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Half bridge converter capacitor selection

Discussion in 'Electronic Design' started by R zaman, Jun 4, 2004.

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  1. R zaman

    R zaman Guest

    I would like to know the exact way to choose the value of the
    capacitors C1 and C2 in a half-bridge DC-DC converter. Should it be
    selected based on the ripple-current?

    Thanks in advance
     
  2. Tam/WB2TT

    Tam/WB2TT Guest

    Are you talking about the voltage doubler capacitors, or the ones in series
    with the transformer?

    Tam
     
  3. Tim Wescott

    Tim Wescott Guest

    Schematic please. I assume you're looking at someone's schematic that
    has two capacitors, which happen to be labeled C1 and C2, but there's a
    lot of different ways to make anything.

    Give a text-only schematic, or a link to a website, or post something
    readable (jpg, gif or pdf) on alt.binaries.schematic.electronic.
     
  4. R zaman

    R zaman Guest


    I am talking about the basic hard-switching half-bridge converter. The
    diagram can be viewed in:
    http://www.coremaster.com/appnotes/an108.pdf

    The half-bridge converter will be used for the (2nd stage) dc-dc stage
    (in buck mode) of a 2-stage universal PFC converter. Electrolytic
    condenser can be used for c1 and c2. But the problem with the
    electrolytic condenser is that the it has very high ESR compare to the
    film condenser and consequently the loss will be very high. SO we
    would like to use film condenser. It would be great if anyone
    enlighten me the procedure for calculating the value of C1 and C2.



    Thanks
     
  5. CBarn24050

    CBarn24050 Guest

    The half-bridge converter will be used for the (2nd stage) dc-dc stage
    The primary current charges/discharges the caps each cycle, so it depends on
    the frequency, the pri current and how much voltage ripple at the mid point you
    are prepared to tolerate. The usual it/c formula applies. Typical values for an
    off line bridge would be 0.47uf to 2uf. most designs have a series capacitor as
    well of same size.
     
  6. If you have a PFC front end you must have a large bulk cap. Either split
    that in half and use the center point as the transformer return or put a
    single cap in series with the transformer to ground. C1 and C2 are not
    necessary just shown for clarity. If you use a single film cap in series
    with the transformer to ground, the voltages and currents thru said cap are
    now easy to calculate because you do not have it in parallel with the bulk
    cap.
    cheers
    harry
     
  7. Yes C1 and C2 set the ripple current (primary).
    They also need to be suitable for the switching frequency of cause.
    Note that the regulator feedback look will decrease ripple in the output too.
    Anyways, you could have electrolitics and an other type of cap in parallel.
    JP
     
  8. Tam/WB2TT

    Tam/WB2TT Guest

    Harry,
    I have seen circuits with a voltage doubler front end, which would have a
    split filter capacitor. Rather than connect the transformer to their center
    tap, they used a pair if 1 ufd caps going from - to + with the center tap of
    the 1 ufd going to the transformer. Not sure why, unless the main filters
    had too much series resistance.

    Tam
     
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