Noise Countermeasures Using Capacitors

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There are many types of noise, and their properties are also varied. Therefore, there are also many noise countermeasures (i.e., methods to reduce noise). Here, we mainly talk about noise related to switching power supplies, so please understand that it is noise with a lower voltage level and higher frequency in the DC voltage. In addition to capacitors, there are also 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 will use LCR to reduce noise according to their circuits and voltage levels.

Schematic diagram of using capacitors to reduce noise

Below is an example of reducing the output voltage noise of a DC/DC converter by adding capacitors.

The waveform on the left shows that when the capacitance of the output LC filter is 22F, there is about 180mVp-p noise (ringing, reflection) in the frequency range of about 200MHz. The waveform on the right shows the result after adding a 2200pF capacitor to reduce this noise. As can be seen from the waveform, adding a 2200pF capacitor reduces the noise by about 100mV.

What we should think about here is “Why is it 2200pF?” The figure below shows the impedance frequency characteristics of the added capacitor.

The reason why a 2200pF capacitor is chosen is that the impedance is lowest near 160MHz. By utilizing this impedance characteristic, the noise amplitude can be reduced by about 2MHz.

This technique is to add capacitors to reduce the impedance at the target noise frequency, thereby reducing the noise amplitude.

When adding a capacitor to reduce noise like this, it is necessary to understand the frequency of the noise (ringing, reflection) and select a capacitor with frequency characteristics that match the impedance.

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