S

#### [email protected]

- Jan 1, 1970

- 0

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