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Reduction of the Mutual Couplings between Two Capacitors with Cancellation Turn

The mutual inductance between two capacitors in an EMI filter makes filter performance much worse than expected. Reducing it so as to improve EMI filter performance is a key for high performance EMI filter design. A ¾ cancellation turn is proposed to integrate with one of the capacitor to cancel the mutual inductance between two capacitors. The cancellation turn is very close to the capacitor and its loop area can be adjusted so that the mutual inductance between the cancellation turn and the other capacitor is equal to the mutul inductance between two capacitors. The induced voltage in cancellation turn can therefore cancel the induced voltag ein the capacitor. Another benefit of the cancellation turn is that it can reduce the ESL of the capacitor since it has a negative inductance with the integrated capacitor. Using Both 90 degree rotated inductor winding structure and the cancellation turn can improve EMI filter's performance by 40dB, a factor of 100, at 30MHz.

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