With the development of modern electronic technology and power devices, switching power supplies are widely used in computer and peripheral equipment communications, automatic control, household appliances and other fields due to their small size, light weight, high performance, and high reliability. The production, life and social construction have provided great help. However, with the rapid development of modern electronic technology and the widespread use of electronic and electrical equipment, various electronic and electrical equipment in the same working environment are getting closer and closer, and the external environment where electronic circuits work is further deteriorating. Since the switching power supply works in a high-frequency switching state, a high current and voltage change rate will be generated internally, which causes the switching power supply to produce strong electromagnetic interference. Electromagnetic interference signals not only cause pollution to the power grid, but also directly affect the normal operation of other electrical equipment and even the power supply itself, and break into the space as radiation interference, causing electromagnetic pollution, restricting people's production and life.

In the 1980s and 1990s in China, in order to strengthen the current domestic electromagnetic pollution control, some standards corresponding to CISPR standards, IEC801 and other international standards were formulated. Since China's mandatory implementation of 3C certification (china compulsory certification) on August 1, 2003, there has been an “electromagnetic compatibility fever”. The research and control of electromagnetic interference at close distances has increasingly attracted the attention of electronic researchers. A new hot spot in the current research field. This article will systematically discuss the related suppression technology for the generation mechanism of electromagnetic interference of switching power supply.

Suppression of electromagnetic interference of switching power supply

The three elements that form electromagnetic interference are the interference source, the propagation path, and the disturbed device. Therefore, the suppression of electromagnetic interference should be done from these three aspects. The purpose of suppressing the interference source, eliminating the coupling and radiation between the interference source and the disturbed device, and improving the anti-interference ability of the disturbed device, thereby improving the electromagnetic compatibility performance of the switching power supply.

Use filters to suppress electromagnetic interference

Filtering is an important method to suppress electromagnetic interference. It can effectively inhibit electromagnetic interference in the power grid from entering the equipment, and it can also inhibit the electromagnetic interference in the equipment from entering the grid. Installing a switching power supply filter in the input and output circuits of the switching power supply can not only solve the problem of conducted interference, but also an important weapon to solve radiated interference. The filtering suppression technology is divided into two ways: passive filtering and active filtering.

Passive filtering technology

The passive filter circuit is simple, low in cost, reliable in performance, and is an effective way to suppress electromagnetic interference. The passive filter is composed of inductance, capacitance, and resistance elements, and its direct function is to solve the conduction emission.

Due to the large capacity of the filter capacitor in the original power circuit, a pulse spike current will be generated in the rectifier circuit. This current is composed of a very large number of high-order harmonic currents, which interferes with the power grid; in addition, the switching tube in the circuit is turned on or off, and the transformer The primary coil will generate pulsating current. Due to the high rate of current change, induced currents of different frequencies are generated to the surrounding circuits, including differential mode and common mode interference signals. These interference signals can be conducted to other lines of 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 can greatly attenuate the electromagnetic interference signal generated by the equipment itself 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 rate of change is, the larger the inductance is required.

The interference signal generated by the pulse current loop through the loop composed of other circuits and the earth or chassis by electromagnetic induction is a common mode signal; a strong electric field is generated between the collector of the switch tube and other circuits in the switching power supply circuit, and the circuit will generate displacement current , And this displacement current is also a common mode interference signal. The common-mode filter in Figure 1 is used to suppress common-mode interference and make it attenuated.

Active filter technology

Active filter technology is an effective method to suppress common mode interference. This method takes measures from the noise source (as shown in Figure 2). The basic idea is to try to take out a compensation signal equal in size and opposite to the electromagnetic interference signal from the main loop to balance the original interference signal to achieve The purpose of reducing interference levels. As shown in Figure 2, the current amplifying effect of the transistor is used to filter the current in the base loop by folding the emitter current to the base. The filter composed of R1 and C2 makes the base ripple very small, so that the emitter ripple is also very small. Since the capacity of C2 is smaller than that of C3, the volume of the capacitor is reduced. This method is only suitable for low-voltage and low-power power supplies. In addition, attention should be paid to frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding and reliability when designing and selecting filters. The installation position of the filter must be appropriate, and the installation method must be correct, in order to achieve the expected filtering effect on interference