Common mode choke, also known as common mode choke, is often used in switching power supply of computer to filter common mode EMI signal. In the design of the board, the common mode inductor also plays the role of EMI filtering, which is used to suppress the electromagnetic wave radiated from the high-speed signal line.
A variety of high-frequency circuits, digital circuits and analog circuits are mixed on the main board of the computer. When they work, they will produce a large number of high-frequency electromagnetic waves to interfere with each other, which is EMI. EMI will also be emitted through the main board wiring or external cable, causing electromagnetic radiation pollution, affecting the normal work of other electronic equipment.
The chip on PC board is not only an EMI object, but also an EMI source. Generally speaking, we can divide these EMI into two categories: series mode interference (differential mode interference) and common mode interference (ground interference). Take the two PCB wires 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 wires; The common mode interference is caused by the potential difference between two wires and PCB ground. 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 wire. The interference current flows through half of the two signal lines in the same direction, and the ground wire is the common loop.
If the common mode current generated by the board is not attenuated and filtered (especially the common mode current on the high-speed interface line such as USB and IEEE 1394 interface), then the common mode interference current can easily generate electromagnetic radiation through the interface data line -- the common mode radiation generated by the common mode current in the cable. FCC of the United States, cispr22 of the International Special Committee on radio interference and GB9254 of our country all have relevant restrictions on common mode conducted interference and radiated emission of communication ports of information technology equipment. In order to eliminate the input interference signal on the signal line and all kinds of induced interference, we must reasonably arrange the filter circuit to filter the common mode and series mode interference. The common mode inductance is a part of the filter circuit.
The common mode inductor is essentially a bidirectional filter: on the one hand, it needs to filter out the common mode electromagnetic interference on the signal line, on the other hand, it needs to restrain itself from emitting electromagnetic interference, so as to avoid affecting the normal operation of other electronic equipment in the same electromagnetic environment.

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