The motherboard inside the computer is mixed with various high-frequency circuits, digital circuits, and analog circuits. When they work, they will generate a large amount of high-frequency electromagnetic waves that interfere with each other, which is EMI. EMI can also emit outward through motherboard wiring or external cables, causing electromagnetic radiation pollution and affecting the normal operation of other electronic devices.

The chip on the PC board is both an electromagnetic interference object and a source of electromagnetic interference during operation. In general, we can divide these electromagnetic interferences into two categories: serial mode interference (differential mode interference) and common mode interference (ground interference). For example, taking the wiring of two PCBs on the motherboard (connecting wires to various components of the motherboard) as an example, serial mode interference refers to the interference between the two wiring; Common mode interference is caused by the potential difference between two wiring and the PCB ground wire. The series mode interference current acts between 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 wire, and the interference current flows through half of each signal line in the same direction, with the ground wire as the common circuit.

If the common mode current generated by the board is not attenuated and filtered (especially the common mode current on high-speed interface cables such as USB and IEEE 1394 interfaces), the common mode interference current can easily generate electromagnetic radiation through the interface data cable - the common mode radiation generated by the common mode current in the cable. The FCC in the United States, CISPR22 from the International Committee on Radio Interference, and GB9254 from China all have restrictions on common mode conducted interference and radiated emissions from communication ports of information technology equipment. In order to eliminate interference signals and various induced interferences on the signal line, we must arrange filtering circuits reasonably to filter common mode and series mode interference. Common mode inductance is a component of the filtering circuit.

A common mode inductor is essentially a bidirectional filter: on the one hand, it filters out common mode electromagnetic interference on the signal line, and on the other hand, it suppresses electromagnetic interference that does not emit outward, avoiding affecting the normal operation of other electronic devices in the same electromagnetic environment