In conventional switching converter, the parasitic capacitance between switching circuit and ground introduces common mode (CM) noise problem. A CM inductor is inserted in the power feeding paths to produce a high impedance to attenuate the CM noise. However, this parasitic capacitance and the CM inductor create low-frequency resonance near the switching frequency and its harmonics. Thus, the filtering performance is diminished. Increasing the CM inductance to shift the resonant frequency to low-frequency range is one of the methods to tackle this problem.

However, this approach leads to increase the power losses (both core and winding losses) of the CM inductor reducing the efficiency of the converter. In this paper, a tunable CM inductor with a small-space auxiliary winding is proposed. The auxiliary winding can be connected to a passive network to alter the frequency response of the CM inductor without affecting the original inductance. As a result, the influence of the low-frequency resonance can be mitigated. A proof-of-concept prototype is constructed and its performance is experimentally measured. Results show that the proposed tunable CM inductor operates as theoretically anticipated.

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