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RE: Low RFI power supply design

Posted by John Swenson (A) on March 31, 2009 at 17:31:02

In Reply to: Low RFI power supply design posted by Ed Sommers on March 30, 2009 at 15:59:51:

I've done a lot of experimentation and listening with this. There are two (at least) differing issues here and different attempts to alleviate them.

One is the RF noise generated by the reverse charge of the diode. The other is the resonance of the transformer secondary. In many cases this resonance falls right in the range of noise generated by the diode. The diode noise excites the resonance which can amplify it to a large degree. Not only is this an issue for the box the PS is in, but the signal from the resonance passes right through the transformer and onto the power cord where it can go to other boxes and be radiated.

This resonance can not only be excited by the diode noise but also from the high current pulses you get from cap input filters.

Part of the confusion is that there are two different circuits called by the same name (snubber) that do very different things.

One type is what you have, a resistor and capacitor in series sitting in parallel with the transformer secondary. This attempts to damp the transformer resonance so it will not resonate no matter how it is "excited". Part of the problem with this solution is that different transformers have very different parameters which theoretically need different values to damp them properly. I did a bunch of tests on every transformer I had and found that a good compromise was 330 ohms and .022uf. With those values I couldn't get any of my transformers to resonate. One big advantage to this "snubber" is that it works for both diode noise and high current pulses.

The other type of snubber is an RC network across with each diode. This attempts to lower the noise at the source. Again you can get different behaviors with different diodes. I found this to be less effective than the secondary damping, it can help but it rarely gets rid of all the noise, and whats left still is amplified by the transformer resonance.

My solution is to use Schottky diodes which don't have reverse charge at all in addition to the secondary damping network. And then just to be safe I carefully design all my supplies to minimize current pulses in general. The overall result is a drastic reduction in PS noise, both fed to the electronics and fed to the outside world.

John S.

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