Triple rail power supply design

Question

Hi, I have a need for a power supply that can deliver +5 @7A min ,-5@7A min and +12v@ >1A min with as little noise as possible , the source is standard 230v@50hz. There has allready been two different designs, the first one used switching psu:s but the output was not good enough (too much noise etc.).

The second one uses Power-One HN5-9/OVP-A x2 and HB12-1.7 x1 Linear psu:s. These work very well but I would like some alternatives to possibly increase the output power with pretty similar device footprint, those HN5 are pretty much as large as I can fit to the case I am using and seem to have been discontinued from farnell catalog. So would a large power switching supply with some sort of efective regulation and noise filtering be the better choice when considering size/price etc. Also what kind of filtering would be efective with the switching supplies, I allready have few of the old versions so they could propably work with some sort of filtering/stabilization circuitry.

enrico.migchels · Answer

Hi there Antto,

I don't know what your ripple and noise demands are, but if they are in the mV-range you could buy 3 SMPS products (2 x 5V, 1 x 12V) and improve the output stage. Start by replacing the output capacitors of the power supplies by really good onces. The main parameter to look for is ESR, the series impedance of a capacitor is the cause for ripple, the ESL is often the cause for the noise. To give you some insight in good capacitors: Supplier NCC, series KZH. If the improvement in ripple and noise is not sufficient, add a second filtering stage after the power supply. Build a 2-orde filter = series coil and after that: capacitor to ground. The inductance should be as high as possible but this is difficult when the current is 7A. A few mH should be no problem. Don't use a common mode choke but a drum core or even a transformer bobbin with a gapped ferrite core. use thick wires to avoid voltage loss across the coil. To damp the noise spikes you can also add ferrite beads (with multiple turns) in series with the outputs.

Best regards,

Enrico Migchels

Power conversion design engineer

Heliox B.V.

Best - The Netherlands

www.heliox.nl