2019.02.07 Switching Noise - EMC
Over three articles we have explained "Effective Use of Decoupling Capacitors". These are extremely important points relating to the use of capacitors to deal with noise, and so are here summarized.
Point 1: Use of Multiple Decoupling Capacitors
In decoupling using multiple capacitors, the effects are different when using several capacitors having the same electrostatic capacitance and when using capacitors with different capacitances in combination.
■When using multiple capacitors with the same capacitance value
The impedance is lowered over all frequency ranges; effective for reducing noise overall.
■When using multiple capacitors with different capacitance values
The impedance can be lowered at higher frequencies; effective for reducing high-frequency noise. However, antiresonance may occur depending on the frequency, and conversely, the impedance may rise and noise may grow worse, so care must be taken.
Point 2: Reducing the Capacitor ESL
If capacitances are the same, the lower the ESL, the higher the resonance frequency rises. Thus reducing the ESL can improve the high-frequency characteristic, and high-frequency noise can be reduced more effectively.
■Using a capacitor that has the same value but is smaller in size
The ESL depends on the structure of the terminal portions; basically, smaller-size capacitors have smaller terminals, and so the ESL is normally smaller. When noise must be reduced at higher frequencies, selecting smaller-size capacitors is one option. However, attention must be paid to the DC bias characteristic.
■Using a capacitor with a lowered ESL
Among multilayer ceramic capacitors, there are types the ESL of which is lowered through innovations in the shape and structure, such as LW reversed type capacitors and three-terminal capacitors.
Effective Use of Decoupling Capacitors: Other Matters to be Noted
■Ceramic capacitors with a high Q factor
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.
■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.
■Virtual capacitor mounting when studying countermeasures
When adding a small-value capacitor to deal with high-frequency noise, placement of the capacitor as close as possible to the place where actual correction is needed should be studied, based on the theory that low-capacitance capacitors should be located as close to the noise source as possible. If placement is different during studies and after correction, impedance may differ, and the characteristics expected from evaluations may not be attained.
■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.
■Temperature characteristics of capacitors
The characteristic of a capacitor changes with temperature, and so when it is obvious from the application that a capacitor will be exposed to high or low temperatures or to extreme temperature changes, a device with a good temperature characteristic should be used.
For further details on each of these subjects, the links can be used to refer to the original articles. From the next article we will discuss noise countermeasures using inductors.