Why should rectifier filter capacitors use high frequency and low resistance?

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Why should rectifier filter capacitors use high frequency and low resistance? [Copy link]

The 120Hz or 100Hz components in the output current components of the switching power supply are "very small", while the current components at and above the switching frequency are the vast majority of components. So what characteristics does the output electrolytic capacitor present at high frequencies?
Taking a 100uF electrolytic capacitor as an example, the capacitive reactance at a frequency of 500kHz is XC=1/(3.14*2*500kHz*100uF)=3.18mΩ, while the inductive reactance of a 10nH inductor is 31.4 mΩ, and the equivalent series resistance of a 100uF low ESR electrolytic capacitor is about 2 Ω. At this frequency, the electrolytic capacitor will mainly show ESR characteristics, that is, resistance characteristics. In fact, the capacitive reactance no longer works, so the actual equivalent circuit of the filter circuit becomes as shown in the figure below. When the load is 5 Ω, the ESR of the filter electrolytic will shunt 5/7 of the ripple current. Assuming that the peak current on the secondary side of the transformer is 3.3A, the ripple current that will be distributed to the load is close to 1A (2/7*3.3A=0.94A), and a peak ripple voltage of more than 5V will be generated on the 5Ω load. If the filter capacitor is an ideal capacitor, that is, ESR=0, only 0.03/(5+0.03)=0.06% of the ripple current (2.1mA) will be distributed to the load, and only a peak ripple voltage of 10.5mV will be generated.

Therefore, it can be seen that some switching power supplies use large-capacity capacitors or more capacitors in parallel in the output filtering. The ultimate effect is not the problem of capacitor filtering requiring such a large capacitance, but the problem of reducing the ESR and parasitic inductance of the filter capacitor. Then, if there is a capacitor filter with extremely low ESR and parasitic inductance, the capacitance of the output filter capacitor does not need to be very large. Therefore, under the conditions allowed, the output filter capacitor should choose a capacitor with low ESR and low parasitic inductance as much as possible. It can also be concluded that the indicators of such a switching power supply must be very poor if cheap ordinary electrolytic capacitors are used as output filter capacitors of switching power supplies, especially flyback switching power supplies, and it is difficult to improve them by other methods.
The selection of capacitors with excellent performance can significantly reduce the output voltage ripple of the switching power supply and significantly improve the efficiency. The reason for the improvement in efficiency is that the number of output filter capacitors is reduced, the leads on the circuit board are shortened, and the loss in this aspect is reduced. At the same time, the reduction of parasitic parameters also improves the efficiency. So don't think that it is equivalent to replace electrolytic capacitors with good performance with multiple capacitors with poor performance in parallel in the circuit.