Generation Mechanism and Suppression Techniques of Electromagnetic Interference in Switching Power Supplies
Suppression of Electromagnetic Interference in Switching Power Supply
The three elements that form electromagnetic interference are the interference source, propagation path, and disturbed equipment. Therefore, suppressing electromagnetic interference should be addressed from these three aspects. The purpose is to suppress interference sources, eliminate coupling and radiation between interference sources and disturbed devices, and improve the anti-interference ability of disturbed devices, thereby improving the electromagnetic compatibility performance of switching power supplies.
Using filters to suppress electromagnetic interference
Filtering is an important method for suppressing electromagnetic interference, which can effectively suppress the entry of electromagnetic interference into equipment in the power grid, and also suppress the entry of electromagnetic interference from equipment into the power grid. Installing switch power filters in the input and output circuits of switch power supplies not only solves the problem of conducted interference, but also plays an important role in addressing radiated interference. The filtering suppression technology is divided into two methods: passive filtering and active filtering.
Passive filtering technology
Passive filtering circuits are simple, cost-effective, and reliable, making them an effective way to suppress electromagnetic interference. Passive filters are composed of inductive, capacitive, and resistive elements, and their direct function is to solve the problem of conduction emission. The schematic diagram of the passive filter used in switch mode power supply is shown in Figure 1.
Due to the large capacity of the filtering capacitor in the original power supply circuit, pulse peak currents are generated in the rectification circuit, which are composed of many high-order harmonic currents and interfere with the power grid; In addition, the conduction or cutoff of the switch tube in the circuit and the primary coil of the transformer will generate pulsating current. Due to the high rate of current change, induced currents of different frequencies are generated in the surrounding circuits, including differential and common mode interference signals. These interference signals can be conducted to other lines in the power grid and interfere with other electronic devices through two power lines. The differential mode filtering part in the figure can reduce the differential mode interference signal inside the switching power supply, and greatly attenuate the electromagnetic interference signal generated by the equipment itself during operation and transmitted to the power grid. According to the law of electromagnetic induction, E=Ldi/dt, where E is the voltage drop across L; L is the inductance; Di/dt is the rate of change of current. Obviously, the smaller the current change rate required, the larger the inductance required.
The interference signal generated by the pulse current circuit through electromagnetic induction with other circuits and the circuit composed of the earth or the casing is a common mode signal; In the switching power supply circuit, a strong electric field is generated between the collector of the switching transistor and other circuits, and the circuit will produce displacement current, which also belongs to common mode interference signals. Figure 1 * The mode filter is used to suppress common mode interference and attenuate it.
