July 31, 2021

The secret of EMC rectification of electronic engineering big coffee

First, the filter function of the capacitor

That is, the larger the frequency f, the smaller the impedance Z of the capacitor.

When the frequency is low, the capacitance C is relatively large due to the impedance Z, and the useful signal can pass smoothly;

When the high frequency is high, the capacitance C is already small due to the impedance Z, which is equivalent to short-circuiting the high frequency noise to the GND.

Second, when will capacitor filtering fail?

In the rectification, components such as capacitors are often used for filtering, and there are often arguments such as "large capacitance filter low frequency, small capacitance filter high frequency".

Taking the common surface-mount MLCC ceramic capacitor as an example, the equivalent model is as follows:

Since the equivalent model has both a capacitor C and an inductor L, which constitutes a second-order system, there is instability. For a circuit loop, resonance occurs, and the resonance point is at the following frequency:

The figure below is an example of a resonance curve:

It is often said that the capacitor is in front of the resonance point, and the capacitor is no longer the capacitor after the resonance point.

Third, when is the LC filter used?

If the series inductance L is connected in parallel to form C, LC filtering is formed:

A single capacitor C is a first-order system, and a single inductor L is also a first-order system with an amplitude attenuation slope of -20 dB. However, in the second-order system composed of LC, the amplitude attenuation slope is -40dB, which is closer to the ideal "steep" cutoff frequency effect, that is, the filtering effect is better.

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