To answer this question, it can be clearly seen from the emission model formulas of common mode radiation and differential mode radiation that the common mode radiation energy is much stronger. However, this is not enough for us to have a deeper understanding of the question "why common mode current is the main cause of EMI", because it is only from the perspective of final energy.
The concise model cannot reflect the actual radiation of differential common mode current in the actual situation. We have not seen how common mode current comes from and why common mode current is easier to radiate out. Therefore, strictly speaking, we have not answered the above questions.
The following content is based on Xiaobian‘s new understanding of common mode current, that is, "common mode current is any current (current difficult to control) in the circuit other than the current flowing in the path we want".
First, what is the differential mode current? Relatively speaking, it is the current that we can control. More accurately, it is the current that we can control its return path. For example, for a typical clock signal, we can control the path of signal current by setting a complete reference ground, which controls the entire return area. The current in this controllable return area is the differential mode current.
According to the differential mode radiation model and the actual test verification, the reflux area is the most critical factor of radiation intensity, and it is not easy to radiate as long as the loop area is reduced. This is why the differential mode current is not the main part in the whole EMI test.
Next, I will answer how the common mode current comes from and why it is easier to radiate out (only some cases, not others). The Xiaobian is from the perspective of converting the differential mode current into the common mode current. It can also be said that most of the common mode current is converted from the differential mode current.
When differential mode current flows in PCB, due to the existence of parasitic parameters (inevitable) or incomplete PCB reference layer, crosstalk coupling "shunting" will occur. If the original signal current is 10mA, the actual current in the desired return path may be only 9ma, and the other 1mA current is left back to the drive end along some other path. This path must make the reflux area of this part larger. The return area is the most important factor in EMI, so common mode current is naturally more likely to cause radiation, even if the current is very small.
The above figure shows the unexpected return path increasing loop area due to incomplete PCB reference ground
The new understanding of common mode current is very beneficial to the public and effective thinking of solving EMI problems in practice. Better understand why crosstalk should be reduced, why the reference layer should be set, why the ground should be covered, and why the loop impedance should be reduced. Therefore, it is an important idea to eliminate any path other than the normal signal flow path.
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