Many electronic designers know the role of filter capacitors in the power supply, but the filter capacitors used at the input end of switching power supplies are not the same as the filter capacitors used in power frequency circuits. For ordinary electrolytic capacitors, the pulsating voltage frequency on it is only 100 Hz, and the charging and discharging time is on the order of milliseconds. In order to obtain a small pulsation coefficient, the required capacitance is as high as hundreds of thousands of microfarads. Therefore, ordinary low-frequency aluminum electrolytic capacitors are used. The purpose of manufacturing is to improve the capacitance. The capacitance, loss tangent value and leakage current of the capacitor are the main parameters to distinguish its quality.
As an electrolytic capacitor for input filtering in a switching regulated power supply, the frequency of the sawtooth wave voltage on it is as high as tens of kilohertz or even tens of megahertz. Its requirements are different from those used at low frequencies. Its advantage is its impedance-frequency characteristic, which requires it to have a low and equal impedance in the working frequency band of the switching regulated power supply. The peak noise can also have a good filtering effect. The ordinary low-frequency electrolytic capacitors are around 10 kHz, and their impedance begins to appear rational, which cannot meet the application requirements of switching power supplies.
The high-frequency aluminum electrolytic capacitor used by the switching regulated power supply has four terminals. The two ends of the positive aluminum sheet are identified as the positive electrode of the capacitor, and the two ends of the negative aluminum sheet are also identified as the negative electrode. The current of the regulated power supply flows from one positive end of the four-terminal capacitor, through the outside of the capacitor, and then flows from the other positive end to the load; the current from the load also flows from one negative end of the capacitor, and then flows from the other negative end to the power supply negative terminal.
Because the four-terminal capacitor has excellent high-frequency characteristics, it provides an extremely advantageous means for reducing the pulsating weight of the input voltage and suppressing the switching peak noise.
High-frequency aluminum electrolytic capacitors also have a multi-core method, which divides the aluminum foil into several short sections, and uses multiple lead-out sheets to connect in parallel to reduce the resistance component in the capacitive reactance. At the same time, low-resistivity materials are used and screws are used as leads terminals to enhance the capacitor's ability to accept large currents.
Laminated capacitors are also called non-inductive capacitors. The cores of ordinary electrolytic capacitors are rolled into a cylindrical shape, and the equivalent series inductance is large; the structure of the laminated capacitors is similar to the original shape of the book. It is offset, so the value of the inductance is reduced, and it has better high-frequency characteristics. This kind of capacitor is generally made into a square shape, which is easy to fix, and can also properly reduce the size of the machine.
In addition, there is also a four-terminal laminated high-frequency electrolytic capacitor that combines four-terminal and lamination, which combines the advantages of both and has better high-frequency characteristics.
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