#012 EMI Suppression ~ There’s a manner in placement of bypass capacitor ~

In the previous article, I mentioned that it is important to arrange the bypass capacitors with different capacitances, so that noise current over a wide frequency range can be flowed into the ground.

So, what should you do next in terms of where to place each bypass capacitor?
Assuming that there are three types of bypass capacitors with large, medium, and small capacitance, which bypass capacitor should be placed where the wiring length on a PCB is shortest?

This can be explained by returning to the principle.

Bypass capacitor with large capacitance is used for low frequency EMI suppression due to the low LC self-resonant frequency. On the other hand, bypass capacitor with a small capacitance has a high self-resonant frequency and is used as high-frequency EMI suppression. The LC resonance frequency is lowered by the combined L value of the internal parasitic inductance of the bypass capacitor and the L component of the PCB wiring.
At this time, regardless of the capacitance value of the bypass capacitor (large, medium, or small), the resonance frequency decreases due to the added effect of the wiring L component.
For a large or medium-capacitance bypass capacitor, the resonance frequency shift caused by wiring ΔL component is relatively small because the original LC product is large. However, in the case of a low-capacitance bypass capacitor, (L + ΔL)*C changes significantly because the original LC product is small, and the degree of frequency decrease becomes larger by that amount.

Therefore, the principle for arranging bypass capacitor is to place a low capacitance one, for the first priority, in a way the wiring being as short as possible.


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