[sdiy] Transformer question ( skin effect )

[Tom Arnold]

On Mon, Apr 11, 2011 at 10:17:28AM +0100, Richie Burnett wrote:
> Are you sure the slight increase in efficiency from operating your transmitter
> at 13.8 volts isn't going to be offset by the dissipation in the boost
> converter.  A well designed boost converter will only be about 90% efficient, so
> you'ld need an 11% efficiency improvement from the transmitter operating at
> 13.8V just to break even.

If I can burn 10% of battery power to get another 15watts out ( 85 or so
watts at 12v, 100+ at 13.8 ) I'll do it.  Also, with the boost regulator you
can run the battery down to safe levels ( 10.5v ) and still maintain your
power output.  Its worth it.

> A boost converter doesn't contain a transformer.  It consists of a switch,
> inductor and diode.  There is no isolation from input to output.

True.  I use "transformer" the same way the designer of the circuit does, to
differentiate from the pure inductors ( chokes ) in the various filters.

You are correct that there is no true isolation.  The center taps of the
windings are connected toegether.  One nice part about this is by just
turning it off, power continues to pass through with only the diode drop and
losses from windings etc.

> If there is a transformer and output choke then it sounds like some variety of
> forward converter, not a boost converter.  If you are talking about output
> currents in the range of 25 amps, i'd recommend winding the transformer with
> bundles of Litz wire or possibly foil.  I'd also recommend powdered iron for the
> output choke as it's more tolerant of the momentary overloads that you mentioned
> without falling foul of saturation.  Micrometals make lots of suitable material
> grades and have an excellent computer aided design tool for their cores.

Well, I already know these cores will work and the company I bought the
board from uses it for both the 25amp version and a 40amp version with the
added windings.   I could just follow their hints and wind using more 24ga
wire but then I got bored and started playing with skin effect calculations
more with the idea of maximizing current handling ( minimizing loss more
like ) when its in bypass mode not when its actually in boost as the "stock"
4x24ga in theory should only be good for 14amps at nominal loss.

> Filtering a SMPSU to feed a sensitive radio receiver can be a real challange. 
> Good layout, screening and filtering are all vital.  Look up terms like
> "common-mode filtering", "Differential-mode filtering" and "ripple steering." 
> These are all techniques that are used to minimise RFI.

I'm lucky that I found someoen who already spent many hours with a spectrum
analyzer on this thing.  He found a few birdies and some noise at approx 8
and 23mhz.  Not a lot but added 40 or so caps on the back of the board
knocks it down 6db and a few toroids on the input and output leads ( along
with a couple caps ) knocked everything down another 25db or so.

Problem I'm fighting is at home I have massive power line and plasma TV
interference, as well as AM radio into any improperly shielded audio lead
which make me paranoid when I build modules.  At least for my ham gear I've
gone mobile.  Adding an aux 100ah battery, isolator, and the boost regulator
so I can drive over to the beach where its radio quiet and relax a bit.  Its
been a worthwhile distraction while I plan how next to attack music gear.  I
got very... disenchanted with the direction I was working on some stuff and
couldn't get refocused.  A couple people on this list have seen what I've
done to my poor car and can vouch for the extent of my latest distraction.
:-)

Thanks everyone for the pointers.

-- 
 -----------------------------------------------------------------
 - Tom Arnold       -  Free Synth DIY webspace http://www.sdiy.org 
 - SynthGeek        -  "...is it a virus, a drug, or a religion?"
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 --------------------         -- Neal Stephenson,  Snow Crash