#010 EMI Suppression ~ Are you saying that a bypass capacitor is not a capacitor? ~

One of the common method for EMI suppression is to insert a bypass capacitor. It’s a strategy to bypass a noise current into a ground.* However, have you ever had an experience that the emission didn’t decrease at all, but instead increased, even though you put in a bypass capacitor?
This is because, depending on how you use it, it doesn’t function as a capacitor but as an inductance.

The bypass capacitor itself has a parasitic inductance component, and the circuit around the bypass capacitor also has parasitic inductance, so the bypass capacitor (or rather, all the capacitors) consists of an LC series circuit.

At frequencies above the resonant frequency of the LC series circuit, it does not “bypass” the noise because it functions as an “L” instead of a “C”. In other words, in this state, it is not a bypass capacitor.

Therefore, when using a bypass capacitor, it is essential to determine the total parasitic L value and take care that the LC resonance frequency is higher than the noise frequency.


*In the strictest sense of the word, the noise current flowing into the ground doesn’t go away. It goes through the return circuit and back to the noise source. Therefore, it has not disappeared. However, the reason a bypass capacitor can suppress an emission is that the bypass capacitor allows a noise current to flow in a minimized closed loop, thus keeping the noise trapped so that it cannot reach other circuit or radiate into space.


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