Most commercial PSU's are tested to EN61000 (if in Europe)which calls up the original CISPR 22 specification. CISPR 22B is more stringent than CISPR 22A by about 10dB. Most commercial equipment is expected to comply with CISPR 22B
In my example, we were testing to 22B.
Note that this test only covers what goes up the AC line, and what's radiated into space. It doesn't care about internal connections in your system, which is why output noise from a PSU module can be higher.
I'm not aware of any tests that specify how PSU modules need to be tested. I didn't look at the standard, and don't know if it covers that sort of thing. I'm sure it covers much more than merely the product we were testing that day...
The spec is very specific about how the test is performed with the idea of ensuring repeatability.
Oddly, the lab tech said he doesn't like the test very much: that it's quite inconsistent on setup methods.
For example, the power and auxiliary cord positions, lengths, and how much is folded up. You're allowed to use whatever stock cable you bring, but the chances are, it won't be the exact length and position and angle specified in the standard.
Especially the ones where you need to stack ferrite beads on cables, and use voltage and current probes, to ensure controlled impedances. Yech, that bothers me just thinking about it. Though I think those pertain to susceptibility standards, not CISPR 22 specifically.
If you fail the conducted test don't even bother with the radiated test as you'll fail if your conducted emissions aren't low.
I don't think this is true. You can very easily add a small value CM filter that controls radiated without affecting the fundamental and first few harmonics. Conversely, you can add a huge CMC that takes care of conducted, but which has so much capacitance, the radiated remains strong. Radiation can also come from the PCB itself, or between wires, without regard to conducted.
Experience has shown me that a common mode choke is generally required on the DC output as well as the AC input. The output diodes generate a huge amount of noise when they recover.
Well, that can be part of it. But schottky diodes are very popular, as are synchronous rectifiers. Of course a poorly timed sync rect will exhibit something like recovery (or do even worse, if it's leading or lagging the ideal switching points), but it's certainly not limited to PN junction diodes.
I've built one flyback power supply, that makes a metric shitload of output-side noise. It appears to be the primary switching edge, pushed through interwinding capacitance and winding resonances, showing up as ground voltage. To the tune of a few hundred volts, if not for the Y caps. (It's so noisy, it's unstable and squeals without the correct selection of Y caps!)
This is peculiar, because with an essentially identical circuit, and transformer windup, but made for 6.3V instead, the circuit is quiet without needing secondary CM filtering!
So it's not just primary coupling, but it's also complicated induction in the secondary winding, too. High voltage outputs seem to be more difficult, at least for the windups I've been using with this series of supplies.
(For reference,
http://seventransistorlabs.com/Images/DistAmp2.jpg shows a 100V model of the PSU I'm talking about. The "high voltage" one is not pictured, but is made for 150-300V output. This one is also noisy, but not quite as bad; you can see I used a nice CMC on this one's output, to deal with that.)
Tim