Re: The step by step design procedure for AN4134

From: Richard (notty_at_emailo.com)
Date: 06/29/04 

Date: Tue, 29 Jun 2004 21:22:26 +0100

"John Popelish" <jpopelish@rica.net> wrote in message
news:40E19790.253BF6D8@rica.net...
> Richard wrote:
> >
> > 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, I'm getting stuck at : Step (3) (b) RCD reset
> >
> > In Excel: You need to choose the Excel *** "Forward with RCD reset": I
> > have a probblem with cell C30. I don't know how to get to a value to put
> > into cell C30, nor do I know what the parameter is. Cell F30 surely is
the
> > nominal snubber capacitor voltage, that is, the voltage across the
snubber
> > capacitor. So, what is the parameter that goes in cell C30 and how do
you
> > determine it's value? TIA.
>
> Second try:
> You get to pick any voltage you like, as long as it is greater than
> the value in F30. Higher voltage minimizes the time wasted to reset
> the core, while stressing the switch and rectifiers with higher
> voltage.

I'm still not getting it.

This is what I am seeing:

So far in the steps, the set input parameters (in blue) I've addressed are:
maximum line voltage; minimum line voltage; line frequency; output volts and
amps; estimated efficiency; dc link capacitor; maximum duty ratio. I
understand these parameters, that is, what they are. But I don't understand
what the nominal snubber capacity voltage is. There are two voltages
associated with row 30, the one in blue and the one in red. I'm looking at
the red one and saying, given all the parameters in blue that are
pertaining, the voltage in red is a voltage that will be present across the
snubber capacitor and is 150.6 volts. Okay, but what is the voltage in blue?
It's not the voltage ascross the snubber, that's in red. This is what I'm
thinking. I'm wondering if things are confusing me because of course these
calculations are not with a fixed input votage, a range is specified. I
could not reconcile the values in C24, C25, and C26 until that dawned on me.