Selection of filter capacitance of switching power supply

Many electronic designers know the influence of filter capacitor in the power supply, but the filter capacitor used at the input end of the switching power supply is different from the filter capacitor used in the power frequency circuit. The common electrolytic capacitor used as filter at the top of the power frequency circuit has a pulse voltage frequency of only 100 Hz, and the charge and discharge time is millisecond. In order to obtain a smaller pulse coefficient, it is necessary to The calculated capacitance is up to hundreds of thousands of Micromethods. Therefore, ordinary low-frequency aluminum electrolytic capacitors are used for manufacturing. The purpose is to improve the capacitance. The capacitance, loss tangent and leakage current of capacitors are the secondary parameters to distinguish them.

As the electrolytic capacitor for input filtering in the switching power supply, the frequency of sawtooth wave voltage on it is as high as tens of kilohertz, even tens of megahertz. Its request and low-frequency use are different. The capacitance is not a secondary goal. What weighs its interest is its impedance frequency characteristic, which requires it to have low equivalent impedance in the task frequency band of the switching power supply, As for the external part of the power supply, because of the peak noise of hundreds of kilohertz which occurs at the beginning of the task of semiconductor devices, there can also be superior filter effect. The impedance of ordinary low-frequency common electrolytic capacitors starts to appear rational at 10 kilohertz, which can not satisfy the application requirements of switching power supply.

The high-frequency aluminum electrolytic capacitor, which is used by the switching voltage stabilizing power supply, has four ends. The two ends of the positive aluminum plate are identified and led out as the positive electrode of the capacitor, and the two ends of the negative aluminum plate are also identified and led out as the negative electrode. The current of the regulated power supply flows in from one positive end of the four terminal capacitor, through the outside of the capacitor, and then from the other positive end to the load; the current from the load flows in from one negative end of the capacitor, and then from the other negative end to the negative end of the power supply.

Because of its superior high frequency characteristics, the four terminal capacitor provides an extremely advantageous means to reduce the fluctuating weight of the input voltage and suppress the switching peak noise.

The high-frequency aluminum electrolytic capacitor also has a multi-core way. It divides the aluminum foil into several short sections, uses multiple lead-in pieces to connect in parallel to reduce the resistance component in the capacitive reactance. At the same time, it adopts the data of low resistivity and uses the screw as the lead-out terminal to strengthen the capacitor's ability to receive large current.

The core of the common electrolytic capacitor is rolled into a cylinder, and the equivalent series inductance is large; The structure of the laminated capacitor is similar to that of the book prototype, and it is offset due to the opposite magnetic flux deviation caused by the current flowing, so the value of the inductance is reduced, and it has more excellent high-frequency characteristics. This kind of capacitor is generally made into a square, which is convenient for fixing, and also can appropriately reduce the occupation volume.

In addition, there is also a kind of four terminal stacked high frequency electrolytic capacitor which combines the advantages of the two, with better high frequency characteristics.