[sdiy] Transformer question ( skin effect )

[Tom Arnold]

On Sun, Apr 10, 2011 at 10:11:43PM +0100, Richie Burnett wrote:
> This doesn't really seem Synth DIY related but I used to design 
> switched-mode power supplies in a previous life so was spurred to reply...

Everything I do one way or another is synth related. :-)  Im taking this
time to do somethig ham radio related to gain some knowledge to abuse for a
synth project.

Just some background on what Im current poking at.  Its a boost regulator.
A simple switching supply that takes in 12 volts from your battery and boosts
it to 13.8 volts where equipment tends to operate most efficiently.
Basically trading battery life for more RF power output from the radio.

In this particular case the design uses trasformer cores from old PC
supplies and you rewind them.  It can just as easily be on new cores (
amidon makes quite a few for example ).  The original rewind spec on the
transformer calls for 4x24ga wires on the secondary and suggests winding
8x24ga to increase current handling.  8x24 is hard to wind so I started
looking into how these transformers are constructed as an exercise and it
looks like a good choice would be 6x22ga which would give 40amps of handling
while the regulator was in bypass mode, and seems to be an optimal match
diameter wise vs skin depth at 35khz.  The idea isn't to make the design
handle twice the current All the time, its just that occasionally I'll
exceed the design spec of 25amps by a few amps.  All the semis can handle it
but the transformer and final choke both need some attention.

The fun part comes in when I have the original design working as I get to
the part that is synth applicable for me and thats figuring out all the
noise sources and how to squish them.  Currently I'm using a bunch ( 12+ )
of PowerOne supplies for my modular and I'd like to move to switchers but
I've yet to find one that wasnt annoyingly noisy.  Its not that switchers are
difficult to figure out, but they are Much Different and it takes some
getting used to, thats why I'm building this project insstead of just buying
the finished one which would have saved me a ton of money.

Skin effect at the lower frequencies is just not something I think we run
into much in the synthdiy world.  At 60hz is like 8mm, even at the upper
range of audio where our max interest for audio work would be its still much
larger then any normal wire we'd be using so its an interesting exercise.
Not to mention learning about the various inductor mixes and sizes and how
to test ones you have to determine the permiability.  Not that we use
inductors too much for synth work, but in the guitar world tranformer
saturation is responsible for quite a lot of the "sound" that people hunt
for.

> Regarding skin depth, the skin depth for a conductor isn't a sharply 
> defined depth after which no current flows...
> [insert rest of text here]
> ...trial designs with different core sets, conductor bundles and possibly 
> air-gaps to see what works best.
> 
> You don't say anything about the specification or application of your 
> supply, or your chosen topology so it's hard to give any specific advice.

Actually that is quite helpful.  I did pick up a couple books on transformer
design including Ferromagnetic-Core Design & Application Handbook which I've
been bashing my way through.  Sometimes I have to send out ideas to make
sure I'm properly grokking what I'm reading.

After tearing apart a bunch of junk supplies to see what inductors they use
and grabbing a few of the common parts, its very intersting to note that a
200watt PC supply will have the same core in it ( note : probably not same
windings, core only ) as a 200watt 12v powerone switcher.  I think we
definiatly see where designers are using economy of scale to keep costs
down.  Its also interesting how even rough board layouts between different
manufacturer supplies are very similar.  Whether this is just because of
where the normal inputs/outputs are and it Just Happens, or everyone is
copying the same supply design from 20 years ago over and over, I dont know.
Same goes for the single voltage switches like Powerone and Meanwell.

I used to think that switchers were not worth fixing, but now that I am
beginning to understand them, I think they might be worth it in some
cases...

-- 
 -----------------------------------------------------------------
 - Tom Arnold       -  Free Synth DIY webspace http://www.sdiy.org 
 - SynthGeek        -  "...is it a virus, a drug, or a religion?"
 - AF6YW            -  Juanita Shrugs. "What's the difference? 
 --------------------         -- Neal Stephenson,  Snow Crash