Switching Noise

Effective Use of Decoupling Capacitors, Other Matters to be Noted


Points of this article

・The relationship between the Q factor and the frequency-impedance characteristic should be understood to use capacitors with different Q factors selectively according to the objective.

・For a high-Q capacitor, the impedance drop is sharp in a narrow band. A low-Q capacitor has a more gentle decline over a broader frequency band.

・The thermal relief pattern of a wiring board and other factors cause inductance components to be increased, shifting the resonance frequency to the low-frequency side.

・Trial mounting when studying noise countermeasures may not result in the effect obtained for the modified board during studies, if the mounting method is not in keeping with the actual modifications.

・If the capacitance change rate is large, the resonance frequency may shift, and noise attenuation at the desired frequency may not be obtained.

・In applications with harsh or fluctuating temperature conditions, the possibility of using capacitors with better temperature characteristics, such as devices with CH/C0G characteristics, should be studied.

In the previous article, in succession to point 1, “Using multiple decoupling capacitors” in “Dealing with Noise Using Capacitor”, we explained point 2, ” Reducing the capacitor ESL”. In this article, we explain the final point, “Other matters to be noted”.

Effective Use of Decoupling (Bypass) Capacitors Point 1
Point 2: Reducing the capacitor ESL (equivalent series inductance)
・Other matters to be noted

Effective Use of Decoupling Capacitors: Other Matters to be Noted

① Ceramic capacitors with a high Q factor

Capacitors have a characteristic value called a Q factor, or simply Q. The following graphs show the relationship between Q and the frequency-impedance characteristic.

When Q is high, the impedance becomes extremely low in a specific narrow band. When Q is low, the impedance does not fall in this extreme manner, but the impedance can be lowered over a broad band. This characteristic is useful for conformance to a specific EMC standard. For example, when using a capacitor that has large variation in the electrostatic capacitance, if the Q factor is high, there is the possibility that the capacitor cannot eliminate noise at the targeted frequency. In such cases, there is the option of using a low-Q capacitor to suppress the effect of such variation.

② Thermal relief and other PCB patterns

Thermal relief and other PCB patterns, which are used with the goal of improving heat dissipation characteristics, increase the inductance component of the pattern. The increase in the inductance component causes the resonance frequency to be shifted to the low-frequency side, and so in some cases the desired noise elimination effect is not obtained.

③ Virtual capacitor mounting when studying countermeasures

After prototyping, measures to counter high-frequency noise are necessary, and the addition of small-value capacitors may be studied. At this time, if capacitors are mounted on a large-value capacitor as shown below (in the example on the left), an excess inductance component is added in the vertical direction, and so the effect of adding the capacitors is not adequately exhibited. In the center example, although not conflicting with the reasoning that “small-value capacitors are brought as close as possible to a noise source”, in actuality the impedance differs from that of the PCB layout for modification. The best method is to study the possibility of placing the capacitors as close as possible to where the modification is actually to be made (example on the right).

It is also possible that a noise countermeasure may be sufficient at the time of noise tests, but ultimately be inadequate when mounted on the modified PCB. Hence actual mounting must be taken into consideration from the start.

④ Capacitance change rate of capacitors

If the capacitance change rate of a capacitor used to deal with noise is high, there may be large fluctuations in the resonance frequency, so that fluctuations and variation may occur in the band to be attenuated, and it may be difficult to achieve the intended noise suppression. Noise countermeasures that require large attenuation in a narrow band require special attention. The following table indicates actual capacitance values for different capacitance change rates and the resulting resonance frequency. The table indicates that, depending on conditions of use, there are many cases in which changes in capacitance cannot be tolerated.

Capacitance change rate (%) Capacitance(pF) Resonance frequency(MHz)
+20 1,200 145
+10 1,100 152
+5 1,050 155
±0 1,000 159
-5 950 163
-10 900 168
-20 800 178

* Calculated assuming L = 1 nH

⑤ Temperature characteristics of capacitors

It is well known that the characteristics of capacitors fluctuate with temperature. At present, there aren’t standardized EMC tests with temperature characteristics, but there are capacitors that, depending on the application, must be used at high or at low temperatures, or that are used under conditions and in environments in which large temperature changes occur.

In such cases, there is a high probability of the occurrence of problems such as those described in ④ above, on the capacitance change rate, and so care must be taken to use capacitors with better temperature characteristics, such as those with CH or C0G characteristics, for noise countermeasures, insofar as possible.

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Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.