A Three-Winding Common Mode Inductor

Electronic products consist of parasitic capacitance between the circuit components and the ground, which introduces common mode (CM) noise. A CM inductor is used to block the noise following into the power supply affecting the others. However, the CM inductor and the parasitic capacitance could create resonance phenomenon that affect the filter performance in low frequency. In this paper, we investigate into this resonance phenomenon by giving a detailed analysis of the CM inductor and filter configuration. A three-winding CM inductor is then proposed to solve the aforementioned problem. This three-winding CM inductor consists of two traditional CM inductor winding and an auxiliary winding connecting to a negative impedance converter. The impedance of the CM inductor is virtually boosted up. Without physically increasing the value of the inductance, the noise attenuation in low frequency is improved and the resonance phenomenon is significantly mitigated. A prototype is constructed and practically tested on a 100-W switched-mode power supply. Experimental results show that the three-winding CM inductor operates as theoretically anticipated. The noise level at the resonant frequency is reduced by 21 dB, whereas the power consumption of the additional circuit only consumes 0.5W.