The high frequency and large capacity of power electronic devices not only lead to an increase in the electrical stress and switching loss of the device, but also produce broadband electromagnetic interference that is difficult to suppress [1-3], causing serious electromagnetic pollution to the power grid and the environment , And even threaten the normal operation of itself and other electronic equipment related to it. This article starts with the mechanism of electromagnetic interference sources of power electronic devices, summarizes the latest foreign research results in recent years, and focuses on analyzing and comparing the electromagnetic interference characteristics of hard switching and soft switching. Keywords: switching converter, electromagnetic compatibility, hard switching, soft switching 1 Introduction power electronic devices are known for their high efficiency for power conversion, and are increasingly widely used in industrial and civil power conversion and drive control. It is estimated that 70% of the electrical energy in industrial production is converted by power electronic devices before being used by humans. In the late 1980s, the practical and large-capacity power field control devices made power electronic devices enter the era of high-frequency and large-capacity. Because of the very steep front and back edges (di/dt up to 1A/ns, dv/dt up to 3V/ns) pulses during the power electronic commutation process, serious electromagnetic interference is caused. These interferences form conduction and radiation interference through the near-field and far-field coupling, which seriously pollutes the surrounding electromagnetic environment and power supply system. This not only reduces the reliability of the conversion circuit itself, but also seriously affects the operation quality of the power grid and adjacent equipment. With the development of the electronic information industry, power electronic devices with switching converters as the core are being widely used in almost all electronic devices such as various terminal equipment and communication equipment led by electronic computers. In the 1997 annual report of the Virginia Power Electronic Center (VPEC), it was written as follows: If it is the advancement of microprocessor technology that promotes the development of computer frequency from 16MHz in 1985 to today 200MHz, then the next leap to GHz is mainly determined by the development of power electronics technology [4]. When the chip is working at GHz, the power supply must supply power to the logic gate at a sufficiently high matching speed (in the case of Pentiumpro, the load current supply rate is required to be 30A/μs), which is why Intel has to slow down the clock speed of the Pentium microprocessor An important reason for this [4]. Therefore, the electromagnetic compatibility problem of power electronic devices needs to be solved urgently. In recent years, with the development of power electronics technology, the capacity of power switching devices has become larger and larger (for example, SCR (Silicon Controllable Rectifier) has 4000A/8000V products, and IGBT (Insulated Gate Bip olar Transistor) has 3500V/ 2400A modules are sold), the switching frequency is getting higher and higher, up to a few MHz, and the size of the device is getting smaller and smaller. Taking the DC-DC power supply as an example, the current domestic level is 30W/in3, while the international level is 120W/in3, and it is expected to reach 240W/in3 in 2000. These factors require further strengthening of the research on the electromagnetic interference characteristics and prevention of power electronic devices. Especially in the design stage, it has become a crucial issue to estimate the interference characteristics of new devices, shorten their development cycle, and improve the electromagnetic compatibility of power electronic devices. 2 Exploratory research on electromagnetic interference sources of power electronic devices In the process of exploring electromagnetic interference sources of power electronic devices, people have conducted a large number of experiments and constantly summed up new experiences. As early as 1983, Schneider developed a technique for testing the source impedance characteristics of a switching power supply in operation. This is a technique that uses a scalar method to measure the noise spectrum to determine the real and imaginary parts of the source impedance. This technology selects the imaginary part oscillation between the reactive load and the noise source, and the reactive part of the noise source can be determined by the oscillation frequency. The real part of the source impedance is determined by the peak value of the oscillating noise current. Impedance testing is mainly carried out in the frequency band of 10kHz～1MHz. Based on the test results, Schneider proposed an equivalent circuit of common mode and differential mode noise describing the characteristics of the AC side noise source of the switching power supply [5]. Since the radiation effect of common mode current is usually much greater than that of differential mode current [6], it is necessary to distinguish between common mode interference and differential mode interference in the system. The VPEC Research Center proposed a power combiner [7] , Quantitative measurement of common mode and differential mode conduction interference in the system. In power electronic devices, the internal mechanisms of common mode noise and differential mode noise are also different. The differential mode noise is mainly caused by the pulsating current of the switching converter; the common mode noise is mainly caused by the high-frequency oscillation caused by the interaction between the higher dv/dt and the spurious parameters. As shown in Figure 1, the common mode current iCM includes the displacement current connected to the ground plane. At the same time, since the dv/dt on the terminal of the switching device is the largest, the stray capacitance Ck between the switching device and the heat sink will also be generated. Common mode current. For different systems, the specific causes of common mode and differential mode interference are not the same. According to the different propagation paths, electromagnetic interference is divided into conducted interference and radiated interference, discussed separately, and the study of the near-field characteristics of the switch converter is explained.
Figure 1 The path of the common mode current of the offline converter
Fig.1 Common-mode current path in a off-line converter
2.1 Research on conducted interference sources conduction is an important way of interference propagation in power electronic devices. Different power electronic devices have different specific causes of conducted interference. For example, in the SCR rectifier system, the generation of differential mode conduction interference is mainly based on two factors [8]: one is the commutation overlap phenomenon caused by the power line inductance; the other is the semiconductor switching characteristics and the physical characteristics that determine its current characteristics. . At the same time, the recovery phenomenon of the thyristor in the SCR rectifier system may have two results: one is to extend the commutation overlap time; the other is to add an exponentially decayed current to the thyristor. The measured thyristor recovery phenomenon can increase the total interference by 4~5dB. For another example, Siemens’ Klotz et al. [9] studied the common mode of 5-10kVA IGBT converters under different operating voltages, operating currents, switching frequencies, module packaging, gate circuits, temperature, grounding conditions, and additional components. Compared with the differential mode conduction interference source, it is concluded that the main differential mode interference source is the reverse recovery current of the freewheeling diode. At the same time, it is pointed out that the spurious parameters of the load will have a certain influence on the interference spectrum. The research of Teuling, Schnaen and Roudet of the French Grenoble Electrical Technology Laboratory (hereinafter referred to as LEG) [10] based on the experimental model of a 400W chopper circuit composed of MOSFETs and a switching frequency of 100kHz shows that common mode noise is related to voltage switching. Mode noise is related to current switching, and both may occur simultaneously. For example, when the MOSFET is turned off in this model, the current is turned off and the voltage exhibits attenuated oscillation. Therefore, common mode noise and differential mode noise coexist at this time. Usually low frequency time difference mode interference is dominant, and common mode interference is dominant at high frequency. Mahdavi of SHARIF University of Technology and Roudet and Scheich of LEG et al. [11] established a 500W Power Factor Preregulator (PFP) single-phase AC/DC converter model. In the research of the emission mechanism, the simulation software MC2 is used to calculate the current harmonics injected into the power supply. The model is in good agreement with the test results in the frequency range of 10 times the switching frequency. In the study of PFP's differential mode conducted EMI, Reis predicted that when the converter works in different modes, the EMI characteristics are also different [13]. Erkuan Zhong and Lip of Wisconsin-Madison University in the United States [12] used an 8kVA PWM inverter system driving a 7.46kW (10hp) induction motor as an experimental model. The study found that the PWM inverter system driven by a high-power high-speed motor feeds the power supply. Into the pulsating current of up to several A, leading to severe conducted EMI (in this experimental model, up to 120dBμV) and power supply voltage waveform distortion (notch voltage up to 50V, 20% more than the rated voltage), the frequency band of interference signals is quite wide, It not only includes the interference components of the switching frequency and its harmonics, but also extends to the radio frequency range. The dv/dt (up to 3kV/μs, lasting a few ns) generated by the power device during the switching process interacts with the stray capacitance between the switching device and the ground, which generates charging and discharging currents at the power supply terminal, causing electromagnetic interference. At the same time, the non-linear switching characteristics of switching devices generate a lot of harmonics. They also pointed out that the diode reverse recovery current is the main source of differential mode interference in the system. Research on conducted interference of power electronic devices, especially common mode and differential mode conducted interference, provides a basis for the design of EMI filters. 2.2 Research on radiated interference sources Compared with conducted interference, the radiated interference of power electronic devices is more complicated. This is because, as an energy conversion device, the conversion capacity ranges from milliwatts to megawatts, and the main loop and control loop are often composed of different components. Compared with the electronic devices concentrated on the printed circuit board, the spatial structure It is more complicated. Therefore, the analysis and calculation of the corresponding spurious parameters and radiated interference are more complicated [2], and there are not many related researches at present. Among them, Orlandi and Scheich [14] studied the radiation interference source of SCR rectifier circuit. They focused on the analysis of the relationship between the common mode current (time domain and frequency domain) and the radiation field, and believed that the common mode current is related to the driving pulse and stray parameters from the control part. The voltage gradient between the stray capacitances promotes the common mode current. Propagation, the voltage gradient on the rising edge of the pulse generates a common-mode current in the stray capacitance. Moreover, the fast current pulse induces unnecessary voltage on the metal parts of the SCR (case and radiator) and becomes a radiation source. In order to determine the radiation model of the switching converter, Professors Antonini and Cristina and Professor Orlandi of the University of Rome established a dipole radiation model for the converter part of the switching power supply with the switching frequency of 75kHz and 150kHz respectively [15]. However, when determining the line current distribution, a transmission line model of equivalent homogeneous medium is used. The result model is in good agreement with the experimental conclusions in the frequency range lower than 10MHz, but in the frequency range higher than 10MHz, the common mode radiation is dominant due to the influence of various spurious parameters. When determining the common mode current distribution, the transmission line model is no longer valid. In fact, the electromagnetic radiation characteristics of power electronic devices are not limited to this. For example, radiators widely used in power electronic devices often exhibit electromagnetic oscillation characteristics, which enhance the RF electromagnetic radiation of power electronic devices. The heat sink usually has a complicated geometry, has multi-band RF radiation characteristics, and is installed outside the device. Therefore, the heat sink is likely to act as an effective radiation antenna on one or more harmonics of the switching frequency. Research work in this area is also underway, such as Ryan, Stone and Chambers [16] using the FDTD method to make a preliminary prediction of the RF electromagnetic radiation pattern from the fin-shaped radiator. 2.3 "Research on near-field characteristics "According to IEC22G-WG4-11, power electronic devices usually consist of two parts, namely the power conversion unit and the control unit. The switching frequency of the switching conversion circuit is generally tens of kHz to hundreds of kHz. The voltage and current transients generated during the switching process are interference sources that produce conduction interference and radiation interference. The energy of electromagnetic radiation generated by the power conversion unit is sufficient to endanger the normal operation of the nearby control unit [15]. Therefore, predicting the near-field characteristics of the power conversion unit and ensuring the normal operation of the control circuit is of great significance to the EMC design of the power electronic conversion device. In order to explore the near-field characteristics of a switching power supply (SMPS), Atonini et al. [15] established a simple SMPS experimental model based on a printed circuit board. When performing near-field calculations, they divided each segment of the experimental circuit into multiple Hertzian dipoles in series. Since the electrostatic term plays a dominant role in the near-field region, it represents the field generated by the electrostatic charge accumulated on a single dipole; when multiple dipoles are connected in series, since the distance r is between the center of the dipole and the test point Therefore, the electrostatic terms cannot be cancelled out, resulting in a large electrostatic field, causing the predicted value to be higher than the actual value. Therefore, when integrating the radiation equation along the circuit, through special processing, the false electrostatic charge effect caused by the integration of the dipole equation is eliminated, and a more accurate near-field (electric and magnetic field) model is established. Calculations show that at a distance of 3m from the experimental model, the electric field difference between the corrected model and the model before the correction is 40dB in the low frequency range, and the two tend to coincide in the high frequency range. The test result shows that in the frequency band below 10MHz, the calculated value is very consistent with the measured value. In the frequency band higher than 10MHz, the influence of common mode current is dominant, and the above calculation model is no longer valid. The main influence on the near-field characteristics of power electronic devices is the main circuit of the power conversion part. Cristina et al. [17] studied the radiation pattern changes, near-field spatial distribution and radiation characteristics of the converter under different load conditions, and concluded that under different load conditions, the switching power supply may exhibit electric dipoles. Or the characteristics of a magnetic dipole. This is very important for selecting and designing a suitable shielding scheme. LEG's Youssef and Roudet et al. [18] used MOSFET as a switching element to establish a simple buck converter model. They assume that the circuit is approximately a thin-line structure, and assume that the current in each part of the circuit is the same, and calculate the near-field distribution based on the time-domain current waveform during the switching operation. At the same time, the mirror image method is used to study the changes of electromagnetic radiation when the grounded conductive plane is under the interference source circuit, and it is concluded that the electromagnetic radiation is reduced under the influence of the conductive ground plane. It can be seen that the research on the near-field characteristics of power electronic devices has just started, and a complete and accurate model has not yet been established. Especially in the high frequency range, the near-field characteristics are more complicated under the influence of various stray parameters. To sum up, in the exploration of electromagnetic interference sources for power electronic devices, most studies use a combination of experiment and analysis. And model the electromagnetic interference characteristics under certain working conditions. However, there are few studies on the characteristics of electromagnetic interference sources of high-power and complex-structured power electronic devices [2]. For an actual power electronic device, it is often common mode and differential
Mode interference coexists, conduction and radiation interference simultaneously. For different systems, the dominant interference factors are also different. Correctly analyzing and predicting the main interference sources in the system is the key to the electromagnetic compatibility design of power electronic devices. 3 Research on the electromagnetic compatibility characteristics of high-frequency soft-switching converters In order to meet the requirements of high-frequency, people have not only improved the withstand capability of the device itself, but also made many efforts to improve the circuit topology to weaken the electrical stress on the device and reduce Small switching loss, eliminating switching surge and peak voltage. The main reason for the interference of power electronic devices is the high di/dt and dv/dt generated during the commutation process of power electronic devices and the stray parameters in the circuit work together to cause high-frequency oscillation. If the conversion process of high di/dt and dv/dt can be reduced as much as possible by selecting the appropriate circuit topology and control technology, it is possible to improve the electromagnetic compatibility characteristics of power electronic devices. So some people speculate that in terms of conducted EMI, the soft switching converter using Zero Voltage Transition (ZVT) should perform better than the hard switching converter [9, 19]. The main basis is that in the ZVT circuit, the main switch works in the zero voltage switching state, and the diode works in the soft switching state. In this way, there is no fast voltage switching in the main switch and no fast current switching in the diode, thereby reducing the high voltage in the circuit. Frequency harmonics. Is this really the case? From the perspective of EMI generation, resonant converters (including soft-switching converters) do have incomparable advantages over PWM hard-switching converters, which can be considered from the following aspects: (1) PWM technology is to interrupt the power flow and The method of controlling the duty cycle transforms the power, resulting in pulse current and pulse voltage; while the resonance technology transforms the power in the form of a sine wave, and its frequency spectrum is usually narrower than the frequency spectrum of a PWM converter. Therefore, compared with the PWM converter, the input should have smaller harmonic interference and larger amplitude of the fundamental component. (2) The working waveform of the resonant switching converter is a quasi-sine wave, with low di/dt and dv/dt. (3) The resonant switching converter uses the junction capacitance of the device and the leakage inductance of the transformer as a part of the resonant LC circuit, and is not sensitive to harmful stray parameters. (4) The resonant switching converter works at a higher frequency, which is convenient for integration and minimization, so it usually has a higher power density, which is very beneficial for reducing the circuit loop and shortening the wiring length. (5) PWM converters often use energy-consuming snubber circuits to limit the stress on the device, and at the same time, it also plays a beneficial role in suppressing electromagnetic interference. Resonant soft-switching converters can reduce or eliminate energy-consuming buffers, thereby improving development efficiency. Based on the above analysis, can a conclusion be drawn easily? In 1996, researchers from the VPEC Research Center conducted a comparative experiment on the conduction interference of two single-phase 400WPFC boost converter experimental models using zero-voltage conversion (ie ZVT) circuits and hard-switching circuits [21]. The test result is unexpected. The EMI difference between the soft switching converter and the hard switching converter using ZVT technology is very small, and even if the additional circuit wiring of the former is improper, the performance will be worse. The difference from the literature [20] is that they compared the common mode and differential mode interference of the two experimental models separately. The result is: in terms of common mode noise, the characteristics of the low frequency band are similar. When the frequency exceeds a few MHz, the hard switching The noise of the ZVT model is several dB higher than that of the ZVT model; at high frequency, the common mode noise of the ZVT model is lower, but in some cases, the noise peak of the ZVT model at individual frequency points exceeds the hard-switching model; in terms of differential mode noise, The noise of hard switching is stronger than that of the ZVT model. The above experimental results can be understood as: common mode noise is mainly coupled through the stray capacitance of the device housing, while the main switch in the ZVT converter is soft switching, and the dv/dt generated during the switching process is small. Therefore, the ZVT converter's High-frequency common-mode interference is smaller than that of hard-switching converters; and the noise peaks of ZVT converters at certain frequency points are caused by incorrect wiring of auxiliary components in ZVT converters. In addition, due to the higher di/dt caused by the diode reverse recovery current in the hard-switching converter, the differential-mode noise of the hard-switching converter is higher than that of the ZVT converter in the high frequency range, but the high di/dt usually does not affect the low-frequency components. Therefore, the interference characteristics of the two are similar in the switching frequency and its low-order harmonics. It can be seen that although the high frequency interference characteristics of the ZVT converter are better than hard switching by several dB, the EMI characteristics of the two are generally similar. As far as the differential mode noise is concerned, the ZVT conversion is better than the hard switching converter, which is the aspect of soft switching better than hard switching. In terms of common-mode noise, the problem is more complicated. The difference between ZVT converters and hard-switching converters is that the former has auxiliary soft-switching elements, including auxiliary switching elements that flow larger peak currents. This switching element may withstand hard and hard switching elements. The main switch tube in the switching converter has the same voltage. The auxiliary switching element in the ZVT converter is hard-switched, which means that the hard switch in the hard-switching converter is transferred to the auxiliary switch of the ZVT converter. Therefore, in the soft switching circuit topology, auxiliary switching elements are important sources of interference, and their location and wiring are particularly important [21]. "In essence, a PWM converter with a snubber circuit does not necessarily have worse noise characteristics than a soft-switching converter. But whether soft switching or hard switching is better depends on the initial stage of circuit design, the appropriate selection of circuit topology and control technology according to the application, the establishment of conduction and radiation interference prediction models, and the guidance of correct circuit layout. 4 Conclusion In summary, the electromagnetic compatibility of power electronic devices is attracting more and more attention from scholars at home and abroad. Since the 1980s, many experimental studies and analytical modeling work have been completed abroad; domestic research in this area has been carried out. There is not much work yet, and no more mature technical reports have been seen yet. Especially in today's rapid development of power electronics technology, how to break the past experience and heuristics in electromagnetic compatibility design and make the electromagnetic compatibility design of power electronic devices onto the track of systematic design is facing domestic and foreign scholars. It is bound to become one of the central topics in the electromagnetic compatibility research of power electronic devices. Only based on the in-depth analysis of electromagnetic interference sources of various power electronic devices, determining the sensitivity of various parameters, studying the electromagnetic compatibility characteristics of various switching topologies and control schemes, and establishing predictive models can the electromagnetic compatibility of power electronic devices be achieved. Systematic design, and adapt to the rapid development of power electronics technology itself