Are you talking about common mode noise that simultaneously affects both pos and neg output on the DC side (due to capacitive coupling in the transformer) or just differential noise i.e. conventional ripple artifacts? Both are treated differently and both these noise sources depend on different things in a design. Are we talking about DC-to-DC isolating converters or just plain ordinary buck/boost regulators?

Can you adjust your question to more fully reflect your aim?

I think you misunderstand it from the view of someone answering. Someone answering can make an answer aimed at a specific design and provide good information. Then, if I were answering, the way my brain seems to work is that I could sit back and think how this might be applied to similar (but not identical) designs. If you expect someone to brainstorm the full gambit of possible scenarios prior to making an answer then your expectations are too high.

You still need to focus your question though. Asking for a schematic is also problematic because it just may not be the topology that is most appropriate to you. If I answered and you then said "yes but what about this or that converter" I'd have been wasting my time on the original answer. You have to be reasonable.

@Andy. See my edit.

The circuit you have posted will not have any significant degree of common mode noise because it is not an isolating DC-to-DC converter because the same GND symbol is used on either side of the transformer. This is so absolutely the reason why a circuit is needed. The circuit will only produce differential noise on the DC output. The devil, as always, is in the details.

The circuit is still problematic. It isn't a conventional flyback converter (that require extensive CM filtering due to a lack of earth) because flybacks don't use a bridge rectifier. So maybe it's a forward converter that deal with higher power transfer (than flybacks) and usually have an incoming earth that can be used to make a decent CM filter work properly yet, the primary is driven by 1 transistor that is hardly ever used in forward converters because of higher power. It's still confusing.

It also names the DC voltage as an error voltage and that is totally wrong. Like I said, the devil is in the detail and, in my humble opinion, if you want this question answering with respect to CM noise you need a believable circuit tied to a recognized power supply topology. The ElecCircuit.com picture doesn't fit this need.

@Andy. This contradicts my experiments. Sure, in theory, you are right: ground is ground. But in my experiments on desktop computer ATX PSUs, I've observed that ground is a fiction. These PSU are so inherently noisy in the high frequency range that a common mode noise arrive SOMEHOW to the terminals (I'm not able to explain how). I was also able to test that whenever the ground has a huge self capacitance, it behaves much like a ground.

@Andy. If you wish, you can do the following experiment: take an ordinary computer PSU and try to filter the noise at the output. You'll see. Regarding your other remarks, I will think about.

Then we need a schematic of the actual power supply. This is exactly the problem behind answering your question - you are changing the goalposts and wasting time. I'm not going to do any physical experiments, after all? Why should I? Like I've said many times here: the devil is in the detail and you need to provide detail and stick with it and not suddenly evolve off into different power supplies. You won't get help from me on this question by doing what you're doing.

Ground = fiction usually means poor o-scope probing.

Sounds like I made everyone angry, by trying to improve things. What a mess! definitely too much for me, I leave.