2018.10.25 Switching Noise - EMC
In the article before last and in the last article, we explained the frequency characteristics of capacitors and how they are used in noise reduction. In this and the next two articles, we will explain the effective use of decoupling capacitors.
Effective Use of Decoupling Capacitors
Broadly speaking, there are two important points to remember for the effective use of decoupling capacitors. Apart from these, there are also some matters to be noted. In this article, among these three topics for discussion, we shall focus on Point 1 below.
Point 1: Use of Multiple Decoupling Capacitors
One effective method of decoupling capacitor use involves use not of a single capacitor, but of multiple capacitors for decoupling. When using multiple capacitors, the effect is different when using capacitors with the same capacitance and when using a combination of capacitors with different capacitance values.
・When using multiple capacitors with the same capacitance value
The ESRs of three capacitors are connected in parallel, and so the impedance at the resonance point is , and if the ESR values of all the capacitors are assumed to be the same, then the ESR is reduced to 1/3 and the impedance is also reduced.
The ESLs of the capacitors in the inductive region beyond the resonance point are also in parallel, so that results, and if the ESL values of the three capacitors are all the same, the ESL is reduced to 1/3 and the impedance is also reduced.
In this way, by using multiple capacitors with the same value, the impedance can be lowered over the entire frequency range, and so noise can be further attenuated.
・When using multiple capacitors with different capacitance values
In general, the value of a capacitance to be added is selected according to the frequency of the noise to be reduced.
The frequency characteristic diagrams shown here are ideal examples, and parasitic components due to pattern wiring in the circuit board and the like have not been considered. When taking actual measures to counter noise, the effects of parasitic components must be included. In the next article, the second point mentioned above will be explained.
・There are two main points to consider for the effective use of decoupling capacitors: (1) Use of multiple capacitors, and (2) lowering capacitor ESL values.
・When using multiple capacitors, the effect differs depending on whether the capacitance values are all the same or are different.