How to choose the size of filter capacitance?

The impedance of the inductor is directly proportional to the frequency, and the impedance of the capacitor is inversely proportional to the frequency. Therefore, the inductor can choke the high-frequency through, and the capacitor can choke the low-frequency through. The proper combination of the two can filter various frequency signals. For example, in the rectifier circuit, the capacitance and the load or the inductor are connected in series on the load can filter out the AC ripple.

Inductive filtering belongs to current filtering, which smoothes the output current by generating electromagnetic induction through the current. The output voltage is low, which is lower than the effective value of AC voltage. It is suitable for large current, the larger the current, the better the filtering effect. Many of the characteristics of capacitance and inductance are just the opposite.

In general, the role of electrolytic capacitor is to filter out the low frequency signal in the current, but even the low frequency signal, its frequency is 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-frequency is relatively speaking).

The low-frequency filter capacitor is mainly used for the filter after the mains filter or transformer rectifier, and its working frequency is 50Hz in line with the mains; while the high-frequency filter capacitor is mainly used for the filter after the switching power supply rectifier, and its working frequency is thousands of Hz to tens of thousands of Hz. When we use the low-frequency filter capacitor in the high-frequency circuit, because the low-frequency filter capacitor has poor high-frequency characteristics, it has a large internal resistance and a high equivalent inductance in the high-frequency charging and discharging. Therefore, in use, it will generate more heat due to frequent polarization of electrolyte. The higher temperature will make the electrolyte inside the capacitor vaporize, and the pressure inside the capacitor will increase, which will eventually lead to the bulge and burst of the capacitor.

The size of power filter capacitance is usually designed. The first stage uses 4.7u to filter low frequency, the second stage uses 0.1U to filter high frequency. The capacitance of 4.7uf is used to reduce output pulse and low frequency interference. The capacitance of 0.1uF is used to reduce high frequency interference caused by instantaneous change of load current. Generally, the larger the front one is, the better. The difference between the two capacitance values is about 100 times. Power filter, switching power supply, depends on how large your ESR (capacitor equivalent series resistance) is, and the best choice of high-frequency capacitor is its self resonance frequency. The mechanism of large capacitance is to prevent surge, just like the flood control ability of large reservoir is stronger; small capacitance filters high-frequency interference, any device can be equivalent to a series parallel circuit of resistance, inductance and capacitance, and there is self resonance. Only in this self resonance frequency, the equivalent resistance is the smallest, so the filter is the best!

The equivalent model of capacitance is the series connection of inductance L, resistance R and capacitance C,

The inductance L is from the capacitor lead, the resistance R represents the active power loss of the capacitor, and the capacitance C

So it can be equivalent to LC circuit in series to find its resonance frequency. The condition of series resonance is WL = 1 / WC, w = 2 * pi * F, Thus, we get the formula F = 1 / (2pi * LC). The reactance at the center frequency of series LC loop is the minimum of pure resistance, so it plays a filtering effect at the center frequency. The size of lead inductance is different due to its thickness, and the inductance of grounding capacitance is generally about 1mm to 10nh, which depends on the frequency of grounding.

Parameters to be considered in the design of capacitor filter:

ESR

ESL

Withstand voltage

resonant frequency