who's cares if the current reverses polarity as long as
    the voltage doesn't..Therefore..polarized cap is ok.

    I might try the math sometime on paralleling electrolytics to compete
    with the poly film.
    But I hate math...so I'm just going to use the film cap. :)



    D from BC
     
  7. Wimpie

    Wimpie Guest

    Hello D from BC

    About derating of foil capacitors,

    For relative low frequencies, the maximum AC voltage is limited
    because of internal corona discharge in air pockets, that may erode
    the dielectric. At high frequencies, heat generation inside the
    dielectric is the limiting factor. At even higher frequencies, the
    limiting factor is current carrying capability of the contacts.
    Mostly the graphs are based on 10 degrees internal temperature rise.

    You also have to derate when the temperature will be higher (for
    example because of heat generated by nearby components.

    Electrolytic capacitors.
    You need a big cap (several 100 uF to mF) to handle a ripple current
    of about 600mA rms. The larger the size, the higher the ripple current
    capability. Ripple current is mostly based on 5..10 degrees
    temperature rise with respect to ambient. Same current at lower
    temperature gives somewhat higher temperature rise because of higher
    ESR at low temperature. The life time is specified at given
    temperature, rated voltage and rated ripple current (I thought).

    When you stay below the maximum ripple current, you gain a factor 2
    for every 10 degrease lower temperature then specified in the data
    sheet. Like with the foil capacitor, check the actual temperature of
    the electrolytic capacitor. When you use them according to the
    specification, they have a predictable lifetime. Life time can be
    shortened significantly by wrong treatment (for example certain
    chemicals for cleaning). Most manufacturers have good information on
    how to calculate the life time based on ripple current, frequency,
    ambient temperature and voltage and how to treat their capacitors (for
    example ELNA, Nichicon, Panasonic).

    Do you also check your design for inrush current and overload
    situations? Standard or SMPS capacitors are not made for fast
    discharging.

    When the value of the capacitor is of importance, a foil capacitor is
    a good option.

    Best Regards,

    Wim
    PA3DJS
     
  8. D from BC

    D from BC Guest

    Cool ...lots of info for me to think about..

    Thanks..
    Especially the bit about film cap derating graphs are based on a 10
    degree temp rise...I didn't know that....
    If needed, I'll check out the capacitor manufacture sites for
    additional reading on capacitor specifications..
    D from BC
     
  9. I calculated : 50x10^-4 = ESR/(1/(2*pi*f*C))

    and got about 8 milliohms.

    tan d = 50x10^-4 so you don't need the tan (50x10^-4) term

    Take a look at the V vs f curves for the caps. These appear to be
    constant power curves (I goes up proportional to f). Pick a V for your C
    and f, then calculate its Xc and I.

    Compare that VI to your current waveform power dissipation through the
    ESR and make sure you don't exceed the curve value.
     
  10. Boris Mohar

    Boris Mohar Guest

    Check out http://www.eci-capacitors.com/
     
  11. D from BC

    D from BC Guest

    Doh!... Newbie goof..Thanks for the correction..
    ....I was thinking angle ..That's why I did tan ( )..
    Good thing I don't make life support systems for a hobby.. :)

    So... it's really a DF number. I was wondering why the spec was
    unitless. If it was an angle it would be in degrees or radians.

    In my case the poly-cap dissipation would be around (1Arms^2)*0.008 =
    8mW = insignificant :)
    D from BC
     
  12. D from BC

    D from BC Guest

    Interesting..
    Electronic Concepts Inc. makes polycarbonate capacitors..
    DF does not exceed 0.3%..from 25C to 125C

    Digikey distributes only these poly's
    Polyester
    Polyethylene
    polypropylene
    polyphenylene
    D from BC
     
  13. Wimpie

    Wimpie Guest

    Hello, D from BC

    Polyethylene and Polypropylene have the lowest maximum operating
    temperature, but lowest dielectric losses and best capacitance versus
    temperature stability.

    Polyester can be used up to 125 degr (derate), and have higher loss.
    Polyphenylene has highest operating temperature and moderate loss.
    Polycabonate is not that popular anymore (but it is not a bad
    dielectric).

    You should check the datasheet. Also you should be aware of the
    construction. Wounded capacitors have some higher inductance then
    "stacked" capacitors. I had it once in a half bridged semi resonant
    converter. Most of the ringing was caused by the (wounded technology)
    capacitor.

    Epcos has a general document on foil capacitors that may help you
    choose the best capacitor.

    Best regards,

    Wim
     
  14. John Larkin

    John Larkin Guest

    Yup, nothing works like doing the math!

    John
     
  15. Oh, I don't know about that. Blowing up ever increasing rated components
    until one holds can be fun too.

    Remind me and some time I'll post my story about destructive tests with
    a bunch of $8000 relays.
     
  16. John Larkin

    John Larkin Guest

    Actually, I do enjoy destructive testing. It can be, well, very
    constructive.
    Now?

    Who makes $8000 relays? Are they surface mount?

    John
     
  17. D from BC

    D from BC Guest

    I have a 1/8W carbon resistor for sale... $500.00 sound ok? :p

    I wanna hear the relay story...
    I know the end!
    "And all the other relays lived happily ever after...The end.."

    $8000 relays....mmmm
    Perhaps liquid cooled copper tube windings on a core the size of a car
    that pulls down a contact the size of a telephone pole???
    D from BC
     
  18. YD

    YD Guest

    Late at night, by candle light, "Paul Hovnanian P.E."
    Do it *NOW*!

    - YD.
     
  19. Years ago, I was working on the design certification for the 747-400
    electrical power system. They had upgraded the system to 4 paralleled 90
    kVA generators, so a lot of the circuit breaker coordination and
    equipment maximum fault duty had to be reviewed.

    One part of the system included a 3PDT relay, rated at 10A, with a
    specified low voltage dropout coil. Its use was to select between a
    primary source and, if that dropped below a threshold, drop to the NO
    position, selecting an alternate 3 phase power bus.

    Since this gizmo had a couple of equipment circuit breakers downstream,
    we decided to test the system coordination by applying a fault at one of
    them, downstream of this relay. With 4 fully loaded generators in
    parallel, we applied a fault and 'Pop'. The relay belched fire and
    smoke. Needless to say we reported our findings back to design
    engineering, who forwarded it to the vendor.

    When the new test sample arrived, we ran it down to the lab and repeated
    the test. 'Poof!', a somewhat smaller ball of fire. After about half a
    dozen tests, each with smaller pyrotechnics, we finally got a unit that
    survived.

    When we were all done, the purchasing department contacted engineering
    management and asked if we were through blowing up relays at $8000 each.
    Due to the organizational philosophy of the company, engineering had no
    visibility of component costs. Had we (or more accurately, design
    engineering) known what these custom relays cost, we (or they) could
    have easily designed a circuit with a standard (probably a couple of
    hundred dollar unit being aviation spec.) part and a voltage sensing
    circuit.

    That's why airplanes cost a couple of hundred million each.
     
  20. YD

    YD Guest

    Late at night, by candle light, "Paul Hovnanian P.E."
    Yabbut, what caused the fireworks, underdimensioned contacts, voltage
    swing or something?

    - YD.
     
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