With the development of modern electronic technology and power devices, switching power supplies are widely used in computer and peripheral communication, automatic control, household appliances, etc. for their small size, light weight, high performance, high reliability, etc. Production, life and social construction have helped a lot. However, with the rapid development of modern electronic technology, the wide application of electronic and electrical equipment, the distance of various electronic and electrical equipment in the same working environment is getting closer and closer, and the external environment of electronic circuit work is further deteriorated. Since the switching power supply operates in the high-frequency switching state, a high current and voltage change rate is generated inside, which causes the switching power supply to generate 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 or even the power supply itself, and also enter the space as radiation interference, causing electromagnetic pollution, which restricts people's production and life.
In the 1980s and 1990s, in order to strengthen the current domestic electromagnetic pollution control, some standards corresponding to international standards such as CISPR standard and IEC801 were formulated. Since the implementation of China compulsory certification on August 1, 2003, China has launched an "electromagnetic compatibility heat". The research and control of electromagnetic interference at close range has attracted the attention of electronic researchers. A new hot spot in the current research field. In this paper, the related suppression techniques will be systematically discussed for the generation mechanism of electromagnetic interference of switching power supplies.
Suppression of electromagnetic interference from switching power supplies
The three elements that form electromagnetic interference are the source of interference, the path of transmission, and the device being disturbed. Therefore, the suppression of electromagnetic interference should be based on these three aspects. The purpose of improving the electromagnetic compatibility of the switching power supply is to suppress the interference source, eliminate the coupling and radiation between the interference source and the victim device, and improve the immunity of the victim device.
Filter 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 can also prevent electromagnetic interference in the equipment from entering the power grid. Installing a switching power supply filter in the input and output circuits of the switching power supply not only solves the problem of conducted interference, but also is an important weapon to solve the radiation interference. Filter suppression technology is divided into passive filtering and active filtering.
The passive filter circuit is simple, low in cost and reliable in work performance, and is an effective way to suppress electromagnetic interference. The passive filter consists of an inductor, a capacitor, and a resistive element, and its direct function is to solve the conducted emission.
Due to the large capacity of the filter capacitor in the original power supply circuit, a pulse spike current is generated in the rectifier circuit. This current is composed of a very large number of high-order harmonic currents, which causes interference to the power grid. In addition, the switch tube is turned on or off, and the transformer is turned on. The primary coil produces a pulsating current. Due to the high current rate of change, induced currents of different frequencies are generated for the surrounding circuits, including differential mode and common mode interference signals, which can be transmitted 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 when the device itself works to the power grid. According to the law of electromagnetic induction, E=Ldi/dt is obtained, where: E is the voltage drop across L; L is the inductance; di/dt is the current change rate. Obviously, the smaller the rate of change of current is required, the greater the inductance is required.
The pulse current loop generates a common mode signal by electromagnetically inducing an interference signal generated by another circuit and a circuit composed of a ground or a casing; a strong electric field is generated between the collector of the switching tube and other circuits in the switching power supply circuit, and the circuit generates a displacement current. And this displacement current also belongs to the common mode interference signal. The common mode filter in Figure 1 is used to suppress common mode interference and to be attenuated.
Active filtering is an effective way to suppress common mode interference. The method adopted by the method from the noise source (as shown in Figure 2), the basic idea is to try to take out a compensation signal with the same magnitude and opposite phase from the main circuit to balance the original interference signal to achieve Reduce the level of interference. As shown in Figure 2, the current amplification of the transistor is used to filter the current in the base loop by folding the current of the emitter to the base. The filter composed of R1 and C2 makes the base ripple small, so the ripple of the emitter is also small. Since the capacity of C2 is less than C3, the volume of the capacitor is reduced. This method is only suitable for low voltage and low power power supplies. In addition, frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding, and reliability should be noted when designing and selecting filters. The installation location of the filter should be appropriate and the installation method should be correct to achieve the desired filtering effect on the interference.