[sdiy] switched mode PSU design

[fmg]

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.


Tim Parkhurst wrote:
> 
> You know, there has GOT to be a way to properly filter and/or shield a
> switcher to keep it from goofing up audio circuits. Old PCs (with a
> power supply) can be had for next to nothing, and someone really
> clever (and with more spare time than I have) could take an old PC
> case and make a really nifty little synth out of it. Where else could
> you get a nice rugged sheet metal case AND power supply for $10 or so?
> Maybe even use the motherboard as a 'backplane' to distribute signals
> and to provide a nice mechanical mounting for synth modules. It sounds
> to me like the toughest part would be figuring out how to quiet down
> that pesky switching supply. Of course, you could just yank the
> switcher, stuff in a linear supply and still use the case, but that
> adds significant cost. Another method might be to 'remote mount' the
> switcher supply in an external box, but there still could be noise on
> the supply lines (and again, this makes the whole mess more costly and
> bulky).
> 
> Any thoughts?
> 
> Tim (messy and costly, but not bulky) Servo
> --
> "Imagination is more important than knowledge." - Albert Einstein