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  1. #1
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    How calculate the input filter capacitors for Half bridge SMPS

    Hello guys,

    In sch attached how to calculate the filter capacitors C4 and C5 ? to a power of 1200W.

    Can anyone help?

    Thanks

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    Re: How calculate the input filter capacitors for Half bridge SMPS

    whatever value keeps the voltage at the mid point within say 1% of the nominal. Also, ensure no overly high ripple current in them.



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    Re: How calculate the input filter capacitors for Half bridge SMPS

    Quote Originally Posted by OrciBorg View Post
    In sch attached how to calculate the filter capacitors C4 and C5 ? to a power of 1200W.
    Do you mean C4 & C7? Those appear to supply the entire circuit.

    They are replenished every 1/100 (or 1/120?) Hz.
    Suppose they are 3mF.
    And suppose the load calculates as 100 ohms.
    Then the RC time constant is .003 x 100 (or 0.3 sec).

    Voltage drops 63% during one time constant.
    Or 6.3% during a tenth of that time (approx).
    Then calculate the drop during 1/100 sec.

    Adjust the Farad value up or down until you get a satisfactory figure.


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  4. #4
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    Re: How calculate the input filter capacitors for Half bridge SMPS

    A decent rule of thumb will get you close:
    C = I x dT/dV

    C (in uF) = Amps x microseconds (between mains half cycles) divided by desired peak to peak ripple.

    Lets see, assume 1,200 watts at 340v dc (= 3.5A) and 34v p/p ripple.
    Assume 100 Hz from rectifier = 10,000uS hold up time between cycles
    This is for both C4 and C7 as they are in series across the dc rail.

    C = 3.5A x 10,000uS / 34v p/p ripple
    1,029uF total

    C5 and C7 will EACH need to be 2,000uF for about 10% ripple in 220v bridge configuration.
    Voltage doubler configuration will require a bit more, but it gets complicated.
    Cheers, Tony.


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