SMPS thoughts/questions

[gootee]

Originally posted by slomatt
Tom,

Thank you for the reply. From looking at the product offerings it seems that the less expensive supplies have lower switching speeds of around 25kHz which is very close to the top of the audible range, I'm looking for reasonable priced supplies with switching frequencies in the 40kHz+ range.

It looks like suitable supplies will run $40-50 each, which of course is getting into the price range of a linear power supply. There are some people on here who claim that switching power supplies should never be used for audio components and will necessarily introduce distortion into the amp, but then there are others that claim that they are the best supply they've tried. I'll keep looking into SMPSs.

- Matt

Hi Matt,

Could you design and build your own switchers? Both linear.com and national.com have automatic power supply design software, which is free. I got good results using LTspice from linear.com, and the built-in automatic smps designer (which might be a separate download, now; I don't know since I don't have the latest version.) I've also tried national.com's on-line Webench psu designer and it seems to work well-enough, and also gives a PCB layout AND a thermal simulation. PCB layout is important, for an SMPS. But there are some good application notes about that, which describe which parts are the most critical, and how to do it, etc. (But, that _would_ tend to make it more daunting, and more perilous,for someone who's never done it before.)

If you look at the designs that use some of the newer smps ICs, they are as simple as a linear supply, having as few as five parts for the actual DC-DC converter.

I did a 60 kHz boost converter using Linear Technolgy's LT1270A, which boosts a 12v-to-30v DC input (rectified from AC mains transformer) to 38v, at up to about 2A.

I added a simple, cheap LC lowpass to the output (forming a CLC Pi-topology lowpass filter with the output C that was already there for the SMPS), followed by a three-terminal adjustable linear regulator, and got rid of virtually ALL of the remaining 60 kHz hash and rectified mains ripple.

The higher you go in frequency, the cheaper they get to build, since the size of the magnetics gets smaller. And smaller inductance values handle more current, and cost less, generally, anyway, in addition to being physically smaller.

60kHz is now considered to be one of the lower frequencies, for an SMPS. They now often use 1 MHz and probably beyond. In my boost-converter case, only an inductor was required, rather than a special transformer. So I was able to use one of the off-the-shelf J.W. Miller toroidal high-current chokes, which are around $2 qty 1.

The only relatively-expensive parts were the mains transformer and the LT1270A IC, which was between $10 and $15 qty 1, if I recall correctly. But National's smps ICs are generally significantly cheaper than Linear's, I think.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

[Raka]

Nuuk,

this a copy of the table I sent to you. These are some measurements I did on one of the skynets.

[Nuuk]

Thanks Raka. But not being a techy, I'm not sure what they are saying to me! Is that good, bad, or purely academic?

I was going to try the +/-24v SMPS supply again but while it works fine in my upstairs system, I get all sorts of nasty noise in the main system! I've given up for now and am building another GC with the Pedja regs instead. (this will be my ninth 'final' GC )