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EMI Filter Improvement: Cancellation of Mutual Coupling Between Filter Capacitors

Fig. 1. The capacitor helping filters achieve high performance.
EMI filters are used to attenuate EMI noise and meet EMI standards in power electronics equipment. Traditional EMI filters fail to achieve attenuation in high-frequency because of unwanted electro-magnetic field coupling effects between filter capacitor. A simple yet novel approach is proposed to help filters mitigate the undesired coupling, thus significantly improving its high frequency noise attenuation. The figure below shows the approach using a film capacitor and a ¾ turn copper foil. In the figure below, a new capacitor is constructed by simply arranging the copper foil next to the capacitor and connecting it to one of the terminals of the capacitor. Using this new technique in EMI filters, it can greatly improve filter's high-frequency performance above 1MHz. As much as 30-40dB improvement can be achieved at 30MHz.

Fig. 2. EMI filter performance improvement after using new capacitor.
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