#011 EMI Suppression ~ Bypass Capacitor: The larger the capacitance, the greater the effect? ~

The role of a bypass capacitor in EMI suppression is to bring a noise current down to a ground.
Since a noise current contains a variety of frequency components, you might think it’s reasonable to select a bypass capacitor that flows all the frequencies into the ground. From this point of view, it seems that you only need to select a bypass capacitor which has large enough capacitance.

Then you might try to suppress noise thinking it can be solved by using e.g 0.1μF capacitor, such as recommended value by a semiconductor manufacturer, though the expectation could end in fail.
Capacitors with large capacitance generally have a large series parasitic L component, so the resonant frequency of the LC resonant circuit is low, and the frequency that it can handle is low.
At the frequency above the LC resonant frequency, it does not function as a bypass capacitor but as an L. As a result, you can’t let the noise current flow to the ground.

So, the larger the capacitance, doesn’t always have the greater the effect.

In order to make the bypass capacitor work, it is very important to combine multiple bypass capacitors with different capacitance to make the total impedance low over a wide frequency range by shifting the each LC resonance frequency.

However, when combining multiple bypass capacitors, attention must be paid to the parallel resonance (antiresonance) caused by the capacitors. When this antiresonance occurs, the ground impedance becomes higher at a particular frequency than it would be with a single capacitor. To prevent this anti-resonance, it is important to select the step of the each capacitance to within 10 times.

 

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