# Ironing out the step by step design procedure for AN4143

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

1. ### RichardGuest

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. ### RichardGuest

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

3. ### RichardGuest

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

4. ### leggGuest

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