Switching power supplies for Dummies Question

[John Fisher]

Greeting to all

I know very little about the nuts and bolts of switching power supplies except the basic concept which is: A high frequency oscillator before the transformer which helps to reduce the core size of the transformer considerably and then later regulated to the desired lower voltage.

Ok! My main question is I need a 200 watt switching power supply for about 25 to 30 volts to power a amplifier.
Is there a basic schematic available for this request?
Just a weird idea, but is it possible to somehow bump up the voltage a little bit on an already made generic computer power supply for this function?
Thanks!

[jrfrond]

You've KIND OF got it right. The key is to avoid lots of iron, and the SMPS is line-operated, meaning that the primary side oscillator is connected directly to line voltage. The transformer THEN steps down the resulting high voltage to the various voltage outputs required, which are then rectified and filtered. Because the AC is at such high frequency on the transformer secondaries, fast recovery diodes are used, but as a cost-reducing bonus, smaller, lower-value filter caps can be used as well.

Now, one thing in this world I would NOT try to build on my own is an SMPS. Most manufacturers don't either, because there are companies (mostly in China of course) that specialize in building them and do it very well, PLUS there are a whole lot of models available. An internet search will turn up PLENTY, but one place I like is Marlin P. Jones & Assoc. (www.mpja.com). Large selection to choose from.

[Rhodesplyr]

I agree with what John says, and I'd add that switch-mode power supplies can radiate a good bit of noise, *if* not optimally designed, noise which can be radiated in both directions--to the load and back into the wall.

That said, I've heard that designers and manufacturers have gotten much better about minimizing SMPS noise, but that leads me back to John's advice: get one that's well-designed from the ground up.

[Enzo]

To get the right SMPS - or any other power source for that matter - you need to specify what you need. 200 watts and 25-30 volts doesnlt really tell me much. First does your amp run on 30 volts? Or does it really run on +30v and -30v? And 200 watts? I don;t think you can get 200 watts very well from 30v rails unless we are going to a 2 ohm load.

I agree with the above, don;t try to build your own SMPS. And I also agree with John that Marlin P Jones is the place to start if you are looking for a power supply unit.

[Sweetfinger]

There was another thread here years ago on switched power supplies in which RG Keen offered similar advice- not a build for someone who doesn't know EXACTLY what they're doing. He said that he(they) used to refer to them as "self eating watermelons" because of their tendency to blow up on initial test.

[km6xz]

The 12 volt supplies of the computer power supplies are not designed to deliver high enough current for your application, although the 5 volt is. Yes, you can increase the output voltage by changing the reference feeding the driver IC comparator. The problem is that everything in the secondary will have to be match the higher output voltage so lots of parts changes regardless of the current demands. A high current switcher for 12 or 15 volts usually can be modified for 20-30 output. My utility Astron 30 amp x 13.8 volts bench supply now, after a little modification is a variable 0-20 volt supply.
Buying a bare switching supply module is the cheapest way to get what you want, it is hard to match the price of a new or surplus module because just the parts to make one would be 2-5 times the cost of a finished, tested and warrantied module.
Switching supplies have become much easier to design with a lot lower part count because of new full function driver ICs. Getting customized inductors/hf transformers is not so easy however because they are not off-the-shelf items.

[John Fisher]

Ok! I wasn't so interested in building one from scratch as I was from trying to convert a computer one to a higher voltage although I did learn a lot from the posts. One idea I had which may be crazy was to get 2 normal computer supplies working at 30 volts each. and then joining the outputs together in series to get a split power supply of 30 plus 30 volts to power an amplifier, maybe a little far fetched but that's what I was thinking. Computer power supplies are cheap and plentiful. Or does anyone have a direct link to an already made one with those specifications?
Thanks so much for all the good comments

[Enzo]

Frondelli already put up a link to m p jones, did you look over their selection?

[km6xz]

Bipolar supplies at higher voltage are harder to find than the computer supplies on that link. One company which makes Class D amp modules and higher voltage/current switching supplies for a reasonable price is CAD Audio in Denmark: Digital amplifier modules from CAD AUDIO DK up to excess of 1000 watts.