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Ironing out the step by step design procedure for AN4143

Discussion in 'Electronic Design' started by Richard, Jun 28, 2004.

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  1. Richard

    Richard Guest

    http://www.fairchildsemi.com/an/AN/AN-4134.pdf

    http://www.fairchildsemi.com/collateral/AN-4134.xls

    The xls file is to be used with the design steps in the pdf file.

    In the pdf file: Step (3) (b) RCD reset

    In Excel: Excel sheet "Forward with RCD reset". I don't know how to
    come to a value to put in cell C30. I don't know what this item is
    supposed to be.

    Cell F30 is the cell that actually shows the nominal snubber capacitor
    voltage, that is the voltage across the snubber capacitor.This I think
    tends to remain fixed.

    Cell C31 is maximum nominal MOSFET voltage stress.This can be altered
    by altering the value of the nominal snubber capacitor voltage, whatever
    that is.

    TIA.
     
  2. Richard

    Richard Guest

    The problem I have is with C30. F30 surely is the nominal snubber capacitor
    voltage, across the snubber capacitor. So, what's C30?
     
  3. Richard

    Richard Guest

    Reposted in sci.electronics.basics. Obviuosly, not suitable for this group.
     
  4. legg

    legg Guest

    Although C30 is an input in the spreadsheet, it has no effect except
    to make the statement " [C30] > [F30] " true or false.

    If the statement is false then the voltage snubber capacitor is not
    being used as the mfr intended, and may fail.

    F30 is the average voltage across the capacitor. This is a function of
    the sum of the known magnetizing energy and leakage inductance energy
    developed at the selected conversion frequency, developing a total
    energy burn-off requirement in the clamp's bleeder resistor.

    As the C value reduces in this dissipating clamp, AC ripple and DC
    peak voltage across it will increase, requiring larger margin between
    the ratio of C30 and F30 spreadsheet values, as the part must be rated
    for the peak voltage value (~ and the dVdT ~ another topic).

    As the R value increases, the average voltage required to burn off the
    known leakage and magnetizing energy will also increase above the
    calculated volt-second reset 'minimum' voltage provided in the
    spreadsheet.

    RL
     
  5. Richard

    Richard Guest

    Thanks for the comments. I'm now setted over at sci.electronics.basic on the
    thread: "The step by step design procedure for AN4134"
     
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