How to select filter capacitor

    The common electrolytic capacitor used in 50 Hz power frequency circuit has a pulsating voltage frequency of only 100 Hz and a charge discharge time of millisecond order. In order to obtain smaller pulsation coefficient, the required capacitance is as high as hundreds of thousands of micro methods. Therefore, the goal of ordinary low-frequency aluminum electrolytic capacitors is mainly to improve the capacitance. The capacitance, loss tangent and leakage current of capacitors are the main parameters to identify their advantages and disadvantages. The output filter electrolytic capacitor in switching power supply has a sawtooth voltage frequency of tens of thousands of Hz, even tens of megahertz. At this time, the capacitance is not the main indicator, and the standard to measure the high frequency aluminum electrolytic capacitance is the "impedance frequency" characteristic. It is required that the switching power supply should have a low equivalent impedance within the operating frequency, and have a good filtering effect on the high-frequency spike signals generated by semiconductor devices.

    Ordinary low-frequency electrolytic capacitors begin to show inductance at about 10000 Hz, which cannot meet the requirements of switching power supply. The high-frequency aluminum electrolytic capacitor for switching power supply has four terminals. The two ends of the positive aluminum sheet are respectively led out as the positive pole of the capacitor, and the two ends of the negative aluminum sheet are also led out as the negative pole. The current flows in from one positive end of the four terminal capacitor, passes through the capacitor, and then flows to the load from the other positive end; The current returned from the load also flows from one negative end of the capacitor, and then from the other negative end to the negative end of the power supply.

    Due to the good high-frequency characteristics of the four terminal capacitor, it provides a very favorable means to reduce the voltage ripple and suppress the switching spike noise. The high-frequency aluminum electrolytic capacitor also has the form of multi-core, that is, the aluminum foil is divided into several short sections, which are connected in parallel with multiple lead out pieces to reduce the impedance component in the capacitance reactance. The low resistivity material is used as the lead out terminal, which improves the capacity of the capacitor to withstand large current.