common mode choke, is often used to filter common mode EMI signals in computer switching power supply

Common mode choke, also known as common mode choke, is often used to filter common mode EMI signals in computer switching power supply. In board design, common-mode inductance also acts as EMI filter, which is used to suppress electromagnetic wave radiation from high-speed signal lines

Small Knowledge of Electromagnetic Interference (EMI)

The main board of the computer mixes various high-frequency circuits, digital circuits and analog circuits. When they work, they will produce a large number of high-frequency electromagnetic waves which interfere with each other. This is EMI. EMI will also emit through the main board wiring or external wiring cables, causing electromagnetic radiation pollution, affecting the normal operation of other electronic equipment.

The chip on PC board is not only an electromagnetic interference object, but also an electromagnetic interference source. Generally speaking, we can divide these electromagnetic interference into two categories: serial mode interference (differential mode interference) and common mode interference (grounding interference). Taking the two PCB lines 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 lines, while the common mode interference refers to the interference caused by the potential difference between the two lines and the PCB ground line. Serial-mode interference current acts on two signal lines, and its direction of transmission is consistent with waveform and signal current. Common-mode interference current acts between signal lines and ground lines, and interference current flows through one-half of the two signal lines and in the same direction, and the ground line is the common circuit.

If the common-mode current generated by the board is not filtered through attenuation (especially the common-mode current on the high-speed interface line such as USB and IEEE 1394), the common-mode interference current can easily generate electromagnetic radiation through the interface data line - the common-mode radiation in the cable caused by the common-mode current. American FCC, CISPR22 of International Radio Interference Special Committee and GB9254 of our country all have relevant restrictions on common mode conducted interference and radiation emission of communication ports of information technology equipment. In order to eliminate all kinds of interference from input signal and induction on signal line, we must arrange filter circuit reasonably to filter common mode and serial mode interference. Common mode inductance is a part of filter circuit.

Common-mode inductance is essentially a two-way filter: on the one hand, it is necessary to filter the common-mode electromagnetic interference on the signal line, on the other hand, it is necessary to restrain itself from sending out electromagnetic interference, so as to 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 common mode inductor. In practical circuit design, multi-stage common mode circuit can be used to better filter electromagnetic interference. In addition, we can also see a patch type common mode inductor on the motherboard (Figure 3). Its structure and function are almost the same as that of the vertical type common mode inductor.