The resistance that obstructs the passage of alternating current at a specific frequency is called impedance. It is closely related to the capacitance and inductance values in the equivalent circuit of capacitors, and is also related to ESR. The capacitance reactance of a capacitor gradually decreases with increasing frequency in the low frequency range, and when the frequency continues to increase and reaches the mid frequency range, the reactance drops to the value of ESR. When the frequency reaches the high frequency range, the inductive impedance becomes dominant, so the impedance increases with the increase of frequency.

The output filtering electrolytic capacitor in a switching power supply has a sawtooth voltage frequency of up to tens of kHz, even tens of MHz. At this time, capacitance is not its main indicator. The standard for measuring the quality of high-frequency aluminum electrolytic capacitors is the "impedance frequency" characteristic, which requires a low equivalent impedance within the operating frequency of the switching power supply and good filtering effect on high-frequency peak signals generated by semiconductor devices during operation.

Characteristics of electrolytic capacitors: The capacitance per unit volume is very large, tens to hundreds of times larger than other types of capacitors.

Characteristic 2 of electrolytic capacitors: The rated capacity can be very large, easily reaching tens of thousands of μ f or even several f (but not comparable to double-layer capacitors).

Characteristic three of electrolytic capacitors: They have an overwhelming advantage in price compared to other types, because the materials used to make electrolytic capacitors are ordinary industrial materials such as aluminum, etc. The equipment for manufacturing aluminum electrolytic capacitors is also ordinary industrial equipment that can be produced on a large scale with relatively low costs.