[sdiy] Switching power supply transformers

[Batz Goodfortune]

Y-ellow Karl Harry et all.

At 10:15 PM 1/12/05 -0800, harrybissell wrote:

>NO NO NO....
>
>switching power supplies run at very high frequencies...
>50/60Hz on one of those transformers is saturation / fire
>for sure

I've seen it. Lots of fun for the whole family. If your going to do that, 
don't forget the marshmallows.

OTOH, Lots of synths use switching PSUs. The latest MOOG offering for one 
it would seem. There are switchers and there are SWITCHERS however. 
Switchers who's only job in life was/is to run a dumb terminal or other 
digital-only device are probably going to be less than impressive. A 
millivolt of noise or two isn't really going to be a problem. Makers of 
general purpose computer supplies had to up their game however because 
people wanted to run audio and other high quality mixed signal devices from 
them.

You should be alright in terms of the 5 volt supply for the logic, Though 
it would be prudent to observe the rules for mixed signal PSU routing 
overall. That is. Separate termination for analogue and digital grounds.

Having said that, you might be surprised at what manufacturers get away 
with in the real world. The Yamaha TX7 for example, has a small, and very 
simple switcher. It produces 5 volts at some current for the mostly digital 
circuitry. But it uses a pair of ordinary 78/79 15 volt regulators for the 
analog. I'd like to say that this arrangement works well but given that 
mine actually did pack up and burst into flames one day seems to suggest 
otherwise. I'm sure that's not exactly an issue with you at this point 
though. (Yet!)

The basic rule of thumb with transformers is. The higher the frequency, the 
smaller the size. (for a given current/voltage output.) But since these 
switchers are often running at RF frequencies these days it also introduces 
several new advantages and some disadvantages. One of the advantages is 
that at frequencies of 1 meg or more, a designer can take advantage of the 
skin effect in the winding of the transformers. Smaller wires in the 
windings but more windings. I don't know off hand what the calculation is 
for that but effectively they double the surface area of the windings and 
at higher frequencies, electrons tend to want to move around on the skin of 
the wire rather than through it.

The disadvantage of course is that if badly designed, you're going to blow 
away radio reception for miles. The people who can deal with these issues 
and many others concerning SMPS design are better men than I Gungadin. This 
is the real voodoo in electronics. There are lots of people now who can do 
this stuff but at one time there were only a handfull of people in the 
world who knew how to cast the spells. And they got paid big wads of cash 
for using their magic. Apparently you can pretty much trace modern SMPS 
design back to just 6 people.
But I digress.

Needless to say, when you enter the world of switchmodes, you're not in 
Kansas anymore Toto. And hopefully before any harm is done, and just in 
case it would be, I should just point out that these things can kill you. 
They are dangerous to poke around inside unless you're absolutely clear 
about what's going on in there. Any mains supply is dangerous, Mains 
powered switchmodes are lethal.

Out of interest, the start-up loading needs only be enough to stop initial 
in-rush overshoot on the output filter caps. But this depends on the 
specific SMPS design as well. An ATX supply for example, needs an auxilury 
switcher or linear PSU in order to provide a constant 5 volts to operate 
the on/off switch. Subsequently the SMPS chip(s) themselves usually have 
their own power supply. Otherwise the PSUs are what they call "Self 
Starting" What that means is that the first few pulses at turn-on are full 
spike. The filter caps charge up pretty quickly. This powers up the SMPS 
circuitry but if there's no load, the filter caps keep on charging. The 
SMPS circuitry detects that it's overshooting it's regulation target and 
immediately pulls back the pulses. The cap remains floating, often 
over-voltage with nothing much to pull it back into line and the chips keep 
sensing an over voltage condition.

The chips don't require much current and so even though there isn't much 
current actually available at the output, there's nothing but the chips to 
use it. Thus every now and then the voltage will fall below the threshold 
and the SMPS circuitry will fire off a few pulses. Which of course, will 
immediately charge the caps up again. If there's something attached to the 
supply that also uses bugger-all juice but is particularly sensitive to 
over-voltage conditions (Say a 5 volt CMOS chip) then this chip will almost 
certainly be toast.

The SMPS chips don't fry because they're designed to handle a wide supply 
range and the transformer ratio is designed that even at full clip, it 
can't spit out more voltage than the ab-max of the circuitry. In single 
supply switchmodes this is not usually so much of a problem because the 
feedback to the switch mode chips is monitoring the actual output. However 
with computer supplies where there are also +/-12 volt rails, these rely on 
the feed back provided by the 5 volt rail. The net effect is that you'll 
get spurious output from your 12 volt rails as the 5 volt rail bounces 
round. The 12 volt rail will bounce over and under voltage and tends to 
destroy things.

Ergo; You must have sufficient load on the output but not necessarily 
attaching a bar-radiator to it.

Take the 5 volts out. Filter it a bit more if you have to, but don't muck 
round inside the things. And if it's not already in a safety cage of some 
kind, put it in one before you use it.

Hope this helps.
Be absolutely Icebox.

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