The resistance that hinders the passage of AC current at a specific frequency is called impedance. It is closely related to the capacitance and inductance values in the capacitance equivalent circuit, and is also related to ESR. The capacitance impedance of the capacitor gradually decreases with the increase of frequency within the low-frequency range, and the reactance decreases to the value of ESR when the frequency continues to increase and reaches the intermediate frequency range. When the frequency reaches the high-frequency range, the inductance becomes dominant, so the impedance increases with the increase of frequency.

The output filtering electrolytic capacitor in the switching power supply has a sawtooth wave voltage frequency as high as tens of kHz, or even tens of MHz. At this time, the capacitance is not its main indicator. The standard to measure the advantages and disadvantages of the high-frequency Aluminium electrolytic capacitor is the "impedance frequency" characteristic, which requires a lower equivalent impedance within the operating frequency of the switching power supply, and has a good filtering effect on the high-frequency spike signals generated by semiconductor devices during operation.

Characteristic 1 of electrolytic capacitor: the capacitance per unit volume is very large, dozens to hundreds of times larger than other types of capacitors.

Characteristic 2 of electrolytic capacitor: the rated capacity can be very large, which can easily reach tens of thousands μ F or even a few f (but cannot be compared to the double layer capacitance).

The third feature of electrolytic capacitor is that its price has an overwhelming advantage over other types, because the constituent materials of electrolytic capacitor are ordinary industrial materials, such as aluminum, etc. The equipment for manufacturing Aluminium electrolytic capacitor is also common industrial equipment, which can be produced in large scale and has relatively low cost.