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Cancelling the Equivalent Parallel Capacitance (EPC) of CM Inductors

It is well known that the equivalent parallel capacitance (EPC) of CM inductors is detrimental to their high frequency (HF) performance. Minimizing EPC so as to greatly improve CM inductor's HF filtering performance is very important for noise suppression and current ripple reduction in power electronics applications. In order to reduce the effects of EPC, traditionally, the distance between the winding turns is increased; however, the performance improvement is still limited. An EPC cancellation technique is proposed in CPES based on network theory. A capacitor whose capacitance is four times of EPC is connected to the center tap of the winding. Based on network theory, the resultant network is a π type filter without the effects EPC. The critical point for a good HF cancellation is the coupling coefficient of two winding halves should be close to unit. A bifilar winding structure is proposed to achieve the coupling coefficient to 0.99995. The experimental results show that after applying this technique and the bifilar winding structure, the inductor's HF filtering performance is improved by 22dB at 30MHz. It is a reduction of EPC by 13 times!
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