Based on the analysis of typical EMI filter topologies, the working principle and characteristics of single-phase common-mode inductors, a single-phase common- and differential-mode magnetically integrated inductor is proposed, and the magnetic integration principle of its common-mode and differential-mode inductors is analyzed. Results show that the single-phase common- and differential-mode magnetically integrated inductor fabricated using an EI-gap core has a common-mode inductance of 833.8 μH (10 kHz), a differential-mode inductance of 42 μH, and a saturation current exceeding 20 A.
Switching power supplies (SMPS) generate two types of noise: common-mode noise and differential-mode noise. A typical topology of the EMI filter for a switching power supply is shown in Figure 1 [1]. A common-mode inductor is usually composed of two independent windings wound on a magnetic core. The two windings have the same number of turns but are wound in opposite directions, as shown in Figure 2. A differential-mode inductor can be achieved by the leakage inductance of the differential-mode current flowing into the common-mode inductor, but more often it is an independent differential-mode inductor. High-permeability ferrite materials and amorphous nanocrystalline materials are widely used to make common-mode inductors due to their high initial permeability [2-3]. Metal powder core materials are used to make differential-mode inductors due to their good DC bias characteristics [4].
As shown in Figure 2, after the common-mode inductor is connected to the circuit, the magnetic flux generated by the differential-mode current and the current on the two windings cancels each other in the magnetic ring, so that the magnetic ring will not reach the saturation state. Furthermore, since the magnetic permeability of the magnetic ring is much higher than that of air, the leakage magnetic flux is small, and the leakage inductance part acts as the differential-mode inductor. For the common-mode current passing between the power line and the ground line, since the direction is the same, the magnetic flux is superimposed, which generates high impedance to the common-mode current, thereby greatly reducing the common-mode noise interference [5].
Reducing the number of magnetic components by using magnetic cores made of special magnetic materials or magnetic circuit structures and magnetic integration technology is one of the hot topics in EMI filter optimization, and there are many reports on this topic.
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