The motherboard inside the computer is mixed with various high-frequency circuits, digital circuits and analog circuits. When they work, they will produce a large number of high-frequency electromagnetic waves to interfere with each other,which is EMI. EMI is also emitted through the wiring of the mainboard or external cables, causingelectromagnetic radiation contamination and Affecting the normal operation of other electronic devices.
The chip on the PC board is both an electromagnetic interference object and an electromagnetic interference source in the working process. Generally speaking, we can divide these electromagnetic interferences into two categories: serial mode interference (differential mode interference) and common mode interference (ground interference). Take two PCB wiring on the motherboard (wires connecting components of the motherboard)as an example, the so-called serial mode interference, refers to the interference between the two lines; The common mode interference is the interference caused by the potential difference between two wires and PCB ground wire. The serial 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 line, and the interference current flows in half and in the same direction on each of the two signal lines, and the ground line is the common loop.
If the common-mode current generated by the boardis not attenuated (especially the common-mode current over high-speed interfaces such as USB and IEEE 1394), then the common-mode interference current can easily pass through the interface data line to generate electromagnetic radiation -- the common-mode radiation in the cable due to common-mode current. FCC of the United States, CISPR22 of the International Special Committee on Radio Interference and GB9254 of China and other standard specifications have related restrictions on common mode conducted interference and radiationemission of communication ports of information technology equipment. In order to eliminate the interference signal input on the signal line and the induction of various interference, we must reasonably arrange the filter circuit to filter the common mode and serial mode interference, the common mode inductor is a component of the filter circuit.
The common mode inductor /common mode filters is essentially a bidirectional filter: on the one hand, it is necessary to filter out the common mode electromagnetic interference on the signal line, on the other hand, it is necessary to suppress the electromagnetic interference from itself, so as to avoid affecting the normal work of other electroniequipment in the same electromagnetic environment.