Power filter capacitor selection

The impedance of the capacitor is inversely proportional to the frequency, so the capacitor can block low-frequency passage. The inductance is the opposite - so if the two are combined properly, the various frequency signals can be filtered. For example, in the rectifier circuit, the capacitor can be connected to the load or the inductor in series, and the AC ripple can be filtered out.

Since the function of the electrolytic capacitor is to filter out the low-frequency signal in the current, even the low-frequency signal has its frequency divided into several orders of magnitude. Therefore, in order to be suitable for use at different frequencies, electrolytic capacitors are also divided into high-frequency capacitors and low-frequency capacitors (where high frequencies are relative).

Low-frequency filter capacitor is mainly used for mains filter or transformer rectification filter, its operating frequency is consistent with the city power is 50Hz; and high-frequency filter capacitor mainly works in the switching power supply rectification filter, its operating frequency is several thousand Hz to several Million Hz. When we use low-frequency filter capacitors for high-frequency circuits, because of the poor high-frequency characteristics of low-frequency filter capacitors, it has a large internal resistance at high frequency charge and discharge, and the equivalent inductance is high. Therefore, during use, large amounts of heat are generated due to the frequent polarization of the electrolyte. The higher temperature will vaporize the electrolyte inside the capacitor and the pressure in the capacitor will increase, eventually causing the capacitor to bulge and burst.

Power filter capacitor size, usually do design, the former with 4.7u, used to filter low frequency, secondary with 0.1u, used to filter high frequency, 4.7uF capacitor function is to reduce output ripple and low frequency interference, 0.1uF The capacitance should be to reduce high-frequency interference due to instantaneous changes in the load current. Generally, the bigger the better, the difference between the two capacitances is about 100 times. Power supply filtering, switching power supply, depends on how large your ESR (equivalent series resistance of the capacitor) is, and the choice of high-frequency capacitance is best at its self-resonant frequency. Large capacitance is to prevent surge, the mechanism is like a large reservoir flood control capability is the same; small capacitor filter high-frequency interference, any device can be equivalent to a resistor, inductor, capacitor series and parallel circuit, also has a self-resonance, Only at this self-resonant frequency, the equivalent resistance is minimal, so the filtering is best.

The equivalent model of the capacitor is an inductor L, a resistor R and a capacitor C in series (below), the inductor L is the capacitor lead, and the resistor R represents the active power loss of the capacitor. Therefore, it is equivalent to find the resonant frequency of a series LC loop. The condition of series resonance is WL=1/WC, W=2*PI*f, so that the formula f=1/(2pi*LC) is obtained, and the series LC loop is obtained. The minimum reactance at the center frequency is purely resistive, so the center frequency acts as a filter. The size of the lead inductance is different due to its coarse and short length. The inductance of the grounding capacitor is generally about 1 MM is about 10nH, depending on the frequency of the grounding.

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