[sdiy] switched mode PSU design

[Tim Parkhurst]

On 8/10/05, fmg <eqys at iposadas.com.ar> wrote:
> Tim, (and list)
> 
> Old-fashion pi filters (C-L-C) can do miracles... don't know why they
> aren't used anymore. (cost? space?..) Try a dual pi scheme, ie:
> 
> _____C______   __L1__   _____C_____    __L2__   _____C______
> 100pF + 1nF ->  50uH -> 1nF + 0.1uF -> 150mH -> 0.1uF + 470uF
> 
> L1 can be done with some copper and one 1/2W resistor, L2 can be done
> from the secondary of a dead wall-wart trafo (choose one with an open
> primary, discard those whose primary is burnt (melt))
> 
> I've seen some designs using Rs instead Ls for the pi. This is bad.
> The inductor on the pi keeps Rsource low while having a hi_z for the ac
> content (including transients), something that doesn't happens with
> resistors. A Pi(c-r-c) and a Pi(c-l-c) are two completely diferent
> things.
> 
> Of course, a good emi shield is another completely diferent thing too.
> 
> As a side note: I always include L-C filtering in power rails of
> every pcb I make. It takes a bit more time and space (just a bit)
> but it extend noise isolation not only between the card and the PS
> but among the cards themselves. (and it doesn't hurt)
> 
> Fabio.
> 
> 

Along these lines, I've done a little research and found what might be
an interesting link for those of you who deal with Pi filters on a
regular basis...
http://www.smeter.net/filters/lpf_hpf.php

It's a short article and includes a link to a downloadable program
that will calculate component values for a Pi filter if you give it a
few variables like the desired cutoff frequency, terminating impedance
(not quite sure what that is), and a few other things.

What I'm looking for is info on winding the inductors. Given the
component values you suggested above, is there a program or a formula
that will show me what gauge wire and how many turns? Are these
inductors (or values fairly close) things that I might find in the
local surplus shops (we have a couple of very good ones here in the
Silicon Valley)? Also, I'm not sure what you mean by the cap values
(e.g., 100pF + 1nF). Are these in series as one 'leg' of the Pi?

TIA for any help or suggestions.


Tim (been one cap short of a Pi most of my life) Servo
-- 
"Imagination is more important than knowledge." - Albert Einstein