Sendust application - Common Mode Choke

Sendust application - Common Mode Choke
 
Common mode Choke, also known as common mode choke, is commonly used to filter common mode electromagnetic interference signals in a computer's switching power supply. In the board design, the common mode inductor also acts as EMI filtering, which is used to suppress the emitted electromagnetic waves generated by high-speed signal lines.
 
The motherboard inside the computer is mixed with a variety of high-frequency circuits, digital circuits, and analog circuits, which produce a large number of high-frequency electromagnetic waves that interfere with each other, which is EMI. EMI also emits through motherboard wiring or external cables, causing electromagnetic radiation contamination that affects the normal operation of other electronic devices.
The chip on the PC board is not only an electromagnetic interference object, but also an electromagnetic interference source in the course of operation. In general, we can divide these electromagnetic interference into two categories: string mode interference (differential interference) and common mode interference (ground interference). Take the two PCB traces on the motherboard (the wires connecting the components of the motherboard) as an example, the so-called string mode interference, refers to the interference between the two traces, while the common mode interference is the interference caused by the potential difference between the two traces and the PCB ground. The string mode interference current acts between the two signal lines, and its conduction direction is consistent with the waveform and signal current, and the common mode interference current acts between the signal line and the ground line, and the interference current flows in one-half and the same direction on the two signal lines, and uses the ground line as the common circuit.
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 the USB and IEEE 1394 interface), then the common mode interference current can easily generate electromagnetic radiation through the interface data line - common mode radiation in the cable due to common mode current. The U.S. FCC, the International Radio Interference Special Committee CISPR22 and China's GB9254 and other standard specifications have related restrictions on the common mode conduction interference and radiation emission of communication ports of information technology equipment. In order to eliminate the interference signal input on the signal line and the various interference in the induction, we must arrange the filter circuit reasonably to filter the interference of the common mode and the string mode, the common mode inductor is an integral part of the filter ingress.

Common mode inductor is essentially a two-way filter: on the one hand, to filter out the common mode electromagnetic interference on the signal line, on the other hand, to suppress itself does not emit electromagnetic interference, to avoid affecting the normal work of other electronic devices in the same electromagnetic environment