How much do you know about common-mode currents as the main cause of EMI

Why is common mode current the main cause of EMI?

To answer this question, it is clear from the emission model formulas of common mode radiation and differential mode radiation that the energy of common mode radiation is much stronger, but this is not enough to give us a deeper understanding of the question "why common mode current is the main cause of EMI", because this is only based on the final energy.

A simple model cannot reflect the actual radiation of the common mode difference current in the actual situation. "We have not seen how common mode currents are generated, nor why common mode currents are more prone to radiation, so we have not strictly answered the above questions.".

The following content is based on the editor's new understanding of common mode current, that is, "Common mode current is any current in a circuit (a current that is difficult to control), except for the current flowing through the desired path.".

First, what is the differential mode current? Relatively speaking, it is the current that we can control. More precisely, it is the current that we can control its return path. For example, for a typical clock signal, we can control the path of the signal current by setting a complete reference position, thereby controlling the entire reflux region. In this controllable reflux region, the current is a differential mode current.

According to differential mode radiation models and practical experiments, the return area is the most critical factor in radiation intensity and radiation intensity, and it is not easy to radiate out. This is why differential mode current is not the main reason for the entire EMI test.

Next, we will answer how common mode currents are generated and why they are more prone to radiation (explaining some cases, not others). The author proceeds from the perspective of converting differential mode currents to common mode currents, which can also be said that most common mode currents are converted from differential mode currents.

When differential mode current flows in the middle of a PCB, "shunting" will definitely occur due to parasitic parameters (unavoidable) or factors such as incomplete reference layers and crosstalk coupling in the PCB. Assuming that the original signal current is 10 mA, in fact, in the expected return path, the current may only be 9 mA, so the remaining 1 mA current is left back to the drive end along other paths. This path will inevitably increase the reflux area of this part. The return area is EMI, so common mode currents are naturally more prone to radiation, even if the current is small.

The figure above shows an unexpected increase in the loop area due to an incomplete PCB reference ground

A new understanding of common mode currents is very helpful for us to solve practical problems with common and effective EMI ideas. We can better understand why to reduce crosstalk, why to set a reference layer, why to package ground, and why to reduce loop impedance.

Therefore, in addition to the normal signal flow path, efforts should also be made to eliminate any path EMI important ideas.