Analysis of capacitor 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 end of switching power supplies are different from the filter capacitors used in power frequency circuits. Common capacitors used for filtering in power frequency circuits The pulsating voltage frequency of electrolytic capacitors 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 manufactured for low frequencies. The main goal is to improve the capacitance. The capacitance, loss tangent value and leakage current of the capacitor are the main parameters to identify its quality.
As an electrolytic capacitor for output filtering in a switching 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. To measure it What is good or bad is its impedance-frequency characteristics, which require it to have low impedance within the operating frequency band of the switching regulated power supply. At the same time, for the inside of the power supply, the peak noise of up to hundreds of kilohertz is generated by the semiconductor device starting to work. , can also have a good filtering effect. Generally, when ordinary electrolytic capacitors are used at low frequencies around 10 kHz, their impedance begins to appear inductive and cannot meet the requirements for switching power supplies.
It is a high-frequency aluminum electrolytic capacitor dedicated to switching power supply. It 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 respectively led out as the negative electrode. The current of the regulated power supply flows from one positive terminal of the four-terminal capacitor, passes through the capacitor, and then flows from the other positive terminal to the load; the current returned from the load also flows from one negative terminal of the capacitor, and then flows from the other negative terminal to the power supply. negative terminal.
Because the four-terminal capacitor has good high-frequency characteristics, it provides an extremely beneficial means for reducing the ripple component of the output voltage and suppressing switching spike noise.
High-frequency aluminum electrolytic capacitors also come in the form of multi-core, which divides the aluminum foil into several shorter segments and connects them in parallel with multiple lead-out sheets 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 withstand large currents.
Laminated capacitors are also called non-inductive capacitors. Generally, the cores of electrolytic capacitors are rolled into a cylindrical shape, and the equivalent series inductance is large. The structure of laminated capacitors is similar to that of books. Because the direction of the magnetic flux generated by the flowing current is opposite, it is called Cancel, 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.
In addition, there is 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.
