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Bridged Amplifier Capacitor Ripple Current

Discussion in 'Electronic Design' started by [email protected], Jun 26, 2007.

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  1. Guest

    In the same way that we have easy conversions for dealing with sine
    waves such as RMS Current = I peak / SQRT(2) and I ave = 2 * Ipeak /
    PI, I would like to know if anyone can help with this question;

    Consider two bridged class B (single supply) amplifiers driving a
    load, the load is connected between each amplifiers output. The power
    supply is fed through a large inductor into a large reservoir
    capacitor.

    For the sake of argument lets say the peak output current is 4 Amps.
    This makes the average current 4 x 2 / PI Amps, and this is the
    [constant] current in the inductor. The ripple current in the
    capacitor is the difference between this average current and the
    instantaneous current being supplied into the load. I get the RMS
    ripple current as 0.31 Ipeak (using a spreadsheet) but am not sure how
    to do the maths to get this answer. Can anyone help?
     
  2. Eeyore

    Eeyore Guest

    You're using a switchmode power supply ?

    Graham
     
  3. Eliot

    Eliot Guest

    I'm not sure what you are getting at - the inductor is "large" and so
    carries a constant current.
     
  4. Eeyore

    Eeyore Guest

    I'm asking a question is what I'm getting at, because I haven't seen such a
    power supply ever for an audio amplifier.

    It'll have to be very large indeed for that.

    Is this some hypothetical scenario ? It doesn't sound like a real world one.

    Graham
     
  5. Eliot

    Eliot Guest

    It is real world. It's an RF amplifier. By "large" I mean the
    inductor has a value so the current barely changes over 1 cycle. It's
    µH's.
     
  6. Eeyore

    Eeyore Guest

    If it's that large then surely there'll be next to no ripple current in the caps.

    Graham
     
  7. Eliot

    Eliot Guest

    I think you have misunderstood this somewhat;
    The inductor supplies the average current,
    The load current varies,
    The capacitor supplies the difference between the load current and the
    average current, hence the current may be current flowing in or out of
    the capacitor. The RMS current in the capacitor is 0.31 * peak load
    current
     
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