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Effects of Mutual Couplings between Two Capacitors on EMI Filter Performance

For a π type EMI filter below, the difference of the noise currents in two capacitors has a slope of 40dB/dec. When the noise current flows through one capacitor, it generates magnetic flux which links to the other capacitor. Although this coupling is very small, its effects are greatly amplified by a number of times which are equal to the difference between the currents in two capacitors. The amplified effects are more significant than the effects of the ESL of the capacitor at high frequencies since the current difference in two capacitor branches are significant. Furthermore, the induced noise voltage on the capacitor is directly added to the output loop of the filter. This makes the noise bypass the inductor at high frequencies. Experimental results show that after the mutual inductance between the input and output capacitors is reduced, filter's performance can be greatly improved at high frequencies. The same rule also applies to the in and out trace loops. They can be easily reduced by minimizing the loop areas of the two trace loops.

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