[sdiy] switched mode PSU design

[Batz Goodfortune]

Y-ellow Peter 'n' all.

At 07:35 AM 8/10/05 -0700, Peter Grenader wrote:
>just a heads up:
>
>I've got a Buchla 259 here (analog)for repair  that's in a homebrew box
>which is powered by a single mutli-tap switcher (not a wallwart).  It's
>noisy as hell.  Replaced power with a linear - noise be gone.

This is most likely because you don't understand the nature of switchmodes. 
There's no reason why they should be inherently noisy if designed 
correctly. Indeed, they are often not. We use them in audio applications 
all the time where size and weight VS grunt are required.

Since I never got round to publishing the switchmode project and have since 
moved on, I'll just mention this. It's mostly got to do with load VS output 
smoothing cap size. A supply insufficiently loaded with an unsuitable 
smoothing cap is probably not a good thing. However, on the other hand, 
when you're talking about switching frequencies of 200Khz to 2meg as we are 
these days, it hardly constitutes a threat to audio.

As for your favorite cheap Chinese wall wart. This Chinese stuff is where 
the Japs were 20 years ago. Build things so they withstand the minimum 
requirement and tell people that fault conditions aren't your problem. But 
it probably doesn't matter too much anyway. A polyswitch isn't going to 
respond fast enough to cope with a strike surge.

Let me put it this way. The diodes and cap on the mains side are probably 
rated at 350 volts. Nice and cheap. With an mains voltage of 240 that 
rectifies out to about 340. A sufficiently long enough spike at just 
another 10 volts is all it takes. Mains switch gear commonly uses spikes of 
over 1000 volts but it's well sub-millisecond in nature. A friend of mine 
who once designed light dimmers for a living, once pulled an experiment 
where he got 15,000 amps across a 1N4048 diode and it lived. This was 
necessary for the system to withstand the slightly infinite impedance 
condition of a light bulb blowing. The catch was, that any more than a few 
microseconds like this and the diode did a good impression of a nuclear 
detonator. It's a scary thought that in most clipsal light dimmers (of that 
era at least) this diode is all that stands between "Happy Happy Joy Joy" 
and a lump of once molten plastic slag on your wall. I've got 2 in this 
house and they haven't done that yet.

Most switchers are happy under their bear minimum conditions. Give them 
something unusual and you're mileage my vary. They like being loaded and if 
you don't, they'll show their lack of appreciation by singing for you.

I don't see the problem really There really isn't much difference between a 
switcher and a linear supply at it's heart. The switch frequency is higher 
and the transformer is smaller. Other than that, there's nothing new under 
the sun.

Be absolutely Icebox.

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