Switching Noise
Measures to Address Noise Using Capacitors
2018.10.25
Points of this article
・Noise amplitudes can be reduced by lowering the impedance at the frequency of the targeted noise.
・A capacitor to be used to address noise is selected for its impedance frequency characteristic rather than for its capacitance value.
In the previous article, the frequency characteristics of capacitors were explained. This time, a summary of measures to address noise using the capacitors discussed here is given, together with related concepts.
Measures to Address Noise Using Capacitors
There are various types of noise, having a wide variety of characteristics. Hence there are also various noise countermeasures, that is, methods for reducing noise. Here we are essentially considering noise relating to switching power supplies, and so the reader should assume high-frequency noise with comparatively low voltage levels, present in DC voltages. Moreover, in addition to capacitors, Zener diodes, suppressors for noise, surge or ESD and the like are also used to deal with noise. There are various components to deal with noise according to the noise properties; when DC-DC converters are assumed, due to the circuits and voltage levels involved, in many cases LCR circuits are used.
Concepts Underlying Noise Countermeasures Using Capacitors
Below is an example in which the noise in the output voltage of a DC-DC converter is suppressed through addition of capacitors.
The waveform on the left is for a case in which the capacitor of the output LC filter is 22 μF; there is noise (ringing, reflection) at about 200 MHz with an amplitude of 180 mVp-p. In order to reduce this noise, a 2200 pF capacitor was added; the result is the waveform on the right. By adding the 2200 pF capacitor, the noise was attenuated to about 100 mV.
What the reader should be asking here is, “why 2200 pF?” The frequency characteristic of the impedance of the added capacitor is shown below.
In this article, we have given a summary explanation of noise countermeasures using capacitors. Next time, we will explain the use of effective decoupling capacitors.
【Download Documents】Switching Power Supply Basic of EMC and Noise Countermeasures
This is a handbook on the basics of EMC (electromagnetic compatibility) and noise countermeasures for switching power supplies. Based on the understanding of the basics of noise, it explains the noise countermeasures using capacitors and inductors in switching power supplies.
List of articles related to the「Measures to Address Noise Using Capacitors」
- Dealing with Noise Using Capacitors
- Understanding the Frequency Characteristics of Capacitors, Relative to ESR and ESL
- Effective Use of Decoupling (Bypass) Capacitors Point 1
- Effective Use of Decoupling Capacitors Point 2
- Effective Use of Decoupling Capacitors, Other Matters to be Noted
- Effective Use of Decoupling Capacitors, Summary
Download Technical Documents
Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.
Switching Noise
- Procedures in Noise Countermeasures
- What is EMC?
-
Dealing with Noise Using Capacitors
- Understanding the Frequency Characteristics of Capacitors, Relative to ESR and ESL
- Measures to Address Noise Using Capacitors
- Effective Use of Decoupling (Bypass) Capacitors Point 1
- Effective Use of Decoupling Capacitors Point 2
- Effective Use of Decoupling Capacitors, Other Matters to be Noted
- Effective Use of Decoupling Capacitors, Summary
-
Dealing with Noise Using Inductors
- Frequency-Impedance Characteristics of Inductors and Determination of Inductor’s Resonance Frequency
- Basic Characteristics of Ferrite Beads and Inductors and Noise Countermeasures Using Them
- Dealing with Noise Using Common Mode Filters
- Points to be Noted: Crosstalk and Noise from GND Lines
- Summary of Dealing with Noise Using Inductors
- Other Noise Countermeasures
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Basics of EMC – Summary