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EPC cancellation technique for EMI filter

In electromagnetic interference (EMI) filters, high frequency filter characteristics are determined by the parasitic parameter of filter components, especially the equivalent parallel capacitor (EPC) of inductors and equivalent series inductance (ESL) of capacitors. In a typical EMI filter, the influence of EPC of CM inductor usually is the major issue due to both large CM inductance value and large EPC.

In order to cancel the effect of the EPC of CM inductor, CM inductor is first center-tapped and one grounded capacitor is inserted to its tap to cancel the EPC. In this EPC cancellation concept, the cancellation capacitor is not necessarily to be inserted in the center point. General capacitance cancellation condition is derived as shown in the figure below. The same concept is also implemented in the integrated EMI filter. An embedded conductive layer is inserted into the CM winding and grounded. The structural capacitance between the embedded layer and CM winding is used to cancel the original EPC of CM inductor.

Parasitic parameters of the inductor and cancellation capacitor, and the non-ideal coupling between each other will degrade the EPC cancellation effect. The cancellation effect is mostly sensitive to the coupling coefficient between two main inductor parts. Higher coupling coefficient insures better cancellation effect. ESL of the inserted cancellation capacitor, along with the coupling between this ESL and main inductors will also degrade the cancellation effects.

Sponsored by Matsushita Electric Works.

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