Read and pay attention to the article. It is talking in those places about
CONDUCTED EMI, not radiated.
..Surely it cant be zero?....
Though we see "two lead" offline SMPS's, and they have common mode chokes, and Y caps across the transformer isolation barrier,
So they do have Common mode noise...so surely a non-isolated DCDC also does?, and can radiate to its surroundings...and when it
does so, its "go" and "return" currents will not be equal and so we have common mode noise?
You are ignoring KCL for a non-isolated DC-DC SMPS. It doesn't matter what the load looks like, it doesn't matter what the SMPS looks like, if you have one wire in and one wire out, the currents MUST always be equal. You don't get electrons magically flying off into the ether unless you are actually getting to where you are turning things into a plasma, and even then they must be returning through another path.
...surely these 3 sentences are filled with innaccurate statements? For starters, conducted EMI involves more strays than just capacitor related
ones, also PCB layout can surely help reduce conducted emissions? High voltage , low current SMPS's can have just as much conducted EMC problem
as Low voltage, high current ones, for example the dv/dt of the switching node can cause severe conducted emissions problems.
You are mixing up conducted and radiated EMI. Conducted EMI is passed through the circuit in question and is not part of the magnetic fields that create
radiated EMI coming from large loops on the board or fields created by switching currents. Remember, at zero current it is impossible to have conducted EMI, there has to be electrons flowing to get conducted EMI.
Please, review your physics.
A simple way to look at this is to think about a linear regulator on a board; conducted EMI is often viewed as the power supply rejection (PSRR) of the part. Conducted EMI should solely be a function of the circuit response and with perfect models will match between simulations and reality. I have personally seen that to hold true in the design of very high PSRR parts. But what was also found to be true is that the PSRR on early boards didn't match what simulations said it should. It was found that having a capacitor right at the input of the part killed the PSRR. This was because at frequency the capacitor was a low impedance and ran significant current. This raised the AC voltage at the input of the part (parasitic R and L of the traces coupled with the cap) and also created a magnetic field that coupled into other sections of the board. We are not talking about a 6dB difference in PSRR, but instead 40dB. It became very important to separate conducted versus radiated EMI and know which was which and how to correct each one.
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