The filter capacitor is an energy storage device connected in parallel to the output of the rectifier power circuit to reduce the ripple coefficient of AC pulsation and smooth the DC output. In the use of electronic circuits that convert AC to DC power supply, the filter capacitor not only makes the DC output of the power supply smooth and stable, reduces the impact of alternating pulsating current on the electronic circuit, but also absorbs the current fluctuations and passages generated during the operation of the electronic circuit The interference of AC power supply makes the working performance of the electronic circuit more stable.

In order to obtain a good filtering effect, the discharge of the capacitor must be slow. The slower the discharge of the capacitor, the smoother the output voltage and the better the filtering effect. The discharge speed of the capacitor is related to the capacitance C and the load R. The larger C and R, the slower the discharge of the capacitor.

At the same time, the capacitance of the filter capacitor selected in the filter circuit is relatively large. The most commonly used electrolytic capacitors are hundreds to thousands of microfarads, and tantalum capacitors or niobium capacitors are also used in high-demand occasions; but even at tens of kilohertz At higher frequencies, the requirements for frequency characteristics are much more important than the requirements for capacity.

Under normal circumstances, the role of electrolytic capacitors is to filter out low-frequency signals in the current, but even for low-frequency signals, the frequency is divided into several orders of magnitude. Therefore, in order to be suitable for use at different frequencies, electrolytic capacitors are also divided into high-frequency capacitors and low-frequency capacitors (here, the high frequency is relatively speaking):
(1) The low-frequency filter capacitor is mainly used for mains filtering or filtering after transformer rectification, and its working frequency is the same as that of mains at 50Hz.
(2) The high-frequency filter capacitor mainly works in the filter after the switching power supply is rectified, and its operating frequency is several thousand Hz to tens of thousands Hz.