Detailed analysis of the selection and calculation of power supply filter capacitor (II)

The size of the power filter capacitor is designed at ordinary times. 4.7u is used in the front stage to filter the low frequency. The two stage uses 0.1U to filter the high frequency. The capacitance of the 4.7uF is to reduce the output pulsation and low frequency interference. The capacitance of the 0.1uF should reduce the high frequency interference caused by the instantaneous change of the load current. Generally, the bigger the front, the better. The difference between the two capacitors is about 100 times. Power filter, switching power supply depends on how large your ESR is (equivalent series resistance of capacitors), and the choice of high frequency capacitance is best on its self resonant frequency. Large capacitance is to prevent surge, and the mechanism is better than the flood control of large reservoirs. Small capacitance filter high frequency interference, any device can be equivalent to a series of resistance, inductance, capacitance series parallel circuit, also have self resonance, only at this self resonant frequency, the equivalent electrical resistance is the least, so filtering is the best.

The equivalent model of capacitance is a series of inductors L, a resistor R and a capacitance C.

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

Therefore, it can be equivalent to a series LC loop to obtain its resonant frequency, and the condition of series resonance is WL=1/WC, W=2*PI*f, so as to get this formula F = 1/ (2pi* LC). The minimum reactance at the center frequency of the series LC loop is pure resistance, so the center frequency plays a filtering effect. The inductance of the lead wire is different in size and length. The inductance of the earth capacitance is usually about 1MM 10nH, depending on the frequency needed to be grounded.

The design of capacitive filter needs to consider the parameters:

ESR

ESL

Withstand Voltage

resonant frequency