When we were studying physics as a student, we always saw a term in the chapter on circuits: capacitance. Capacitors, as one of the most widely used electronic components, have many functions. Among them, filtering is one of the very common functions of capacitors. So do you know what a filter capacitor is? And what is the role of the filter capacitor?
1. Filter capacitor
Capacitors are two conductors that are close to each other and insulated from each other.
Filter capacitor refers to a kind of energy storage device installed at both ends of the rectifier circuit to reduce the ripple coefficient of AC pulsation and improve the high efficiency and smooth DC output. Because the filter circuit requires a large capacity for the energy storage capacitor. Therefore, most filter circuits use electrolytic capacitors. Electrolytic capacitors get their name because they use electrolyte as the electrode (negative electrode).
Filter capacitors are connected with other accessories, such as reactors and resistors, to provide a capacitor (or capacitor bank) with a low impedance path for one or more harmonics. It has the characteristics of small size, light weight, convenient carrying, and low leakage current.
The function of the filter capacitor: The function of the electrolytic capacitor is to filter out the low-frequency signal in the current, but even for the low-frequency signal, its 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).
Low-frequency filter capacitors are mainly used for electrical filtering or filtering after transformer rectification, and their working frequency is 50Hz consistent with the mains; while high-frequency filter capacitors are mainly used for filtering after switching power supply rectification, and their working frequency is several thousand Hz to tens of thousands. Hz. When we use low-frequency filter capacitors in high-frequency circuits, due to the poor high-frequency characteristics of low-frequency filter capacitors, it has a large internal resistance and a high equivalent inductance during high-frequency charging and discharging. Therefore, in use, a large amount of heat is generated due to frequent polarization of the electrolyte. The higher temperature will vaporize the electrolyte inside the capacitor and increase the pressure in the capacitor, which will eventually cause the capacitor to bulge and burst.
2. What is the function of the filter capacitor?
The function of the filter capacitor is mainly 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.
The function of the filter capacitor produces different operating frequencies according to the difference between low frequency and high frequency:
Low-frequency filter capacitors are mainly used for mains filtering or filtering after transformer rectification, and their operating frequency is the same as that of mains power at 50Hz; while high-frequency filter capacitors are mainly used for filtering after switching power supply rectification, and their operating frequency is several thousand Hz to several thousand Hz. Ten thousand Hz.
The filter capacitor plays a very important role in the switching power supply. How to select 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 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. The capacitance, loss tangent value and leakage current of the capacitor are distinguished 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 pros and cons 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 during the operation of the semiconductor device.
Ordinary low-frequency electrolytic capacitors begin to appear 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. 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.
The role of the filter capacitor as the name says is obviously mainly used for filtering.
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