#013 EMI Suppression ~ Be careful of capacitance variation in a bypass capacitor ~

As I have explained so far, a bypass capacitor cannot be used without considering a resonance frequency LC. In addition to a LC self-resonant frequency, it is important to understand a resonance frequency with an L component of a peripheral circuit added.
Therefore, it is important to know the exact capacitance value and parasitic inductance component of the bypass capacitor. On the other hand, there is always a variation in any product, so a capacitor, as an electronic component, also have capacitance variations. The square root of the variation will lead to the resonance frequency shift.

For example, a bypass capacitor with a ±20% tolerance will show a resonance frequency variation of about 10%. It would be good if this was acceptable for the product, but if not, a bypass capacitor with less variation should be used. Otherwise, some products may not meet the noise standards. In addition, because of the temperature characteristics of the bypass capacitor, the operating state may vary or even fall out of specification depending on the temperature at which the product is used.

So, it is necessary to pay attention not only to a capacitance and a parasitic L-component, but also to a capacitance tolerance and temperature characteristics of the bypass capacitor.

 

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