With the development of modern electronic technology and power devices, switching power supplies, with their small size, light weight, high performance, and high reliability, are widely used in fields such as computer and peripheral equipment communication, automatic control, and home appliances, greatly assisting people's production, life, and social development. However, with the rapid development of modern electronic technology and the widespread application of electronic and electrical equipment, various electronic and electrical devices in the same working environment are getting closer and closer, further deteriorating the external environment for electronic circuit operation.
In the 1980s and 1990s, China formulated several standards corresponding to international standards such as CISPR and IEC 801 to strengthen the control of electromagnetic pollution. Since China implemented mandatory 3C certification (China Compulsory Certification) on August 1, 2003, an "electromagnetic compatibility fever" has emerged, with research and control of near-field electromagnetic interference attracting increasing attention from electronics researchers and becoming a new hot topic in the field. This paper will systematically discuss the generation mechanism of electromagnetic interference in switching power supplies and related suppression techniques.
1. Suppression of electromagnetic interference in switching power supplies
The three essential elements for electromagnetic interference (EMI) are the interference source, the propagation path, and the affected device. Therefore, suppressing EMI should address these three aspects: suppressing the interference source, eliminating coupling and radiation between the interference source and the affected device, and improving the immunity of the affected device, thereby improving the EMI performance of the switching power supply.
1.1 Using filters to suppress electromagnetic interference
Filtering is an important method for suppressing electromagnetic interference (EMI). It effectively prevents EMI from entering equipment from the power grid and also prevents EMI from entering the power grid from within the equipment. Installing switching power supply filters in the input and output circuits of switching power supplies not only solves conducted interference problems but is also an important tool for combating radiated interference. Filtering suppression techniques are divided into two types: passive filtering and active filtering.
1.1.1 Passive Filtering Technology
Passive filter circuits are simple, inexpensive, and reliable, making them an effective way to suppress electromagnetic interference. Passive filters consist of inductors, capacitors, and resistors, and their direct function is to address conducted emissions. The schematic diagram of a passive filter used in a switching power supply is shown in Figure 1.
Because the filter capacitor in the original power supply circuit has a large capacitance, pulse spike currents are generated in the rectifier circuit. This current consists of many high-order harmonic currents, which interfere with the power grid. In addition, the switching transistors and the primary coil of the transformer in the circuit will generate pulsating currents. Due to the high rate of change of current, different frequencies of induced current will be generated in 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 the two power lines. The differential-mode filter section in the diagram can reduce the differential-mode interference signal inside the switching power supply and greatly attenuate the electromagnetic interference signals generated by the device itself during operation that are 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; and di/dt is the rate of change of current. Obviously, the smaller the rate of change of current, the larger the inductance needs to be.
The interference signal generated by the pulse current loop through the electromagnetic induction of other circuits and the loop formed by the ground or chassis 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 generate a 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 for suppressing common-mode interference. This method addresses the noise source (as shown in Figure 2). Its basic idea is to extract a compensation signal from the main circuit that is equal in magnitude but opposite in phase to the electromagnetic interference signal to balance the original interference signal, thereby reducing the interference level. As shown in Figure 2, the current amplification effect of the transistor is utilized, filtering is achieved by refracting the emitter current to the base circuit. The filter composed of R1 and C2 minimizes the base ripple, thus also minimizing the emitter ripple. Since the capacitance of C2 is smaller than that of C3, the capacitor size is reduced. This method is only suitable for low-voltage, low-power power supplies. Furthermore, when designing and selecting filters, attention should be paid to frequency characteristics, withstand voltage, rated current, impedance characteristics, shielding, and reliability. The filter's installation location and method must be appropriate to achieve the desired filtering effect against interference.
1.2 Shielding and grounding technologies
Shielding technology can effectively suppress electromagnetic radiation interference from switching power supplies. Shielding is generally divided into two types: electrostatic shielding, mainly used to prevent the influence of electrostatic fields and constant magnetic fields; and electromagnetic shielding, mainly used to prevent the influence of alternating electric fields, magnetic fields, and alternating electromagnetic fields. Shielding technology is divided into shielding of parts that emit electromagnetic waves and shielding of components affected by electromagnetic waves. In switching power supplies, components that emit electromagnetic waves include transformers, inductors, and power devices. These are typically surrounded by copper or iron plates as shielding to attenuate the electromagnetic waves.
Furthermore, to suppress the radiation emitted by the switching power supply and reduce the impact of electromagnetic interference on other electronic devices, overall shielding should be adopted. The shielding cover can be manufactured using methods similar to those for magnetic field shielding, and then the entire shielding cover can be integrated with the system chassis and ground to effectively shield the electromagnetic field. However, when using overall shielding, electromagnetic leakage at the seams of the shielding material, the input/output terminals of the wires, and the wire exit points should be fully considered, as well as factors such as poor heat dissipation and a significant increase in structural cost.
To ensure that electromagnetic shielding also functions as electrostatic shielding, thereby enhancing the shielding effect and protecting personnel and equipment, the system should be connected to the earth—this is grounding technology. Grounding refers to the design of establishing a conductive path between a selected point in the system and a ground plane. This process is crucial; properly combining grounding and shielding can better solve electromagnetic interference problems and improve the anti-interference capability of electronic products.
1.3 PCB Design Technology
To better suppress electromagnetic interference (EMI) in switching power supplies, the anti-interference technology of their printed circuit boards (PCBs) is particularly important. To reduce electromagnetic radiation and crosstalk between circuits on the PCB, careful attention must be paid to PCB layout, routing, and grounding. For example, reducing radiated interference involves minimizing the path area, reducing the loop area between interference sources and sensitive circuits, and using electrostatic shielding. To suppress the coupling of electric and magnetic fields, the distance between traces should be maximized.
Grounding is an important method for suppressing interference in switching power supplies. There are three basic types of grounding: safety grounding, functional 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 low-voltage ground; separate analog circuit power ground from digital circuit power ground; and use thicker ground wires whenever 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, with harmonic energy decreasing exponentially with increasing frequency. Since the bandwidth of the nth harmonic is n times that of the fundamental frequency, spread spectrum technology can distribute the harmonic energy over a wider frequency range. Because the fundamental frequency and harmonic energy are reduced, their emission intensity should also be reduced accordingly. To use a spread spectrum clock signal in a switching power supply, the pulse signal output by the power supply's switching pulse control circuit needs to be modulated to form a spread spectrum clock (as shown in Figure 3). Compared with traditional methods, using spread spectrum technology to optimize the EMI of a switching power supply is both efficient and reliable, requiring no bulky filtering components or cumbersome shielding, and does not negatively impact the power supply's efficiency.
1.5 Adding a Power Factor Correction (PFC) Network to a Primary Rectifier Circuit
For DC regulated power supplies, the grid voltage is stepped down by a transformer and then directly rectified by a rectifier circuit. Therefore, the harmonic components generated during rectification directly affect the AC grid waveform, causing distortion and a low power factor. To address input current waveform distortion and reduce harmonic content, applying power factor correction (PFC) technology to switching power supplies is essential. PFC technology makes the current waveform follow the voltage waveform, correcting it to an approximate sine wave, thereby reducing harmonic content, improving the input characteristics of the bridge rectifier capacitor filter circuit, and increasing the power factor of the switching power supply. Passive power factor correction circuits use inductors and capacitors to form filters, performing phase shifting and shaping of the input current waveform to improve the power factor. Active power factor correction circuits, on the other hand, rely on the principle of a control circuit forcing the input AC current waveform to track the input AC voltage waveform, achieving sinusoidal AC input current synchronization with the AC input voltage. Both methods improve the power factor, with the latter being more effective but more complex.
2 Conclusion
The design method presented in this paper is correct, and the simulation results are normal. It overcomes some problems existing in traditional solutions and further optimizes electromagnetic interference suppression technology. From the perspective of the mechanism of electromagnetic interference generation in switching power supplies, there are multiple ways to suppress electromagnetic interference. In addition to the main methods analyzed in this paper, opto-isolators, LSA series surge absorbers, and soft-switching technology can also be used. Suppressing the electromagnetic interference of switching power supplies aims to enable their effective application in various fields while minimizing electromagnetic pollution, thus achieving effective control of electromagnetic pollution problems. In practical design, various electromagnetic interferences of switching power supplies should be comprehensively considered, and multiple methods for suppressing electromagnetic interference should be selected and used in combination to minimize electromagnetic interference, thereby improving the quality and reliability of electronic products.
