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uc3852 pfc question

Discussion in 'Electronic Design' started by [email protected], Sep 20, 2005.

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  1. Guest

    Hi all.
    I am looking to make a pfc cct using the uc3852 chip and have a few
    questions.The info I have is the U-132 app note from Unitrode and the
    uc3852 datasheet.
    I am just going through some equations and have a question.
    In the datasheet they have an application for a 100W power factor
    preregulator(page 5)
    I can't figure out how they get some of the values thay do.It's getting
    late in the day and maybe I'm missing something :0(.
    The cap and resistor (C4 and R4) that are used to determine the max on
    time are 13.3K and 180pF.That give a t(on)Max of 4.2uS - That relates
    to a min frequency of 237KHz ?? (formula from u-132:
    T(on)Max = Rset*C(ramp)) / 0.568
    The value of the inductor is 680uH.
    I can't get this to tie up with the formula to calculate the inductance
    (formula 16A)
    L = (VP(min)^2 * [Vo - VP(min)] ) / (( 4*Po(max)* Vo * F(conv)min)
    Po =100
    Vo =400
    Vp = 127 , I assume as they say 90 - 160V rms input.
    I have a feeling that I am missing something obvious.
    Any help appreciated.
    Cheers
    Rob
     
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