Electromagnetic interference suppression technology of switching power supply Time: 20 10-07-22 09:27:42 Source: Modern Electronic Technology Author: Yu, He Zhongyue, Xu.
Introduction to 0
With the development of modern electronic technology and power devices, switching power supply is widely used in computer and peripheral communication, automatic control, household appliances and other fields with its advantages of small size, light weight, high performance and high reliability, which has provided great help for people's production, life and social construction. However, with the rapid development of modern electronic technology and the wide 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 in which electronic circuits work is further deteriorated. Because the switching power supply works in high frequency switching state, it will produce high current and voltage change rate, which will lead to strong electromagnetic interference. Electromagnetic interference signals not only pollute the power grid, but also directly affect the normal work of other electrical equipment and even the power supply itself, and break into the space as radiation interference, causing electromagnetic pollution and restricting people's production and life.
In 1980s and 1990s, in order to strengthen the control of electromagnetic pollution in China, some standards corresponding to CISPR standards, IEC80 1 and other international standards were formulated. Since the compulsory implementation of 3C certification in August 2003 1 China, there has been a "EMC craze". The research and control of short-range electromagnetic interference has been paid more and more attention by electronic researchers and has become a new hot spot in the current research field. Aiming at the electromagnetic interference mechanism of switching power supply, this paper systematically discusses the related suppression technologies.
Suppression of electromagnetic interference of switching power supply
The three elements of electromagnetic interference are interference source, propagation path and interfered equipment. So the suppression of electromagnetic interference should start from these three aspects. Suppress the interference source, eliminate the coupling and radiation between the interference source and the disturbed equipment, improve the anti-interference ability of the disturbed equipment, and thus improve the electromagnetic compatibility of the switching power supply.
1. 1 Use filters to suppress electromagnetic interference.
Filtering is an important method to suppress electromagnetic interference, which can effectively prevent electromagnetic interference from entering the equipment in the power grid and also prevent electromagnetic interference from the equipment from entering the power grid. Installing switching power supply filter in the input and output circuit of switching power supply can not only solve the problem of conduction interference, but also be an important weapon to solve radiation interference. Filter suppression technology can be divided into passive filtering and active filtering.
1. 1. 1 passive filtration technology
Passive filter circuit is simple, low-cost and reliable, which is an effective way to suppress electromagnetic interference. Passive filter is composed of inductance, capacitance and resistance elements, and its direct function is to solve conducted emission. The principle structure diagram of passive filter applied to switching power supply is shown in figure 1.
Because of the large capacity of filter capacitor in the original power supply circuit, pulse peak current will be generated in the rectifier circuit, which is composed of a large number of high-order harmonic currents, which will interfere with the power grid. In addition, the on-off of the switch tube in the circuit and the primary coil of the transformer will produce pulsating current. Due to the high current change rate, induced currents with different frequencies will be generated to the surrounding circuits, including differential mode and * * * mode interference signals, which can be conducted to other lines of the power grid and interfere with other electronic equipment through two power lines. The differential mode filtering part in the figure can reduce the differential mode interference signal in the switching power supply, greatly attenuate the electromagnetic interference signal generated when the equipment itself works, and transmit it 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 inductance; Di/dt is the current change rate. Obviously, the smaller the required current change rate, the greater the required inductance.
The interference signal generated by the pulse current loop through the electromagnetic induction of other loops and the loop formed by the earth or bushing is a * * * mode signal; In the switching power supply circuit, a strong electric field is generated between the collector of the switching tube and other circuits, and the circuit will generate displacement current, which also belongs to the * * * mode interference signal. * * * mode filter in figure 1 is used to suppress and attenuate * * * mode interference.
1. 1.2 active filtration technology
Active filtering technology is an effective method to suppress * * * mode interference. The basic idea of this method is to take out a compensation signal with the same size and opposite phase as the electromagnetic interference signal from the main loop to balance the original interference signal, thus reducing the interference level. As shown in fig. 2, the emitter current is converted to the base by current amplification of the transistor and filtered in the base loop. The filter composed of R 1 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 supply. In addition, when designing and selecting filters, we should also pay attention to frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding and reliability. The installation position and method of the filter should be appropriate, so as to play the expected filtering role for interference.
1.2 shielding technology and grounding technology
Using shielding technology can effectively suppress the electromagnetic radiation interference of switching power supply. There are generally two kinds of shielding: one is electrostatic shielding, which is mainly used to prevent the influence of electrostatic field and constant magnetic field; The other is electromagnetic shielding, mainly used to prevent the influence of alternating electric field, magnetic field and alternating electromagnetic field. Shielding technology can be divided into shielding components that emit electromagnetic waves and shielding components that are affected by electromagnetic waves. In switching power supply, the components that can emit electromagnetic waves refer to transformers, inductors, power devices and so on. Copper or iron plates are usually used as shielding around to attenuate electromagnetic waves.
In addition, in order to restrain the radiation generated by switching power supply from spreading outward and reduce the influence of electromagnetic interference on other electronic devices, integral shielding should be adopted. The shield can be processed completely according to the method of shielding magnetic field, and then the whole shield can be connected with the shell and ground of the system as a whole, so that the electromagnetic field can be effectively shielded. However, when using integral shielding, we should fully consider the electromagnetic leakage at the joints of shielding materials, the input/output ends of wires and the outlet of wires, which is not easy to dissipate heat and the structural cost is greatly increased.
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 personnel and equipment, the system should be grounded, which is grounding technology. Grounding refers to the design of the conductive path between the selected point of the system and the ground plane. This process is very important. The correct combination of grounding and shielding can better solve the problem of electromagnetic interference and improve the anti-interference ability of electronic products.
Design technology of 1.3 PCB
In order to better suppress the electromagnetic interference of switching power supply, the anti-interference technology of its printed circuit board (PCB) is particularly important. In order to reduce the electromagnetic radiation of PCB and crosstalk between circuits on PCB, great attention should be paid to the layout, wiring and grounding of PCB. For example, reducing radiation interference means reducing the path area, reducing the loop area of interference sources and sensitive circuits, and adopting electrostatic shielding. In order to suppress the coupling between electric field and magnetic field, the distance between lines should be increased as much as possible.
Grounding of switching power supply is an important method to suppress interference. There are three basic types of grounding: safety grounding, working grounding grounding and shielding grounding. Attention should be paid to the following points when designing the ground wire: AC power supply is separated from DC power supply; Power grounding is separated from weak current grounding; The power supply ground of analog circuit is separated from that of digital circuit; 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 in series with the increase of frequency. Because the bandwidth of the nth harmonic is n times the bandwidth of the fundamental frequency, the harmonic energy is distributed in a wider frequency range by spread spectrum technology. With the decrease of fundamental frequency and harmonic energy, its emission intensity should also decrease accordingly. In order to use the spread spectrum clock signal in the switching power supply, it is necessary to modulate the pulse signal output by the switching pulse control circuit of the power supply to form the spread spectrum clock (as shown in Figure 3). Compared with traditional methods, using spread spectrum technology to optimize the electromagnetic interference of switching power supply is efficient and reliable, without adding bulky filters and cumbersome shielding treatment, and will not bring any negative impact on power supply efficiency.
Adding power factor correction (PFC) network in 1.5 primary rectifier circuit.
For DC stabilized power supply, the grid voltage is directly rectified by the rectifier circuit after being stepped down by the transformer, so the harmonic components generated in the rectification process directly affect the waveform of AC power grid as interference, which distorts the waveform and leads to low power factor. In order to solve the input current waveform distortion and reduce the current harmonic content, it is very necessary to apply the 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 to approximate sine wave, thus 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 is a filter composed of inductance and capacitance, which carries out phase-shifting shaping on the input current waveform and improves the power factor. Active power factor correction circuit is based on the principle that the control circuit forces the input AC current waveform to follow the input AC voltage waveform to realize sinusoidal AC input current and synchronize with AC input voltage. Both methods improve the power factor, the latter is more effective, but the circuit is complicated.
2 conclusion
The design method in this paper is correct and the simulation results are normal, which overcomes some problems existing in the traditional scheme and further optimizes the electromagnetic interference suppression technology. From the mechanism of electromagnetic interference of switching power supply, there are many ways to suppress electromagnetic interference. In addition to the main methods analyzed in this paper, photoelectric isolator, LSA series surge absorber and soft switching technology can also be used. The purpose of suppressing electromagnetic interference of switching power supply is to make it be effectively applied in various fields, and at the same time minimize electromagnetic pollution and realize effective control of electromagnetic pollution. In practical design, we should comprehensively consider all kinds of electromagnetic interference of switching power supply and comprehensively use various methods to suppress electromagnetic interference, so as to minimize electromagnetic interference and improve the quality and reliability of electronic products.