The Application of Common Mode Inductance in Switching Power Supplies?
A common mode inductor, also known as a common mode choke, is a coil symmetrically wound on a closed magnetic ring with opposite directions and the same number of turns. An ideal common mode choke has a suppressive effect on the common mode interference between L (or N) and E, while it has no inductive suppression effect on the differential mode interference between L and N. However, incomplete symmetry in actual coil winding can lead to the generation of differential mode leakage inductance. When signal current or power current flows in opposite directions between two windings, the magnetic flux generated cancels out each other, and the choke presents low impedance. The common mode noise current (including the disturbance current caused by the ground loop, also known as the longitudinal current) flows through two windings in the same direction, generating magnetic flux that is added in the same direction, and the choke presents high impedance, thereby suppressing the common mode noise.
The common mode inductor is essentially a bidirectional filter: on the one hand, it needs to filter out the common mode electromagnetic interference on the signal line, and on the other hand, it also needs to suppress the electromagnetic interference itself from emitting outward to avoid affecting the normal operation of other electronic devices in the same electromagnetic environment.
The common mode choke can transmit differential mode signals, and both direct current and low frequency differential mode signals can pass through. However, for high-frequency common mode noise, it presents a large impedance, so it can be used to suppress common mode current disturbances.
The Application of Common Mode Inductance in Switching Power Supplies?
Common-mode inductance, sometimes also known as common-mode choke, is used to suppress EMI in switching power supply circuits. It forms various filters to filter EMI and suppress the electromagnetic waves generated by various high-speed signals. As shown in the following figure, there is a set of parallel circuits in the circuit, which are basically unaffected when normal signals pass through. However, when there is a common-mode current flowing through, due to the directionality of the common-mode current, A magnetic field in the same direction will be generated within the coil, increasing the inductance of the coil, resulting in high impedance and strong damping effect. At this time, the common mode current will be attenuated, achieving the purpose of suppressing interference.
1. Schematic diagram of signal passing through common mode inductance
The schematic diagram of the front stage of the switching power supply, where L1 is a common mode inductor. In order to suppress common mode interference, we know that the common mode signal is a signal with equal amplitude and phase, and the noise it generates is ground noise, which is the noise caused by two wires being grounded separately. By understanding common mode noise, we can understand why the common mode inductor is connected to the AC side.
Common mode inductance on the AC side of the switching power supply circuit
