“There are many kinds of noise, and the nature is also varied. Therefore, noise countermeasures (that is, methods for reducing noise) are also varied. The noise related to switching power supply is mainly discussed here, so please understand it as noise with lower voltage level and higher frequency in DC voltage. In addition to capacitors, there are noise reduction components such as Zener diodes and noise/surge/ESD suppressors. Different noise properties require different noise reduction components. In the case of DC/DC converters, most use LCRs to reduce noise depending on their circuit and voltage levels.
1. Noise countermeasures using capacitors
1) Use capacitors to reduce noise
There are many kinds of noise, and the nature is also varied. Therefore, noise countermeasures (that is, methods for reducing noise) are also varied. The noise related to switching power supply is mainly discussed here, so please understand it as noise with lower voltage level and higher frequency in DC voltage. In addition to capacitors, there are noise reduction components such as Zener diodes and noise/surge/ESD suppressors. Different noise properties require different noise reduction components. In the case of DC/DC converters, most use LCRs to reduce noise depending on their circuit and voltage levels.
2) Schematic diagram of using capacitors to reduce noise
Below is an example of reducing DC/DC converter output voltage noise by adding capacitors.
The waveform on the left shows the noise (ringing, reflection) of about 180mVp-p in the frequency range of about 200MHz when the capacitance of the LC filter at the output is 22µF. The waveform on the right is the result of adding a 2200pF capacitor to reduce this noise. As can be seen from the waveforms, adding a 2200pF capacitor reduces the noise by about 100mV.
What should be considered here is “why 2200pF”. The lower right graph shows the impedance frequency characteristics of the added capacitors.
The 2200pF capacitor was chosen because the impedance is lowest around 160MHz, and with this impedance characteristic, the noise amplitude can be reduced by about 2MHz.
This is a technique to reduce the noise amplitude by adding capacitors to lower the impedance at the target noise frequency.
To reduce noise by adding capacitors in this way, it is necessary to grasp the frequency of the noise (ringing, reflection) and select a capacitor with frequency characteristics corresponding to the impedance.
This article briefly introduces measures to reduce noise using capacitors. The next article will describe the effective use of decoupling capacitors.
2. Frequency characteristics of capacitors
In the previous article, the common mode filter for switching power supply input, including capacitors, inductors, ferrite beads and resistors, was introduced. Next, countermeasures in which capacitance and inductance are used for noise reduction will be introduced, which may also be referred to as “basics of noise countermeasures”. A simple four-element model is used here. More component models may be required to further express high-frequency resonances.
1) Frequency characteristics of capacitors
When discussing noise reduction with capacitors, it is important to fully understand the characteristics of capacitors. The figure below on the right is a schematic diagram of the relationship between the impedance and frequency of a capacitor, which is one of the most basic characteristics of a capacitor.
In the capacitor, not only the capacitance C, but also the resistance component ESR (equivalent series resistance), the inductive component ESL (equivalent series inductance), and the EPR (equivalent parallel resistance) existing in parallel with the capacitance exist. EPR has the same meaning as insulation resistance IR between electrodes or leakage current between electrodes. May generally use “IR” more.
C and ESL form a series resonant circuit, and in principle, the impedance of the capacitor has the V-shaped frequency characteristic shown in the figure above. Up to the resonant frequency, it is capacitive and the impedance drops. The impedance at the resonant frequency depends on the ESR. After the resonant frequency is passed, the impedance characteristic becomes inductive, and the impedance increases as the frequency increases. The inductive impedance characteristic depends on the ESL.
The resonance frequency can be calculated by the following formula.
It can be seen from this formula that the smaller the capacitance value and the lower the ESL of the capacitor, the higher the resonant frequency. If it is applied to noise cancellation, a capacitor with a smaller capacitance value and a lower ESL has a higher frequency and a lower impedance, so it can eliminate high frequency noise well.
Although the order described here is somewhat reversed, the countermeasures to reduce noise by using capacitors are to take advantage of the basic characteristic of capacitors that “the higher the frequency, the easier it is to pass when AC passes.” Bypass to GND etc.
The figure below shows the impedance frequency characteristics of capacitors with different capacitance values. In the capacitive region, the larger the capacitance, the lower the impedance. In addition, the smaller the capacitance value, the higher the resonance frequency and the lower the impedance in the inductive region.
The frequency characteristics of capacitor impedance are summarized below.
• The smaller the capacitance and ESL, the higher the resonant frequency and the lower the impedance in the high frequency region.
• The larger the capacitance value, the lower the impedance of the capacitive region.
• The smaller the ESR, the lower the impedance at the resonant frequency.
• The smaller the ESL, the lower the impedance of the inductive area.
In short, capacitors with low impedance have excellent noise canceling ability, and the frequency characteristics of impedance of different capacitors are different, so this characteristic is a very important point to confirm. When selecting a capacitor for noise reduction, select it based on the frequency characteristics of the impedance (not the capacitance).
When selecting a capacitor for noise reduction, it is necessary to recognize that the series resonant circuit of the LC (not the capacitor) is connected to confirm the frequency characteristics.