Richard said:

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 sheet "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 am working on understanding this statement from page 3 of the pdf:

"winding and reset winding, respectively.

Since the snubber capacitor voltage is fixed and almost

independent of the input voltage, the MOSFET voltage

stress can be reduced compared to the reset winding

approach when the converter is operated with a wide input

voltage range."

The snubber voltage is a function of the average magnetization current

times the snubber resistor value. The resistor value is constant. If

the transformer were perfect, and the output voltage was perfectly

regulated and there were no resistive drops related to various load

currents, then the duty cycle is inverse to line voltage, so as the

voltage gets higher the percent of on time goes down. The

magnetization current thus climbs faster, but has less time to climb,

so it reaches about the same value, independent of line voltage.

I am looking at the voltage and current waveforms on page 3 and so

far, it all makes sense. The magnetization current rises while the

switch is on (and output current is being sent through the output

rectifier), and decays while the current must push up to snubber

voltage. But then, the current, Isn, reaches zero, and the inductor

no longer pushes current into the snubber cap through the diode and

the winding voltage falls toward zero. At this point (t3 to t4) the

graph shows the magnetization current decreasing below zero much

faster than it fell while Isn was flowing. A faster rate of change

implies a larger winding voltage, but the voltage waveform, in red,

shows a decreasing voltage heading toward zero as the transformer

waits for the next on pulse.

So I don't understand where the expression Vsn/(Lm/Coss) comes from

that I think is describing the value of voltage being applied ot the

winding while waiting for the next pulse t4 to t5). I have no idea

what Coss is. My guess is that this is the value of magnetization

that develops as the winding capacitance charges from Vsn down to just

below zero, where the output rectifier comes on an shorts the

secondary, allowing that value of accumulated magnetization current to

circulate with little change till the next on time occurs.

So to get back to the approximately fixed Vsn, I guess the on time is

inverse to line voltage, so the peak snubber current is fixed, and the

energy dumped into the snubber is fixed per cycle, regardless of line

voltage and is almost fixed independent of load current, so as long as

the cycle time is fixed, there is a constant average flow of energy

through the snubber resistor, so a fixed average voltage. And the

snubber capacitor keeps the instantaneous voltage near the average.

So back to the spread sheet, you get to pick any value for the snubber

nominal voltage (that will stay pretty close to that by the above

reasoning) as long as it is greater than the specified minimum value

if F30. The higher voltage you choose, the higher the stresses on the

switch and rectifiers, but the more of the cycle time the transformer

is able to transform power to the secondary (the higher the

transformer utilization). If you pick a generous core size for the

transformer, this is not much of a concern, so you can treat your

switch and rectifiers better by picking a lower voltage.