Abstract: The increasing frequency and capacity of power electronic devices not only lead to increased electrical stress and switching losses, but also generate uncontrollable broadband electromagnetic interference [1-3], causing serious electromagnetic pollution to the power grid and the environment, and even threatening the normal operation of the devices themselves and other related electronic equipment. This paper starts with the mechanism of electromagnetic interference generation in power electronic devices, summarizes the latest research results abroad 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 in power conversion and are increasingly widely used in power conversion and transmission control in industry and civil applications. 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 application and large capacity of power field control devices ushered in the era of high frequency and large capacity for power electronic devices. Because the power electronic commutation process generates pulses with very steep leading and trailing edges (di/dt can reach 1A/ns, dv/dt can reach 3V/ns), it causes serious electromagnetic interference. These interferences are coupled through near-field and far-field to form conducted and radiated interference, 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 operating quality of the power grid and nearby equipment. With the development of the electronic information industry, power electronic devices with switching converters as the core are widely used in almost all electronic devices, such as various terminal equipment and communication equipment dominated by electronic computers. The annual report of the Virginia Power Electronic Center (VPEC) in 1997 stated: If the progress of microprocessor technology has driven the computer clock frequency from 16MHz in 1985 to 200MHz today, then the next leap to GHz mainly depends on the development of power electronic technology [4]. When a chip operates at GHz, the power supply must provide power to the logic gates at a sufficiently high matching speed (for example, the Pentium Pro requires a load current supply rate of 30A/μs), which is also an important reason why Intel had to slow down the clock speed of the Pentium microprocessor [4]. Therefore, the electromagnetic compatibility problem of power electronic devices urgently needs to be solved. In recent years, with the development of power electronics technology, the capacity of power switching devices has become larger and larger (for example, there are 4000A/8000V products for optically controlled SCRs (Silicon Controllable Rectifiers), and 3500V/2400A modules for IGBTs (Insulated Gate Bipolar Transistors) are available), the switching frequency is becoming higher and higher, reaching up to several MHz, while the size of the devices is becoming smaller and smaller. Taking DC-DC power supplies 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 by 2000. These factors all require further strengthening of research on the electromagnetic interference characteristics of power electronic devices and their prevention. Especially in the design phase, predicting the interference characteristics of new devices, shortening their development cycle, and improving the electromagnetic compatibility of power electronic devices become crucial issues. 2 Exploratory Research on Electromagnetic Interference Sources of Power Electronic Devices In the process of exploring the electromagnetic interference sources of power electronic devices, people have continuously summarized new experiences through a large number of experiments. As early as 1983, Schneider developed a technique for testing the source impedance characteristics of switching power supplies in operation. This technique uses a scalar method to measure the noise spectrum to determine the real and imaginary parts of the source impedance. This technique selects the imaginary oscillation of 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. The impedance test is mainly carried out in the 10kHz to 1MHz frequency band. Based on the test results, Schneider proposed equivalent circuits for common-mode and differential-mode noise to describe 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 essential to distinguish between common-mode interference and differential-mode interference in a system. The VPEC Research Center proposed a power combiner [7] to quantitatively measure common-mode and differential-mode conducted interference in the system. In power electronic devices, the internal mechanisms of common-mode noise and differential-mode noise are also different. Differential-mode noise is mainly caused by the pulsating current of the switching converter; common-mode noise is mainly caused by the high-frequency oscillations generated by the interaction between the high dv/dt and stray parameters. As shown in Figure 1, the common-mode current iCM includes the displacement current from the connection 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 generate common-mode current. The specific causes of common-mode and differential-mode interference are also different for different systems. According to the different propagation paths, electromagnetic interference is divided into conducted interference and radiated interference, which will be discussed separately, and the research on the near-field characteristics of the switching converter will be described. Fig.1 Common-mode current path in an off-line converter 2.1 Research on conducted interference sources Conducted interference is an important path for interference propagation in power electronic devices. The specific causes of conducted interference vary among different power electronic devices. For example, in an SCR rectifier system, differential-mode conducted interference is mainly based on two factors [8]: one is the commutation overlap phenomenon caused by the inductance of the power line; the other is the semiconductor switching characteristics and the physical characteristics that determine its current characteristics. At the same time, the thyristor recovery phenomenon in the SCR rectifier system may produce two results: one is that it prolongs the commutation overlap time; the other is that it adds an exponentially decaying current to the thyristor. It has been measured that the thyristor recovery phenomenon can increase the total interference by 4 to 5 dB. For example, Klotz et al. [9] of Siemens studied the common-mode and differential-mode conducted interference sources of 5 to 10 kVA IGBT converters under different operating voltages, operating currents, switching frequencies, module packages, gate circuits, temperatures, grounding conditions, and additional components, and concluded that the main differential-mode interference source is the reverse recovery current of the freewheeling diode. It was also pointed out that the stray parameters of the load have a certain impact on the spectrum of interference. Teuling, Schnaen and Roudet of the Grenoble Electrical Technology Laboratory (hereinafter referred to as LEG) [10] studied the experimental model of a 400W chopper circuit with a switching frequency of 100kHz composed of MOSFETs. The results showed that common-mode noise is related to voltage switching and differential-mode noise is related to current switching. The two may be generated at the same time. For example, when the MOSFET is turned off in the model, the voltage exhibits decaying oscillation at the same time the current is turned off. Therefore, common-mode noise and differential-mode noise coexist at this time. Usually, differential-mode interference dominates at low frequencies and common-mode interference dominates at high frequencies. Mahdavi of SHARIF University of Technology, Roudet and Scheich of LEG [11] established a 500W power factor preregulator (PFP) single-phase AC/DC converter model. They focused on the study of the generation and transmission mechanism of radio frequency conducted interference and used the simulation software MC2 to calculate the current harmonics injected into the power supply. The model showed good agreement with the test results within a frequency range of 10 times the switching frequency. In the study of differential-mode conducted EMI for PFPs, Reis predicted that the EMI characteristics would differ when the converter operated in different modes [13]. Erkuan Zhong and Lip et al. [12] of the University of Wisconsin-Madison used an 8kVA PWM inverter system driving a 7.46kW (10hp) induction motor as an experimental model. They found that the PWM inverter system driven by the high-power high-speed motor fed up to several A of pulsating current into the power supply, resulting in severe conducted EMI (up to 120dBμV in this experimental model) and power supply voltage waveform distortion (short voltage up to 50V, exceeding the rated voltage by 20%). The interference signal had a wide bandwidth, including not only the interference components and harmonics of the switching frequency, but also extending to the radio frequency range. The dv/dt (up to 3kV/μs, lasting several ns) generated by power devices during switching and the stray capacitance between the switching devices and ground generate charging and discharging currents at the power supply end, causing electromagnetic interference. At the same time, the nonlinear switching characteristics of the switching devices generate a large number of harmonics. They also pointed out that the diode reverse recovery current is the main source of differential-mode interference in the system. The study of conducted interference of power electronic devices, especially the study of common-mode and differential-mode conducted interference, provides a basis for the design of EMI filters. 2.2 Study of Radiated Interference Sources Compared with conducted interference, the radiated interference of power electronic devices is more complex. This is because, as energy conversion devices, the conversion capacity ranges from milliwatts to megawatts, and the main circuit and control circuit are often composed of different components. Compared with electronic devices concentrated on printed circuit boards, the spatial structure is more complex. Therefore, the analysis and calculation of the corresponding stray parameters and radiated interference are more complex [2]. At present, there are not many related studies. Among them, Orlandi and Scheich [14] are more representative in their study of the radiated interference sources of SCR rectifier circuits. They focused on analyzing the relationship between common-mode current (time domain and frequency domain) and radiation field, believing that common-mode current is related to the driving pulse from the control section and stray parameters. The voltage gradient between stray capacitors promotes the propagation of common-mode current, and the voltage gradient at the rising edge of the pulse generates common-mode current in the stray capacitor. Moreover, fast current pulses induce unwanted voltages on the metal parts (casing and heat sink) of the SCR, becoming radiation sources. To determine the radiation model of the switching converter, Professors Antonini and Cristina and Professor Orlandi of the University of Rome established dipole radiation models for the converter section of switching power supplies with switching frequencies of 75kHz and 150kHz [15]. However, because a transmission line model with an equivalent homogeneous medium was used to determine the line current distribution, the model showed good agreement with experimental results in the frequency range below 10MHz, but in the frequency band above 10MHz, common-mode radiation dominated due to the influence of various stray parameters. The transmission line model was no longer effective in determining the common-mode current distribution. In fact, the electromagnetic radiation characteristics of power electronic devices are determined by more than just this. For example, heat sinks, which are widely used in power electronic devices, often exhibit electromagnetic oscillation characteristics, which enhance the RF electromagnetic radiation of power electronic devices. Heat sinks usually have complex geometries, multi-band RF radiation characteristics, and are installed outside the device. Therefore, heat sinks are likely to act as effective radiating antennas on harmonics of one or more switching frequencies. Research in this area is also underway. For example, Ryan, Stone, and Chambers [16] used the FDTD method to make preliminary predictions on the RF electromagnetic radiation modes from finned heat sinks. 2.3 Study of near-field characteristics According to IEC22G-WG4-11, power electronic devices usually consist of two parts: a power conversion unit and a 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 sources of conducted interference and radiated interference. The energy of the electromagnetic radiation generated by the power conversion unit is sufficient to endanger the normal operation of the control unit nearby [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 for the EMC design of power electronic conversion devices. In order to explore the near-field characteristics of switching power supplies (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 wire segment of the experimental circuit into multiple Hertzian dipoles connected 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 the distance between the dipole center and the test point, the electrostatic terms cannot cancel each other out, thus generating a large electrostatic field, causing the predicted value to be higher than the actual value. Therefore, when integrating the radiation equation along the circuit, the spurious electrostatic charge effect caused by the integration of the dipole equation was eliminated through special processing, and a more accurate near-field (electric and magnetic field) model was established. Calculations show that at a distance of 3m from the experimental model, the electric field difference between the corrected model and the uncorrected model is 40dB in the low-frequency band, while the two tend to overlap in the high-frequency band. Test results show that the calculated and measured values ​​are in good agreement in the frequency band below 10MHz. In the frequency band above 10MHz, the common-mode current influence is dominant, and the above calculation model is no longer valid. The main circuit of the power conversion section is the main factor affecting the near-field characteristics of power electronic devices. Cristina et al. [17] studied the radiation mode changes, near-field spatial distribution and radiation characteristics of the converter section under different load conditions, and concluded that the switching power supply may exhibit electric dipole or magnetic dipole characteristics under different load conditions. This is very important for selecting and designing a suitable shielding scheme. Youssef and Roudet et al. [18] of LEG established a simple buck converter model using MOSFETs as switching elements. They assumed that the circuit was approximately a thin-wire structure and that the current in each part of the circuit was the same, and calculated the near-field distribution based on the time-domain current waveform during the switching operation. Meanwhile, the change in electromagnetic radiation when the grounded conductive plane is below the interference source circuit was studied using the mirror method, and it was concluded that 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 is still in its infancy, and there is no complete and accurate model yet. Especially in the high-frequency band, the near-field characteristics under the influence of various stray parameters and common-mode current are more complex. In summary, in the exploration of electromagnetic interference sources of power electronic devices, most studies adopt a combination of experimental and analytical methods and model the electromagnetic interference characteristics under certain operating conditions. However, there is still very little research on the electromagnetic interference source characteristics of high-power and complex-structured power electronic devices [2]. For a practical power electronic device, common-mode and differential-mode interference often coexist, and conducted and radiated interference occur 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 Electromagnetic Compatibility Characteristics of High-Frequency Soft-Switching Converters To meet the requirements of high-frequency operation, in addition to improving the withstand capability of the devices themselves, researchers have also focused on improving circuit topologies to reduce the electrical stress on the devices, decrease switching losses, and eliminate switching surges and voltage spikes. The main cause of interference in power electronic devices is the interaction between the high di/dt and dv/dt generated during the commutation process and stray parameters in the circuit, leading to high-frequency oscillations. If the high di/dt and dv/dt conversion process can be minimized by selecting appropriate circuit topologies and control techniques, then the electromagnetic compatibility characteristics of power electronic devices can be improved. Therefore, some have speculated that, in terms of conducted EMI, soft-switching converters using zero-voltage transition (ZVT) should perform better than hard-switching converters [9, 19]. The main basis for this is that in a ZVT circuit, the main switch operates in a zero-voltage switching state, and the diode operates in a soft-switching state. Thus, there is no rapid voltage switching in the main switch and no rapid current switching in the diode, thereby reducing high-frequency harmonics in the circuit. Is this really the case? From the perspective of EMI generation, resonant converters (including soft-switching converters) do indeed have advantages that PWM hard-switching converters cannot match. Specifically, this can be considered from the following aspects: (1) PWM technology converts power by interrupting power flux and controlling duty cycle, resulting in pulse current and pulse voltage; while resonant technology converts power in the form of a sine wave, and its spectrum is usually narrower than that of a PWM converter. Therefore, compared with PWM converters, it should have smaller harmonic interference and a larger amplitude base component at the input. (2) The operating waveform of a resonant switching converter is a quasi-sine wave with lower di/dt and dv/dt. (3) Resonant switching converters use the junction capacitance of the device and the leakage inductance of the transformer as part of the resonant LC circuit, and are not sensitive to harmful stray parameters. (4) Resonant switching converters operate at higher frequencies, which facilitates integration and minimization, and therefore usually have higher power density, which is very beneficial for reducing circuit loops and shortening wiring length. (5) PWM converters often use energy-dissipating buffer circuits to limit the stress on the devices, and at the same time, they also play a beneficial role in suppressing electromagnetic interference. Resonant soft-switching converters can reduce or eliminate energy-dissipating buffers, thereby improving development efficiency. Based on the above analysis, can we easily draw a conclusion? In 1996, researchers at the VPEC Research Center conducted a comparative experiment on the conducted interference of two single-phase 400WPFC boost converter experimental models using zero-voltage conversion (ZVT) circuits and hard-switching circuits, respectively [21]. The test results were unexpected. The EMI difference between soft-switching converters and hard-switching converters using ZVT technology was very small. In fact, if the additional circuit wiring of the former was not properly arranged, the performance would be worse. Unlike reference [20], they compared the common-mode and differential-mode interference of the two experimental models separately. The results showed that: in terms of common-mode noise, the two models had similar characteristics in the low-frequency range, but when the frequency exceeded several MHz, the noise of the hard-switching model was several dB higher than that of the ZVT model; in the high-frequency range, the common-mode noise of the ZVT model was lower, but in some cases, the noise peak of the ZVT model exceeded that of the hard-switching model at certain frequency points; in terms of differential-mode noise, the noise of the hard-switching model was stronger than that of the ZVT model. The above experimental results can be understood as follows: common-mode noise is mainly coupled through the stray capacitance of the device casing, while the main switch in the ZVT converter is a soft switch, and the dv/dt generated during the switching process is small. Therefore, the high-frequency common-mode interference of the ZVT converter is less than that of the hard-switching converter; and the noise peak of the ZVT converter at certain frequency points is due to the incorrect wiring of the auxiliary components in the ZVT converter. In addition, due to the higher di/dt caused by the reverse recovery current of the diodes 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. However, the high di/dt usually does not affect the low-frequency components, so the interference characteristics of the two are similar at the switching frequency and its low harmonics. It can be seen that although the high-frequency interference characteristics of the ZVT converter are a few dB better than those of the hard-switching converter, the overall EMI characteristics of the two are similar. In terms of differential-mode noise, the ZVT converter is better than the hard-switching converter, which is the advantage of soft switching over hard switching. In terms of common-mode noise, the problem is more complicated. The ZVT converter is different from the hard-switching converter in that the former has auxiliary soft-switching components, including auxiliary switching components that carry a larger peak current. These switching components may withstand the same voltage as the main switching transistor in the hard-switching converter. The auxiliary switching components in the ZVT converter are hard-switched, which means that the hard switching in the hard-switching converter is transferred to the auxiliary switching in the ZVT converter. Therefore, in the soft-switching circuit topology, the auxiliary switching components are important sources of interference, and their location and wiring are particularly important [21]. In essence, PWM converters with buffer circuits do not necessarily have worse noisy characteristics than soft-switching converters. However, whether soft-switching or hard-switching is better depends on the initial circuit design phase. Appropriate circuit topology and control techniques should be selected based on the application, and a predictive model for conducted and radiated interference should be established to guide the correct circuit layout. 4. Conclusion In summary, the electromagnetic compatibility (EMC) problem of power electronic devices is increasingly attracting the attention of scholars both domestically and internationally. Since the 1980s, many experimental studies and analytical modeling works have been completed abroad; however, domestic research in this area is still limited, and no mature technical reports have yet been seen. Especially with the rapid development of power electronics technology today, how to break away from the empirical and trial-and-error methods in EMC design and put power electronic device EMC design on a systematic design track is a major problem facing scholars both domestically and internationally. It will inevitably become one of the central topics in power electronic device EMC research. Only through in-depth analysis of the electromagnetic interference sources of various power electronic devices, determination of the sensitivity of various parameters, research on the EMC characteristics of various switching topologies and control schemes, and establishment of predictive models can the systematic design of power electronic device EMC be achieved and adapted to the rapid development of power electronics technology itself. 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