Analysis and Research on the Electromagnetic Interference Suppression Technology of Switching Power Supply

0 Introduction

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 with their small size, light weight, high performance and high reliability, providing great help for people's production and life and social construction. However, with the rapid development of modern electronic technology and the widespread application of electronic and electrical equipment, the distance between various electronic and electrical equipment in the same working environment is getting closer and closer, and the external environment of electronic circuits is further deteriorating. Since the switching power supply works in a high-frequency switching state, a very high current and voltage change rate will be generated inside, resulting in strong electromagnetic interference from the switching power supply. Electromagnetic interference signals not only pollute the power grid, but also directly affect the normal operation of other electrical equipment and even the power supply itself, and break into space as radiation interference, causing electromagnetic pollution, which restricts people's production and life.

In the 1980s and 1990s, in order to strengthen the control of domestic electromagnetic pollution, some standards corresponding to international standards such as CISPR and IEC801 were formulated. Since China's compulsory certification was implemented on August 1, 2003, "electromagnetic compatibility fever" has been set off. The research and control of close-range electromagnetic interference has attracted more and more attention from electronic researchers, and has become a new hot spot in the current research field. This article will systematically discuss the relevant suppression technology based on the generation mechanism of switching power supply electromagnetic interference.

1 Suppression of switching power supply electromagnetic interference

The three elements that form electromagnetic interference are interference source, propagation path and interfered equipment. Therefore, electromagnetic interference suppression should start from these three aspects. The purpose is to suppress the interference source, eliminate the coupling and radiation between the interference source and the interfered equipment, and improve the anti-interference ability of the interfered equipment, thereby improving the electromagnetic compatibility performance of the switching power supply.

1.1 Use filters to suppress electromagnetic interference

Filtering is an important method to suppress electromagnetic interference. It can effectively suppress electromagnetic interference in the power grid from entering the equipment, and can also suppress 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 can not only solve the problem of conducted interference, but also is an important weapon to solve the problem of radiated interference. Filtering suppression technology is divided into two methods: passive filtering and active filtering.

1.1.1 Passive filtering technology

Passive filter circuit is simple, low-cost, reliable, and is an effective way to suppress electromagnetic interference. Passive filter is composed of inductor, capacitor, and resistor components, and its direct function is to solve the conducted emission. The principle structure diagram of the passive filter used in the switching power supply is shown in Figure 1.

Due to the large capacity of the filter capacitor in the original power supply circuit, a pulse peak current will be generated in the rectifier circuit. This current is composed of a lot of high-order harmonic currents, which will 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 change of current, induced currents of different frequencies will be generated in the surrounding circuits, including differential mode and common mode interference signals. These interference signals can be transmitted to other lines of the power grid and interfere with other electronic devices through two power lines. The differential mode filter 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 device itself when it is working and transmitted 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 current change rate is required, the larger the inductance is required.

The interference signal generated by the pulse current loop through electromagnetic induction of other circuits and the ground or the housing 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, which is also a common mode interference signal. The common mode filter in Figure 1 is used to suppress common mode interference and attenuate it.

1.1.2 Active filtering technology

Active filtering technology is an effective method to suppress common-mode interference. This method takes measures from the noise source (as shown in Figure 2). Its basic idea is to try to extract a compensation signal with the same size and opposite phase as the electromagnetic interference signal from the main circuit to balance the original interference signal, so as to achieve the purpose of reducing the interference level. As shown in Figure 2, the current amplification effect of the transistor is used to filter in the base circuit by converting the emitter current to the base. The filter composed of R1 and C2 makes the base ripple very small, so 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, when designing and selecting filters, attention should be paid to frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding and reliability. The installation position of the filter should be appropriate and the installation method should be correct to play the expected filtering role on interference.

1.2 Shielding technology and grounding technology

Shielding technology can effectively suppress the electromagnetic radiation interference of switching power supplies. Shielding is generally divided into two types: one is electrostatic shielding, which is mainly used to prevent the influence of electrostatic fields and constant magnetic fields; the other is electromagnetic shielding, which is mainly used to prevent the influence of alternating electric fields, magnetic fields and alternating electromagnetic fields. Shielding technology is divided into shielding of the parts that emit electromagnetic waves and shielding of components affected by electromagnetic waves. In switching power supplies, components that can emit electromagnetic waves refer to transformers, inductors, power devices, etc., and copper plates or iron plates are usually used as shielding around them to attenuate electromagnetic waves.

In addition, in order to suppress the radiation generated by the switching power supply from radiating to the outside and to reduce the impact of electromagnetic interference on other electronic equipment, overall shielding should be adopted. The shielding cover can be processed completely according to the method of magnetic field shielding, and then the entire shielding cover is connected to the system casing and ground as a whole, which can effectively shield the electromagnetic field. However, when using overall shielding, the electromagnetic leakage at the seams of the shielding material, the input/output terminals of the wires, and the outlets of the wires should be fully considered, and it is not easy to dissipate heat, and the structural cost increases significantly.

In order to make electromagnetic shielding play the role of electrostatic shielding at the same time, strengthen the shielding effect, and ensure the safety of people and equipment, the system should be connected to the earth, which is the grounding technology. Grounding refers to the design of establishing a conductive path between a selected point of the system and a grounding surface. This process is crucial. The correct combination of grounding and shielding can better solve the problem of electromagnetic interference and improve the anti-interference ability of electronic products.

1.3 PCB Design Technology

In order to better suppress the electromagnetic interference of the switching power supply, the anti-interference technology of its printed circuit board (PCB) is particularly important. In order to reduce the electromagnetic radiation of the PCB and the crosstalk between the circuits on the PCB, it is necessary to pay great attention to the PCB layout, wiring and grounding. For example, to reduce radiation interference, the path area is reduced, the loop area of ​​the interference source and sensitive circuit is reduced, and electrostatic shielding is used. To suppress the coupling of electric field and magnetic field, the distance between lines should be increased as much as possible.

Grounding is an important method to suppress interference in switching power supplies. There are three basic types of grounding: safety grounding, working grounding, and shielding grounding. The following points should be noted in grounding design: separate AC power ground from DC power ground; separate power ground from weak current ground; separate power ground for analog circuits from digital circuits; and make the ground wire as thick as possible.

1.4 Spread spectrum modulation technology

For a periodic signal, especially a square wave, its energy is mainly distributed in the fundamental frequency signal and harmonic components, and the harmonic energy decreases exponentially with the increase of frequency. Since the bandwidth of the nth harmonic is n times the bandwidth of the fundamental frequency, the harmonic energy is distributed over a wider frequency range through spread spectrum technology. As the energy of the fundamental frequency and each harmonic is reduced, its emission intensity should also be reduced accordingly. To use a spread spectrum clock signal in a switching power supply, it is necessary to modulate the pulse signal output by the power switch pulse control circuit to form a spread spectrum clock (as shown in Figure 3). Compared with traditional methods, the use of spread spectrum technology to optimize the EMI of switching power supplies is both efficient and reliable, without the need to add bulky filter components and cumbersome shielding processing, and will not have any negative impact on the efficiency of the power supply.

1.5 Adding a Power Factor Correction (PFC) Network to the Primary Rectification Circuit

For DC regulated power supplies, the grid voltage is directly rectified through the rectifier circuit after being stepped down by the transformer. Therefore, the harmonic components generated during the rectification process directly affect the waveform of the AC grid as interference, causing the waveform to be distorted and the power factor to be low. In order to solve the distortion of the input current waveform and reduce the harmonic content of the current, it is very necessary to apply power factor correction (PFC) technology to the switching power supply. PFC technology makes the current waveform follow the voltage waveform and corrects the current waveform into an approximate sine wave, thereby reducing the current harmonic content, improving the input characteristics of the bridge rectifier capacitor filter circuit, and improving the power factor of the switching power supply. Among them, the passive power factor correction circuit uses components such as inductors and capacitors to form a filter, and performs phase shifting and shaping processes on the input current waveform to achieve the improvement of the power factor. The active power factor correction circuit is based on the principle that the control circuit forces the input AC current waveform to track the input AC voltage waveform to achieve the sinusoidalization of the AC input current and synchronize it with the AC input voltage. Both methods improve the power factor, and the latter has a more obvious effect, but the circuit is complicated.

2 Conclusion

The design method of this paper is correct, the simulation results are normal, and some problems existing in the traditional solution are overcome, so that the electromagnetic interference suppression technology is further optimized. From the perspective of the mechanism of electromagnetic interference generated by the switching power supply, there are many ways to suppress electromagnetic interference. In addition to the main methods analyzed in this paper, optoelectronic isolators, LSA series surge absorbers, soft switching technology, etc. can also be used. The purpose of suppressing the electromagnetic interference of the switching power supply is to enable it to be effectively applied in various fields while minimizing electromagnetic pollution, thus achieving effective control of electromagnetic pollution problems. In actual design, various electromagnetic interferences of the switching power supply should be fully considered, and a variety of methods for suppressing electromagnetic interference should be selected for comprehensive utilization to minimize electromagnetic interference, thereby improving the quality and reliability of electronic products.