Common-mode inductance

 

1. When common-mode current flows through, the magnetic flux in the magnetic ring is superimposed on each other, so that it has a considerable inductance, which suppresses the common-mode current. When the differential-mode current flows through the two coils, the magnetic flux in the magnetic ring cancels each other, and there is almost no inductance, so the differential-mode current can pass without attenuation.

Therefore, the common-mode inductor can effectively suppress the common-mode interference signal in the balanced line, and has no effect on the differential-mode signal transmitted normally by the line.

2. The chip on the PC board is both an electromagnetic interference object and an electromagnetic interference source during operation. In general, we can divide these electromagnetic interferences into two categories: series-mode interference (differential-mode interference) and common-mode interference (ground interference). Taking the two PCB traces on the motherboard (the wires connecting the components of the motherboard) as an example, the so-called series-mode interference refers to the interference between the two traces; while the common-mode interference is the interference caused by the potential difference between the two traces and the PCB ground wire. The series mode interference current acts between the two signal lines, and its conduction direction is consistent with the waveform and signal current; the common mode interference current acts between the signal line and the ground line, and the interference current flows through half of each of the two signal lines in the same direction, and the ground line is used as a common loop.

3. If the common mode current generated by the board is not attenuated and filtered, the common mode interference current can easily generate electromagnetic radiation through the interface data line. In order to eliminate the interference signal input on the signal line and various induced interferences, we must reasonably arrange the filter circuit to filter the common mode and series mode interference. The common mode inductor is a component of the filter circuit.

4. The common mode inductor is essentially a two-way filter: on the one hand, it is necessary to filter out the common mode electromagnetic interference on the signal line, and on the other hand, it is necessary to suppress itself from emitting electromagnetic interference to avoid affecting the normal operation of other electronic equipment in the same electromagnetic environment.