Common Mode Inductance

What is a common mode inductor?
Common mode Choke, also known as common mode choke, is often used in switching power supplies of computers to filter common mode electromagnetic interference signals. In the design of the board, the common mode inductor also plays the role of EMI filter, which is used to suppress the outward radiation of electromagnetic waves generated by high-speed signal lines.

Theoretical knowledge of common mode inductors
Various high-frequency circuits, digital circuits, and analog circuits are mixed on the motherboard inside the computer. When they work, they will generate 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 cables, causing electromagnetic radiation pollution and affecting 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 working process. In general, we can divide these electromagnetic interferences into two categories: series mode interference (differential mode interference) and common mode interference (ground interference). Take the two PCB traces on the motherboard (the wires connecting the various components of the motherboard) as an example, the so-called series mode interference refers to the interference between the two traces; The common mode interference is the interference caused by the potential difference between the two traces and the PCB ground. The series 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 wire. The interference current flows through half of the two signal lines and 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 high-speed interface traces such as USB and IEEE 1394 interfaces), then the common-mode interference current can easily generate electromagnetic radiation through the interface data line— —Common mode radiation due to common mode currents in cables. US FCC, CISPR22 of the International Special Committee on Radio Interference, and my country's GB9254 and other standards and regulations. Both have related restrictions on the 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 various induced interferences, we must reasonably arrange the filter circuit to filter the common-mode and series-mode interference, and the common-mode inductor is a component of the filter circuit.
A 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, and on the other hand, it is necessary to restrain itself from sending out electromagnetic interference. Avoid affecting the normal work of other electronic equipment in the same electromagnetic environment.

Signal Common Mode Inductor
Part Number	Common Mode Impedance (Ω)	Rated current (mA)	DC resistance (Ω)	encapsulation
CMF0605UD120MFR	12	100	2	0605
CMF0605UD350MFR	35	100	2.7	0605
CMF0605DH900MFR	90	100	5	0605