Selection of filter capacitors for switching power supplies

　Many electronic designers know the role of filter capacitors in power supplies, but the filter capacitors used at the input of switching power supplies are different from those used in power frequency circuits. Ordinary electrolytic capacitors used for filtering in power frequency circuits have a pulsating voltage frequency of only 100 Hz, and a charge and discharge time of milliseconds. In order to obtain a smaller ripple coefficient, the required capacitance is as high as hundreds of thousands of microfarads. Therefore, ordinary low-frequency ordinary aluminum electrolytic capacitors are manufactured with the purpose of increasing capacitance. The capacitance, loss tangent and leakage current of the capacitor are the main parameters to distinguish its quality. The frequency of the sawtooth voltage on the electrolytic capacitor used as input filter in the switching power supply is as high as tens of kilohertz or even tens of megahertz. Its requirements are different from those for low-frequency use. Capacitance is not the main indicator. What is good or bad is its impedance-frequency characteristics. It is required to have low and equal impedance within the working frequency band of the switching power supply. At the same time, for the outside of the power supply, the peak noise of up to hundreds of kilohertz generated by the semiconductor device starting to work can also have a good filtering effect. The impedance of ordinary electrolytic capacitors used in ordinary low frequencies begins to become irrational at around 10 kilohertz, which cannot meet the requirements of switching power supply use. The high-frequency aluminum electrolytic capacitor used in the switching power supply has four terminals. The two ends of the positive aluminum sheet are respectively led out as the positive electrode of the capacitor, and the two ends of the negative aluminum sheet are also led out as the negative electrode. The current of the regulated power supply 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 returning from the load also flows in from one negative end of the capacitor, and then flows from the other negative end to the negative end of the power supply. Since the four-terminal capacitor has excellent high-frequency characteristics, it provides a very advantageous means for reducing the pulsation weight of the input voltage and restraining the switching spike noise. High-frequency aluminum electrolytic capacitors also have a multi-core method, which divides the aluminum foil into several shorter segments, and connects them in parallel with multiple lead sheets to reduce the resistance component in the capacitive reactance. At the same time, low-resistivity materials are used and screws are used as lead terminals to enhance the capacitor's ability to withstand large currents. Stacked 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 stacked capacitor is similar to the original book, and the magnetic flux generated by the current flowing through is offset in the opposite direction, thereby reducing the value of the inductance and having more excellent high-frequency characteristics. This capacitor is generally made into a square shape, which is easy to fix and can also appropriately reduce the volume of the machine. In addition, there is a four-terminal stacked high-frequency electrolytic capacitor that combines four terminals and stacks. It combines the advantages of both and has better high-frequency characteristics.