# uc3852 pfc cct one more try.

Discussion in 'Electronic Basics' started by [email protected], Sep 21, 2005.

1. ### Guest

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

2. ### GenomeGuest

Seems to me that what you can do is.....

Ramp valley is 0.2V. Ramp peak is, arbitrarily, chosen as 9V. So ramp valley
to ramp peak is 8.8V.

Let's say your minimum line is 90V and your output voltage is 400V. Since
this is critical mode conduction the operating duty cycle is

d = (VOUT - VIN)/VOUT

Or 0.775.

Pick your minimum operating frequency, say 40KHz which gives your period as
25uS so the on time is 19uS.

Pick CT and call it 470pF. 8.8V in 19uS on 470pF makes the timing current
217uA so your timing resistor is 22K.

Say your output power is 100W. 100W from 90V is an input current of 1.1A.
With critical mode conduction the peak current is twice this or 2.2A. For
2.2A with 90V and 19uS you required an inductor of 750uH.

DNA

3. ### Guest

Thanks.
You make it sound so easy.

Should d not be 0.682 , not 0.775
The 90V is rms which is 127V Pk.
I still can't explain the values thay have come up with in the
datasheet app.
The app note u132 has values that come closer to what you have.The
inductor formula comes out at 688uH (90V rms , 100W , 400V , 40Khz) ,
which is at least in the same ballpark.