What is a filter capacitor and the function of a filter capacitor

一、What is a filter capacitor

An energy storage device installed at both ends of the rectifier circuit to reduce the ripple coefficient of the ac pulsation and improve the smooth dc output. This device is usually called a filter capacitor.Since the filter circuit requires the energy storage capacitor to have a relatively large capacitance.Therefore, most filter circuits use electrolytic capacitors. Electrolytic capacitors get their name from the fact that they use an electrolyte as the electrode (negative pole).One end of the electrolytic capacitor is the positive pole and the other end is the negative pole, which cannot be reversed. The positive terminal is connected to the positive terminal of the rectified output circuit, and the negative terminal is connected to the negative terminal of the circuit.In all circuits that need to convert alternating current to direct current, setting filter capacitors will make the performance of electronic circuits more stable, and at the same time reduce the interference of alternating pulsation ripples on electronic circuits.The symbol of the filter capacitor in the circuit is generally represented by "C", and the capacitance should be determined according to the load resistance and the output current. When the filter capacitor reaches a certain capacity, increasing the capacitor capacity will have harmful effects on other indicators.

二、The function of filter capacitor

The filter capacitor is used in the power rectification circuit to filter out the AC component. Make the output DC smoother. And for precision circuits, the combination of parallel capacitor circuits is often used at this time to improve the efficiency of filter capacitors.

The low-frequency filter capacitor is mainly used for mains filtering or filtering after transformer rectification, and its working frequency is 50Hz consistent with the mains;The high-frequency filter capacitor mainly works in the filter after the switching power supply is rectified, and its working frequency is from several thousand Hz to tens of thousands of Hz.Filter capacitors play a very important role in switching power supplies. How to correctly select filter capacitors, especially the selection of output filter capacitors, is a matter of great concern to every engineer.

Common electrolytic capacitors used in 50 Hz power frequency circuits have a pulsating voltage frequency of only 100 Hz, and charge and discharge times are on the order of milliseconds.In order to obtain a smaller pulsation 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.Capacitance, loss tangent and leakage current of capacitors are the main parameters to identify its advantages and disadvantages.The output filter electrolytic capacitor in the switching power supply has a sawtooth wave voltage frequency up to tens of thousands of hertz, possibly tens of megahertz.At this time, capacitance is not the main indicator, and 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 have a good filtering effect on the high-frequency spike signals generated when the semiconductor device is working.

Ordinary low-frequency electrolytic capacitors begin to show inductance at about 10,000 Hz, which cannot meet the requirements of switching power supplies.The high-frequency aluminum electrolytic capacitor dedicated to the switching power supply has four terminals. The two ends of the positive aluminum sheet are respectively drawn out as the positive electrode of the capacitor, and the two ends of the negative aluminum sheet are also respectively drawn out as the negative electrode.The current flows in from one positive terminal of the four-terminal capacitor, passes through the inside of 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.