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.