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Common mode inductance theory
Aug 06, 2018

Common mode inductance theory

The main board of the computer is mixed with various high-frequency circuits, digital circuits and analog circuits. When they work, a large number of high-frequency electromagnetic waves are generated to interfere with each other. This is EMI. EMI will also be emitted through the motherboard wiring or external cables, causing electromagnetic radiation pollution, which not only affects the normal operation of other electronic devices, but also harmful to the human body.

The chip on the PC board is both an electromagnetic interference object and a source of electromagnetic interference during operation. In general, we can classify these electromagnetic interferences into two categories: series mode interference (differential mode interference) and common mode interference (grounding interference). Take the two PCB traces on the motherboard (the wires connecting the components of the motherboard) as an example. The so-called serial-mode interference refers to the interference between the two traces; the common-mode interference is the two traces and the PCB ground. Interference caused by the potential difference between the lines. 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 the two signal lines. One and the same direction, and the ground line is a common circuit.

vertical common mode inductor.png

Common mode inductance

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

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

Figure 2 is a schematic diagram of the internal circuit of our common common mode inductor. In the actual circuit design, multi-level common mode circuit can also be used to better filter out electromagnetic interference. In addition, we can also see a chip-type common mode inductor (Figure 3) on the motherboard, the structure and function is almost the same as the vertical common mode inductor.

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