Hi all. In got no responses to the last post , so I'll try again , in case my news server did not let it through.
I am looking to make a pfc cct using the uc3852 chip and have a few questions.The info I have is the U-132 app note from Unitrode and the uc3852 datasheet. I am just going through some equations and have a question. In the datasheet they have an application for a 100W power factor preregulator(page 5) I can't figure out how they get some of the values thay do.It's getting
late in the day and maybe I'm missing something :0(. The cap and resistor (C4 and R4) that are used to determine the max on time are 13.3K and 180pF.That give a t(on)Max of 4.2uS - That relates to a min frequency of 237KHz ?? (formula from u-132: T(on)Max = Rset*C(ramp)) / 0.568 The value of the inductor is 680uH. I can't get this to tie up with the formula to calculate the inductance
(formula 16A) L = (VP(min)^2 * [Vo - VP(min)] ) / (( 4*Po(max)* Vo * F(conv)min) Po =100 Vo =400 Vp = 127 , I assume as they say 90 - 160V rms input. I have a feeling that I am missing something obvious. Any help appreciated. Cheers Rob