Analysis of Capacitance Characteristics in EMC Design of Switching Power Supply
Many electronic designers know the role of filter capacitors in power supplies, but the filter capacitors used at the output of switching power supplies are different from the filter capacitors used in power frequency circuits. Electrolytic capacitors, the frequency of the pulsating voltage on it is only 100 Hz, and the charge and discharge 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 aluminum electrolytic capacitors are generally used for low frequencies. The goal is mainly to increase the capacitance. The capacitance, loss tangent and leakage current of the capacitor are the main parameters to identify its pros and cons.
As an electrolytic capacitor for output filtering in a switching regulated power supply, the frequency of the sawtooth voltage on it is as high as tens of kilohertz, or even tens of megahertz. Its requirements are different from those in low-frequency applications. Capacitance is not the main indicator. The good or bad is its impedance-frequency characteristic, which requires it to have a low impedance in the working frequency band of the switching regulated power supply. , It can also have a good filtering effect. Generally, ordinary electrolytic capacitors used for low frequencies are around 10 kHz, and their impedance begins to appear inductive, which cannot meet the requirements of switching power supplies.
The high-frequency aluminum electrolytic capacitor dedicated to the switching power supply has four terminals. The two ends of the positive aluminum sheet are respectively drawn out as the positive electrode of the capacitor, and the two ends of the negative aluminum sheet are also drawn 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 from the other positive end to the load; the current returned from the load also flows in 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 good high-frequency characteristics, it provides an extremely favorable means for reducing the pulsating component of the output voltage and suppressing the switching spike noise.
High-frequency aluminum electrolytic capacitors also have a multi-core form, which divides the aluminum foil into several short segments and connects them in parallel with multiple leads to reduce the resistance component in the capacitive reactance. At the same time, it uses low-resistivity materials and uses screws as lead terminals to enhance the ability of the capacitor to withstand large currents.
Laminated capacitors are also called non-inductive capacitors. Generally, the cores of electrolytic capacitors are rolled into cylindrical shapes, and the equivalent series inductance is large; the structure of laminated capacitors is similar to that of books. Offset, thus reducing the value of the inductance and having better high-frequency characteristics, this kind of capacitor is generally made into a square shape, which is easy to fix, and can also appropriately reduce the volume of the machine.
