The filter capacitor is used in the power rectifier circuit to filter out AC components. Make the output DC smoother. And for precision circuits, a combination of parallel capacitor circuits is often used at this time to improve the working effect of the filter capacitor.
Low-frequency filter capacitors are mainly used for mains filtering or filtering after transformer rectification, and their working frequency is the same as that of mains at 50Hz; while high-frequency filter capacitors are mainly used for filtering after switching power supplies, and their working frequency is several thousand Hz to several Ten thousand Hz. The filter capacitor plays a very important role in the switching power supply. How to choose the filter capacitor correctly, especially the choice of the output filter capacitor, is a problem that every engineering and technical personnel is very concerned about.
Common electrolytic capacitors used in 50 Hz power frequency circuits have a pulsating voltage frequency of only 100 Hz, and the charging and discharging time is on the order of milliseconds. In order to obtain a smaller ripple coefficient, the required capacitance is as high as hundreds of thousands of microfarads. Therefore, the goal of ordinary low-frequency aluminum electrolytic capacitors is to increase the capacitance. The capacitance, loss tangent value and leakage current of the capacitor are the identification The main parameters of its pros and cons. The output filter electrolytic capacitor in the switching power supply has a sawtooth voltage frequency as high as tens of thousands of hertz, or even tens of megahertz. At this time, the capacitance is not its main indicator. The standard to measure the quality of high-frequency aluminum electrolytic capacitors is the “impedance-frequency” characteristic. It is required to have a lower equivalent impedance within the operating frequency of the switching power supply, and at the same time, it has a good filtering effect on the high-frequency spikes generated when the semiconductor device is working.
Ordinary low-frequency electrolytic capacitors begin to be inductive around 10,000 Hz, which cannot meet the requirements of switching power supplies. The special high-frequency aluminum electrolytic capacitor for switching power supply has four terminals. The current flows in from one positive terminal of the four-terminal capacitor, passes through the capacitor, and then flows from the other positive terminal to the load; the current returning from the load also flows in from one negative terminal of the capacitor, and then flows from the other negative terminal to the negative terminal of the power supply.