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Effects of Mutual Couplings on Inductor's Performance

The inductive couplings between inductors and capacitors in EMI filters can not only affect the filtering performance of capacitors but also inductors. Although the inductance of inductors in EMI filters is usually much larger than the mutual inductance between inductors and capacitors, the currents in inductors and capacitors can be much different. As a result, a very small coupling between an inductor and a capacitor can have significant effects on inductor's performance. The effects of the coupling can be represented by a current controlled voltage source in the inductor branch. Depending on the values and polarities of the mutual inductance, the inductor behaves differently at high frequencies. Its impedance could be inductive or capacitive at high frequencies. The equivalent inductance could be larger or smaller than the original inductance of the inductor. Within certain frequency regions, its impedance may also proportional to frequency f or f3. Since the winding capacitance of the inductor could be positive or negative, the inductor may have increased or decreased resonant or minimal impedance frequencies. The 90 degree rotated winding structure is proposed to reduce the mutual inductance between the inductor and the capacitor.

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