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At a given frequency, the resistor that blocks the AC current is called the impedance. It is closely related to capacitance value and inductance value in capacitance equivalent circuit, and is also related to ESR. Capacitance resistance in the low frequency range as the frequency increases gradually decreased, the frequency continues to increase to reach the intermediate frequency when the reactance reduced to the ESR value. When the frequency reaches the high frequency range, the inductance becomes dominant, so the impedance increases with the increase of the frequency.
Output filter capacitor in switching power supply, its sawtooth wave voltage frequency is up to dozens of khz, even dozens of MHz, at this time, the capacitance is not its main indicator, the standard of high frequency aluminum electrolytic capacitors is the "impedance-frequency" characteristics, requires the switching power supply in the operating frequency to have a lower equivalent impedance, while the semiconductor devices in the work of high-frequency spike signal has a good filtering effect.