How to use a common mode inductor in a switching power supply?
Common mode inductor (Common mode Choke), also called common mode choke, is a coil wound symmetrically on a closed magnetic ring with opposite directions and the same number of turns. An ideal common mode choke has the effect of suppressing the common mode interference between L (or N) and E, but has no inductive effect on the differential mode interference existing between L and N. However, the incomplete symmetry of the actual coil winding will lead to the generation of differential mode leakage inductance. Signal current or supply current flows in opposite directions in the two windings, the resulting magnetic fluxes cancel each other out, and the choke exhibits low impedance. Common-mode noise currents (including disturbance currents caused by ground loops, also called longitudinal currents) flow in the same direction when flowing through the two windings. The generated magnetic fluxes add in the same direction, and the choke coil exhibits high impedance, thereby suppressing the common mode noise current. The effect of modulo noise.
The common mode inductor is essentially a two-way filter: on the one hand, it must filter out common mode electromagnetic interference on the signal line; on the other hand, it must suppress itself from emitting electromagnetic interference to avoid affecting the normal operation of other electronic equipment in the same electromagnetic environment. .
The common mode choke can transmit differential mode signals, and both DC and differential mode signals with very low frequency can pass through. However, it presents a large impedance to high-frequency common mode noise, so it can be used to suppress common mode current disturbance.
What are the applications of common mode inductors in switching power supplies?
Common mode inductors are sometimes called common mode chokes because their function is to suppress. They are mostly used in switching power supply circuits to form various filters to filter EMI and suppress the electromagnetic waves generated by various high-speed signals. Transmission, as shown in the figure below, there is a set of parallel lines in the circuit, which is basically unaffected when normal signals pass through. However, when a common mode current flows through it, due to the isotropic nature of the common mode current, a magnetic field in the same direction will be generated in the coil. Increasing the inductive reactance of the coil causes the coil to exhibit high impedance and produce a strong damping effect. At this time, the common mode current will be attenuated to suppress interference.
1. Schematic diagram of signal passing through common mode inductor
Schematic diagram of the front-end part of the switching power supply, in which L1 is the common mode inductor. In order to suppress common mode interference, we know that the common mode signal is a signal with equal amplitude and the same phase. The noise it generates is ground noise, which is the difference between the two lines. Regarding ground noise, if we understand the common mode noise, we will know the common mode inductor, and we will know why the common mode inductor is connected to the AC side.
