Power filter capacitor how to choose - Database & Sql Blog Articles

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How to choose the power filter capacitor, master the essence and method, in fact, it is not difficult.
1) Theoretically ideal capacitors whose impedance decreases with increasing frequency (1/jwc), but due to the inductive effect of the pins at both ends of the capacitor, the capacitor should be considered as an LC series resonant circuit, the self-resonant frequency The FSR parameter of the device, which means that when the frequency is greater than the FSR value, the capacitor becomes an inductor.
If the capacitor is filtered to ground, when the frequency exceeds the FSR, the suppression of interference is greatly reduced, so a smaller capacitor is required to be connected in parallel to the ground. Can you think about why?
The reason is that the small capacitor has a large SFR value and provides a ground path for the high frequency signal. Therefore, in the power supply filter circuit, we often understand that the large capacitor considers the low frequency and the small capacitor considers the high frequency. The fundamental reason is the SFR (self-resonance). The frequency value is different, of course, you can think about why? If you think about this angle, you can understand why the capacitor in the power supply filter should be as close as possible to the ground.
2) So in the actual design, we often have questions, how do I know the SFR of the capacitor? Even if I know the SFR value, how do I choose the capacitance value of different SFR values? Is it a capacitor or two capacitors?
The SFR value of the capacitor is related to the capacitance value, and is related to the pin inductance of the capacitor. Therefore, the SFR value of the 0402, 0603, or the in-line capacitor of the same capacitance value will not be the same. Of course, there are two ways to obtain the SFR value. The device data sheet, such as the 22pf0402 capacitor SFR value is about 2G, 2) directly measure its self-resonant frequency through the network analyzer, think about how to measure? S21?
The essence of the capacitor is through AC, DC blocking, theoretically speaking, the larger the capacitor for power supply filtering, the better.
But due to the lead and PCB wiring, the capacitor is actually a parallel circuit of inductor and capacitor.
(There is also the resistance of the capacitor itself, sometimes it can not be ignored)
This introduces the concept of resonant frequency: ω = 1 / (LC) 1/2
The capacitance is capacitive below the resonant frequency, and the capacitance above the resonant frequency is inductive.
Therefore, the general capacitor filter low frequency wave, small capacitor filter high frequency wave.
This also explains why the capacitor filter frequency of the same value STM package is higher than the DIP package.
As for how much capacitor is used, this is a reference capacitor resonant frequency capacitance value DIP (MHz) STM (MHz)
1.0μF 2.5 5
0.1μF 8 16
0.01μF 25 50
1000pF 80 160
100 pF 250 500
10 pF 800 1.6 (GHz)
But just a reference, in the words of the old engineer - mainly by experience.
A more reliable approach is to connect two large and one small capacitors in parallel.
It is generally required to differ by more than two orders of magnitude to obtain a larger filter band.