Aimed at the problem that DC-DC converters with coreless transformers must operate at high frequen-cies, a DC-DC converter with a coreless transformer based on free decay oscillation is proposed. First, a circuit model in series-series topology with no excitation sources is established, the evolution of the eigenvalue is discussed, and the free decay oscillation behavior of the system is analyzed. Second, the DC-DC converter with a coreless transformer based on free decay oscillation is designed. The switching frequency of this converter can be significantly lower than the operating frequency of the coreless transformer, and the output power from the converter and the operating frequency of the core-less transformer can be described by the eigenvalue of the free decay oscillation system. Finally, an experimental proto-type was constructed, which was under constant duty control. The switching frequency of the converter was reduced to one half（103 kHz） and one third（69 kHz） of the operating frequency of the coreless transformer（206 kHz）, respective-ly. The converter efficiency achieved 91.2%, and the transformer efficiency was always higher than 96%.

A design method for small- and medium-power DC/DC converters suitable for low-orbit commercial aerospace is given in this paper. A synchronous rectifier flyback topology and surface mounted devices are adopted, which can meet the demands for common single- and multi-output DC/DC converters applied in aerospace, with advantages of low cost, high performance, high reliability, and mass production. To further improve the conversion efficiency and power density, gallium nitride（GaN） FETs are also used. In addition, the losses of main power devices in the topology under different operating conditions are calculated and compared. Finally, a DC/DC converter with a wide range of input voltage（23-47 V） and output of 5 V and 30 W was built for verification.

Owing to its obvious advantages such as high control flexibility, no commutation failure and strong dynamic reactive power support capability, the voltage sourced converter based high-voltage direct-current（VSC-HVDC） transmission technology has been widely applied in scenarios including point-to-point transmission, back-to-back interconnections and DC grids. As a core piece of equipment in VSC-HVDC transmission engineering, the VSC valve achieves AC/DC energy conversion through frequent switching of power electronic devices. In this paper, the key design requirements for VSC valves in different application scenarios are systematically summarized by combining with practical experiences accumulated in engineering, the commonly used power devices and VSC valve topologies in VSC-HVDC transmission engineering are compared and analyzed, and their development trends are projected. In addition, different schemes for two typical application scenarios in the future are also compared, providing reference for the applications of VSC-HVDC transmission technology in high-voltage, large-capacity and long-distance transmission scenarios.

Silicon carbide metal-oxide-semiconductor field effect transistor (SiC MOSFET) has attracted attention from the industry owing to its excellent characteristics such as high voltage, high frequency and low conduction loss. However, compared with the silicon-based IGBT, the problem of gate oxide reliability caused by the high defect density at the SiC/SiO₂ gate oxide interface has become a key bottleneck restricting the large-scale applications of SiC MOSFET devices. By sorting out and analyzing the research results of the gate oxide reliability of SiC MOSFET at home and abroad in recent years, the causes of the gate oxide reliability problems at present were elaborated upon, and various commonly-used gate oxide reliability evaluation methods were summarized and compared. Finally, the gate oxide reliability of SiC MOSFET under extreme operating conditions and the development status of technologies for improving its performance were discussed.

Accurately obtaining the electromagnetic characteristics of high-voltage and high-power switching devices is crucial for predicting the electromagnetic interference in a system in which the devices are located. Research is focused on an equivalent method of switch waveforms for analyzing the electromagnetic characteristics of high-voltage and high-power switching devices. Aimed at the problem that the existing equivalent waveforms are too ideal to reflect the complex spectral components in the switching transients of devices, an analytical model for the electromagnetic characteristics of devices considering their switching processes is proposed. Starting from the time-domain analytical formula for the analytical model and based on the Fourier transform theory, the frequency-domain analytical formula for the analytical model is derived, and the spectral envelope characteristic parameters are analyzed to obtain the spectral characteristics of the analytical model. The theoretical analysis was verified by using the measured switching waveforms of Si IGBT and SiC MOSFET devices.

The design of DC-DC converters which are applied to vehicle auxiliary power modules (APMs) is taken as a research object, the topology of a two-stage DC-DC converter consisting of a three-level Boost (TL-Boost) topology and a half-bridge LLC resonant topology is proposed, and its working principle is analyzed. The front-stage TL-Boost topology converts a wide range of input voltage into a stable voltage, ensuring the high-efficiency operation of the back-stage half-bridge LLC resonant topology. The feasibility and correctness of the proposed DC-DC converter were verified by establishing an experimental platform and carrying out relevant experiments.

With the rapid development of new energy technology, the performance of DC-DC converters continuously increases. In this paper, a novel high-gain DC-DC converter is proposed, which is improved based on the quasi-Z-source topology. Owing to the use of the topology in which three capacitors discharge together to the load, a higher voltage gain is obtained while the voltage stress of capacitors is reduced. The proposed converter has advantages of the traditional quasi-Z-source converter such as simple control, continuous current and small current ripple. The working principle for this converter is analyzed. In addition, its performance was verified through simulation experiments and prototype experiments.

The advancements in research on automotive power device packaging have significantly improved the dynamic performance and driving range of electric vehicles, making them more efficient and reliable. With the continuous optimization of automotive power device packaging, the electric vehicle industry is expected to embrace a broader market prospect and development space. In recent years, power device packaging modeling, packaging structure and optimization, thermal management and junction temperature monitoring, gate drive and applications, reliability analysis, and online monitoring have become current research hotspots and have received sustained attention from both the academic and industrial sectors. To promote discussions on the challenges and hot issues related to automotive power devices packaging and their applications, a special issue titled “High Reliability Power Device Packaging and Assistant Technology in EV Application” has been launched in the Journal of Power Supply.

A soft-switching DC-DC converter with low current ripple and high gain is proposed, which can be applied to new energy generation systems. Based on the conventional interleaved Boost converter, the proposed converter can achieve high gain by introducing a coupled inductor, diodes and a capacitor Boost unit. The coupled inductor transmits energy during the entire switching cycle, thus improving the utilization rate of magnetic core. The input Boost stage works in an interleaved mode, and the current ripple of the two-phase inductor can cancel each other, so as to obtain a lower input current ripple. Due to the existence of leakage inductance of the coupled inductor, the reverse recovery problem of rectifier diodes are alleviated. Meanwhile, an active clamp circuit is adopted to absorb the leakage inductance energy, thereby achieving the zero-voltage soft-switching of all switches, restraining the turn-off voltage spike of switches, and improving the converter’s conversion efficiency. The working principle, circuit characteristics and soft-switching realization method of the converter are analyzed in detail. Finally, a 200 W experimental prototype was built to verify the theoretical analysis.

The dual-active-bridge (DAB) converter is a key device in a bidirectional power transmission system. In this paper, its fundamental operation principle and topologies are reviewed at first. Then, four basic modulation strategies for the DAB converter are introduced, including single-phase-shifted, dual-phase-shifted, extended-phaseshifted and triple-phase-shifted strategies. Moreover, the modeling and optimization methods based on these four modulation strategies are compared and analyzed. Finally, some problems faced by practical applications and the corresponding solutions are discussed. Along with the development of DC power distribution, energy storage and distributed energy resources, DAB converters will have broad application prospects.

As a novel and extensively applied switching device, silicon carbide metal-oxide-semiconductor field-effect transistor（SiC MOSFET） offers a faster switching speed and lower device loss in practical applications, thereby enhancing the converter efficiency and delivering a superior performance. Aimed at the driving characteristics of SiC MOSFET, the influence of parasitic parameters on its performance was analyzed. To investigate the relationship between the gate-source voltage and turn-on time of SiC MOSFET, a two-pulse experimental platform was also established. However, there are certain drawbacks with the existing domestic SiC MOSFET. Based on the experimental platform and other power products, the changes in conduction time, driving loss and negative voltage amplitude after replacing the imported SiC MOSFET with domestic devices were analyzed.

For evaluating the capacity of wind powers, photovoltaics and other power electronic grid-connected units supporting power systems, the core foundation is to correctly understand the unit’s functional role（i.e., the unit characteristics） that unit adjusts its own internal voltage amplitude/frequency according to the active/reactive power imbalance. However, the mainstream PLL-based grid-connection structure in power electronic units seriously hinders the understanding of the unit’s functional role. In particular, based on a specific PLL-based grid-connection structure, the industry and academia at present form a “grid following” role perception that the internal voltage of unit follows the grid voltage or terminal voltage, and have not recognized the functional role that the unit should take during the system operation. Therefore, through an in-depth understanding of the independent excitation-response mechanism of current control which is hidden under the PLL-based grid-connection structure, i.e., the internal voltage response depends on current excitation alone, the functional role of PLL-based grid-connected units in which the active/reactive power imbalance independently adjusts the internal voltage amplitude/frequency is clarified. Afterwards, a role characterization method for unit is pro-posed based on the relationship between active/reactive power imbalance excitation and internal voltage amplitude/fre-quency response, i.e., the amplitude-frequency motion equation. Finally, the inevitability of characterizing the role of PLL-based grid-connected units through the relationship between power excitation and internal voltage response is elab-orated on, and the existing limitations in the understanding of the role of PLL-based grid-connected units in industry and aca-demia are pointed out.

LLC resonant converters are widely applied in on-board power supplies owing to their high power density, high efficiency and small size, and their reliability is critical to the driving safety and passenger experiences. However, the complicated working conditions and harsh environment under which vehicles operate have become a huge challenge to power devices. When a switch failure occurs, the resonant converter cannot maintain a stable output voltage while operating at a resonant point, and both efficiency and output capability of the system will decrease substantially. To make the converter more compatible with fault occasions, an improved LLC topology and its control strategy are proposed in this paper, which can ensure that the output voltage remains unaffected when a switch failure occurs and the converter operates near the resonant frequency. Additionally, an optimized Burst control strategy is designed to suppress the overvoltage of the resonant capacitor during the fault tolerance transition and guarantee a smooth fault tolerance process. Finally, simulation and experimental results verified the effectiveness of the proposed method.

The silicon carbide（SiC） device is considered as a semiconductor device with high temperature resistance, and a careful study on its loss and heat dissipation is required when it is applied to high-power-density and high-tem-perature scenarios. The maximum current conduction capability of SiC MOSFET power module at high temperature is stud-ied, and the relationship between electrical performance and heat dissipation is taken into account. Based on an electro-thermal coupling model of SiC MOSFET device and a heat dissipation model of the cooling system, the mechanism of thermal runaway process is analyzed. A co-simulation is conducted to determine the current conduction capability of one SiC power module at high temperature, and the simulation error with respect to the experimental result is about 4%, which verifies the effectiveness of the proposed method.

Aimed at the problem of DC voltage fluctuations caused by load switching, power fluctuations and dou-ble-frequency injection in DC microgrids, a dynamic compensation control strategy for a Buck-type bidirectional DC-DC converter based on model predictive control（MPC） is proposed. First, the corresponding discrete state space matrix is es-tablished, and the input current is used as the disturbance. Second, the model-based inner-loop current predictive control and outer-loop voltage control of the Buck-type bidirectional DC-DC converter is designed. Third, a dynamic compensa-tion control structure based on a residual generator is designed for the current disturbance, and the dynamic compensa-tion controller Q（z） is solved. At the same time, the recursive least squares algorithm is used for parameter identification to reduce the influence of model uncertainty on the dynamic compensation control strategy. Finally, a comparative ex-periment was designed on the PSCAD/EMTDC simulation platform to verify the effectiveness of the proposed control strategy. Experimental results show that the compensation control structure can effectively solve the problem of DC bus voltage fluctuations and enhance the robustness of the entire system without changing the original predictive control.

As power lithium-ion batteries play a key role in the electric vehicle industry, ensuring their working reliability has become a research hotspot at present. In this paper, the materials and manufacturing processes of lithium-ion batteries are reviewed. The battery state estimation and remaining useful life prediction methods are summarized in detail, and the advantages and disadvantages of these methods are discussed. From the perspective of battery management system, the relevant knowledges of equalization management system and thermal management system are sorted out in turn. From the perspective of electric vehicle hybrid energy storage system, the performance degradation mechanism under actual working conditions and the relevant technologies are elaborated upon. Finally, the status quo of key technologies related to the reliability of power lithium-ion batteries used in electric vehicles is summarized from four aspects, and the development possibilities in the future are forecasted.

Enhancing the power density of vehicle-grade power modules is of significance for the performance of electric vehicles. The two-dimensional layout used in conventional power modules results in large parasitic inductance, which limits the switching speed and bus voltage and further affects the increase in power density. To solve this problem, an IGBT power module with EconoDUAL packaging was taken as the research object, and a three-dimensional layout was designed using the stacked DBC method to develop a 1 200 V/1 200 A IGBT power module. The layout structure of the proposed power module was introduced in detail. Compared with those obtained using the conventional two-dimensional layout methods, the parasitic inductance decreased by 58%. Additionally, electrical performance tests including a double-pulse test with pulse current of 1 200 A under bus voltage of 800 V were conducted on the power module, thereby verifying the improved power density of the module. To maintain the heat dissipation performance while increasing the power density, a water-cooled PinFin heatsink was used at the bottom of the power module, and heat dissipation simulation and thermal resistance test were carried out, revealing an IGBT thermal resistance of 0.084 K/W and a diode thermal resistance of 0.124 K/W. These results show no significant difference compared with a commercially available 1 200 V/900 A module with the same packaging, confirming the correctness and effectiveness of the proposed design approach.

The new generation of wide bandgap power semiconductors such as SiC and GaN are driving the rapid high-frequency, high-efficiency, and small volume development of power electronic equipment. However, they are also more likely to interfere with sensitive loads, affect wireless communication, and even endanger their own safety and reliable operation, which poses great pressure and challenges to the electromagnetic compatibility（EMC） performance of power electronic equipment. In recent years, the radiated frequency（RF） characteristics of power switches, wideband electromagnetic models of magnetic components, electromagnetic radiation mechanisms of switched mode power supplies, near-field characteristics of wireless power transmission（WPT）, and the new designs of electromagnetic interference（EMI） filters have become current research hotspots and received continuous attention from academia and industry. The Journal of Power Supply has specially released the album "Electromagnetic Compatibility in Power Electronic Systems" to promote the exploration of difficult and hot issues in the field of EMC analysis and design of power electronic systems.

In a weak grid, due to the existence of grid impedance, the natural resonant frequency of a new energy grid-connected LCL filter will shift, and the traditional active damping control strategy cannot guarantee the system stability. Moreover, as the proportion of new energy power generation in the power system continues to grow, how to reduce the operating costs is a hot topic for research. Therefore, a novel control strategy based on grid-connected current and common coupling voltage feedback is proposed in this paper, which not only provides active damping to suppress LCL resonance, but also reduces the use of sensors. In addition, it has a strong adaptability under wide-ranging changes in grid impedance. Simulation and experimental results show that, compared with that under the traditional control strategy, the practical range of weak grid under the improved strategy increases, the system stability is enhanced, and the capability to suppress harmonics is raised, indicating that the quality of grid-connected current is well improved.

To ensure the safety of new energy vehicles during the entire period of use, it is necessary to conduct health monitoring for the full life cycle of lithium-ion batteries. Aimed at the low learning rate due to the small capacity of training data set for the remaining useful life（RUL） prediction model based on neural network and the duplicate collinearity of the extreme learning machine（ELM） method, a method for augmenting the training data set is proposed. In addition, based on the improved ELM, an RUL prediction model for the full life cycle of lithium-ion battery is built. First, the early operation data of battery is extracted to formulate health factors, and the Akima interpolation method is used to augment the amount of training data. Then, the salp swarm algorithm is used to improve the ELM network, and the RUL prediction model for the full life cycle of lithium battery is established. Finally, the NASA battery data set is used to validate the model. Experimental results show that the proposed method for augmenting the training data capacity is effective, the capacity tracking capability of the RUL prediction model in full life cycle is strong, and the prediction error is small.

Based on the fact that inductance and capacitance are of fractional-order, the nonlinear dynamic characteristics of a fractional-order Boost converter are studied. The predictor-corrector model of the Boost converter is established using the predictor-corrector algorithm of fractional-order calculus. On this basis, the bifurcation diagrams with the reference current, input voltage and orders of capacitance and inductance as bifurcation parameters are obtained. The period doubling bifurcation and chaotic behaviors of the fractional-order Boost converter are studied, and its nonlinear dynamic behavior is compared with that of an integer-order Boost converter at the same time. Results show that under certain operating conditions, some nonlinear phenomena such as bifurcation and chaos will appear in the fractional-order Boost converter with changes in some circuit parameters. Under the condition of the same circuit parameters, the parameter stability domains of integer- and fractional-order converters are different. Compared with that of the integer-order converter, the parameter stability region of the fractional-order converter is smaller, which more truly reflects the nonlinear dynamic characteristics of the Boost converter.

In the application of wide output voltage range, some problems exist in an LLC resonant converter under the traditional frequency control, such as a wide switching frequency regulation range, a large circulation current and difficulty in high-efficiency operation. To solve these problems, a dual-full-bridge LLC resonant converter under a fixed-frequency phase-shift and fixed-frequency PWM hybrid control strategy is proposed, which is suitable for the wide range of output voltage. This converter is composed of two full-bridge LLC resonant converters sharing one bridge arm, and the output voltage is modulated by the hybrid control, so that a quadruple voltage gain and a wider output voltage range of the resonant converter can be realized. Meanwhile, the problems that the large circulation current exists under the traditional frequency control and the soft switching cannot be realized at a small phase shift angle are solved, thus improving the system efficiency. The switching frequency of the converter is always equal to the resonance frequency, and the voltage gain is independent of load, which is helpful for the design of the magnetic element. At the same time, the soft switching can be realized in the full load range. Finally, a detailed analysis of the circuit principle was given, which was further verified by simulations. An experimental platform was also established to validate the feasibility and effectiveness of the proposed converter.

Owing to its advantages including lower switching stress, harmonic components and a better anti-interference capability, the diode neutral point clamped（NPC） three-level inverter has become a prominent topology for DC-AC converters used in new energy fields such as photovoltaic and energy storage. The NPC three-level insulated gate bipolar transistor（IGBT） power semiconductor module which is widely used in high-power applications is studied. The commutation circuit in the NPC three-level power module is analyzed, and a precise simulation and evaluation method for the corresponding parasitic parameters is given. According to the principle of minimizing the parasitic parameters of the commutation circuit, a dynamic characteristic test circuit suitable for the NPC three-level power semiconductor module is designed. Based on the commutation circuit and the operating principle of circuit, a drive circuit for the NPC three-level power module is designed, and a driving scheme that enhances drive current, prevents shoot-through and allows for adjustable dead time is formulated. Finally, through dynamic testing of the NPC three-level IGBT module, a comprehensive assessment of the dynamic loss in power devices under various operating conditions is conducted.

Due to the duty cycle constraint on the traditional Boost converter, its applications to high-voltage-gain power supply are limited to certain degree. In this paper, a DC-DC converter with high voltage gain based on an isolated Boost converter and Cockcroft-Walton voltage multiplier cell（VMC） is studied, and its working principle and characteristics are analyzed. This converter achieves a conversion with an ultra-high step-up ratio by integrating the isolated Boost converter with the VMC. Compared with the traditional Boost converter, this topology has a high voltage gain in a low duty cycle, a low voltage stress of active switching device, and a simple control circuit with one single switch. Finally, a 35 W prototype with an efficiency of 89.5% was built to achieve a high step-up conversion from 24 V to 1 000 V, and the theoretical analysis results was verified by experimental results.

To study the degradation mechanism of silicon carbide metal-oxide-semiconductor field effect transistors（SiC MOSFETs） under dynamic drain-source stress, a dynamic reverse bias test platform with an adjustable dV_ds /dt capability up to 80 V/ns was developed. A dynamic high-temperature reverse bias test of commercial SiC MOSFET was carried out, and the effect of dynamic drain-source stress with a high voltage change rate on the electrical characteristics of SiC MOSFET was discussed. Experimental results show that the threshold voltage and forward conduction voltage of the bulk diode increased, indicating that the gate oxygen layer and the bulk diode above the JFET region of the device may be degraded. Sentaurus TCAD was used to analyze the weak position of plane-gate SiC MOSFET under high drain-source voltage and a high voltage change rate, and hole traps were set at the gate oxygen layer junction and the body diode region to simulate the effect of dynamic high-temperature reverse bias on the dynamic and static parameters of SiC MOSFET.

With the improvement of the integration degree of power modules, the optimization of their heat transfer structures has become a focus in the development. The topology optimization（TO） can maximize the cooling performance by transforming the morphology and structure of heat sinks, thus receiving extensive attention. However, in the TO process, the temperature distribution of modules and heat sinks needs to be calculated in each iteration step, consuming a large amount of computing resource and calculation time. To accelerate the TO process of traditional heat sinks, a fast iterative method combining neural network（NN） synchronous learning and the traditional solid isotropic material with penalization （SIMP）-based TO methods is put forward. First, an NN prediction model based on the encoder-decoder structure is constructed, which can iteratively evolve the shape of heat sinks to achieve a fast prediction of optimized structures. Second, the NN model is integrated into the TO process of the heat sink based on the SIMP method, and the NN is trained synchronously using the intermediate morphology obtained in the iteration process. Finally, aimed at the single-chip and dual-chip modules, the results obtained by the new method and traditional iterative methods are compared to validate the accuracy and rapidity of the proposed NN synchronous leaning method.

Silicon carbide（SiC） devices possess advantages such as high voltage resistance, low losses and high thermal conductivity, making them of significant importance for the development of the electric vehicle industry. A design for a SiC MOSFET power module utilizing large-chip packaging was proposed, and experiments were conducted to analyze the module’s electrical performance. Simulations were set up to compare the module temperature under two conditions, i.e., electrical characteristics only and a combination of electrical characteristics and temperature feedback. Simulation results indicate that under identical operating conditions, the SiC MOSFET power module designed with large-chip packaging exhibited stronger conduction current capability, smaller temperature variations and improved electrical performance.

The two-switch Buck-Boost converter has been widely applied in step-up and step-down scenarios. However, it usually operates under hard switching conditions in the existing various control and modulation modes. In addition, its interleaved control circuit is usually complicated. A three-switch interleaved Buck-Boost circuit with co-directional coupling inductor and its control method are given based on the characteristics of co-directional coupling inductor. First, the coupling process of the coupling inductor during the switching process is analyzed under a large coupling coefficient, based on which the circuit’s fundamental operating principle is given in detail. Then, it is concluded that the extended duty cycle and soft switching of Boost-side power switches can be achieved in the discontinuous self-induction current mode, so as to avoid the synchronous and current-sharing circuits in the two-phase interleaved control circuit, thus obviously simplifying the control circuit. Finally, simulation and experimental results verified the analysis results.

Aimed at the problem of low efficiency of a dual active bridge（DAB） converter in the wide voltage range of single phase shift modulation strategy, a one-sided asymmetric duty modulation strategy is proposed in this paper, which significantly improves the efficiency of DAB converter, especially in the case of light load. First, the principle of one-sided asymmetric duty modulation scheme is described, and two operation modes are obtained according to the relationship of control degrees of freedom. Second, based on the time-domain analysis, the steady-state characteristics in the two operation modes are derived, including inductance current and transmission power. Third, in order to find the optimal combination of control degrees of freedom, the peak-to-peak value of inductance current is selected as the optimization objective, and the optimal one-sided asymmetric duty modulation strategy is obtained by applying the KKT condition. Finally, an experimental platform for DAB converter based on SiC device was built, and experimental results verified the effectiveness of the proposed one-sided asymmetric duty modulation strategy.

Building a complete spacecraft electrical power and distribution standard system is an important tool for improving the design of spacecraft electrical power and distribution system and ensuring the safety and reliability of satellite energy. In this paper, the electrical power and distribution standard systems published by European Cooperation for Space Standardization（ECSS）, National Aeronautics and Space Administration（NASA）, Japan Aerospace Exploration Agency（JAXA）, International Standard Organization（ISO） and American Institute of Aeronautics and Astronautics（AIAA） were investigated. The focuses of their respective standard specialties were analyzed, the corresponding content was discussed, and the standard systems of various organizations were summarized. Combined with the actual situation in China, the suggestion and reference for the spacecraft electrical power and distribution standard system are put forward.

The large grid inductance in weak grid may cause a grid-connected converter to be unstable. Therefore, an impedance model of grid-connected converter is built at first, and the influence of grid inductance on the stability of grid-connected converter is analyzed according to the impedance ratio criterion. Then, aimed at the problem of low adaptability of the grid-connected converter to inductive grid impedance, a virtual impedance control strategy based on band-pass filter is proposed, and the influence of virtual resistance value on the adaptability of grid-connected converter to weak grid is studied. Furthermore, a selection principle for the virtual resistance value is also given. Finally, a system simulation model is built, and simulation results verify the correctness of theoretical analysis and the effectiveness of the proposed control strategy.

With the rapid development of power generation by renewable energy and the grid-connection technology, the microgrid dominated by power electronic converters has attracted more and more attention in recent years. Owing to the low inertia and high nonlinearity of power electronic converters, an islanded microgrid under large disturbances is more likely to lose its transient stability. Considering the interactions between grid-forming and grid-following converters in the microgrid, a transient stability criterion based on the equal area criterion（EAC） and an improved control strategy for transient stability are proposed. First, the simplified second-order dynamic model of the islanded microgrid is established, which contains a nonlinear damping term relying on the power angle. Then, the impact of the nonlinear damping term on the acceleration and deceleration areas is revealed from the energy perspective. Considering the distribution characteristics of nonlinear damping, a transient stability criterion is formulated for the positive damping region. In addition, according to the stable boundary conditions, an improved control strategy for transient stability based on voltage feedforward is also put forward. Finally, simulations are carried out with MATLAB/Simulink to verify the effectiveness of the proposed stability criteria and the improved control strategy. The results show that the microgrid transient stability criterion and the improved control strategy proposed can provide a theoretical basis for the parameter optimization design of power electronic converters and the improvement of the stable operation capability of microgrid.

Power semiconductor devices are the core of electric energy conversion and electric drive based on the power electronics technology, which have broad application prospects in new energy generation, transportation, aerospace and other fields. However, the problems such as degradation, failure and reliability caused by heat generation have become bottlenecks that limit their further development, and it is urgent to explore effective thermal management methods to improve their reliability and service life. In this paper, based on the introduction of thermal management methods for power modules, the research progress in active thermal management methods is reviewed in detail, and these methods are divided into device-level, system-level and multi-parameter comprehensive methods according to the difference in control parameters. In addition, various methods are analyzed and compared. Finally, the development trend and prospect of technologies for power devices which are related to junction temperature are put forward, providing a reference for the subsequent research and applications.

The advantages of a Boost-APFC circuit operating in critical conduction mode are introduced. Aimed at the disadvantages of the traditional single-phase CRM-Boost APFC voltage mode control method, such as a long PI parameter debugging time, a poor adjustment effect and increasing unstable factors, a single-phase CRM-Boost APFC voltage mode control method with a static operating point is proposed, and the advantages of this method are verified by PSIM simulations. Considering the shortcomings of the novel interleaved control method, such as a long PI parameter debugging time, increasing unstable factors and the need to use an additional voltage-controlled current source, an improved two-phase interleaved parallel CRM-Boost APFC voltage mode control method is put forward, and the PSIM simulations are completed, with a power factor as high as 99.96%. A 4 kW two-phase interleaved parallel CRM-Boost APFC experimental prototype was made, and it was experimentally debugged, with a power factor of 99.66% and an efficiency of 98.02%.

With the widespread applications of insulated gate bipolar transistors（IGBTs） in power electronic systems, the accurate acquisition of junction temperature which affects their reliability has become crucial. However, one of the main forms of module failure is the aging of the solder layer, which can have a significant impact on the junction temperature. To accurately estimate the junction temperature, the advantages of two traditional thermal network models（i.e., Cauer and Foster） are combined in this paper, and an interface method for the two models is studied, so that the combination is completed. The aging of the chip solder layer is taken into account, and a hybrid thermal network model is proposed. Finally, through the comparison of finite element simulation and experimental test with the calculation results of the hybrid thermal network model, it is verified that the hybrid thermal network model can achieve an accurate junction temperature estimation, providing a basis for monitoring the operating status of the module.

A flux-weakening control method for permanent magnet synchronous motor（PMSM） used in electric vehicles is put forward on the basis of current prediction to improve the dynamic performance in the flux-weakening region of PMSM. The voltage boundary problem in the flux-weakening region is analyzed in detail, and the stability problem under different voltage selection criteria is also introduced. On this basis, a dynamic overmodulation strategy considering the stability and dynamic characteristics is proposed. Furthermore, a model-based predictive current control algorithm is investigated, in which the advantages of fast dynamic response and manageable constraints help to improve the dynamic performance in the flux-weakening region while guaranteeing the stability. Finally, the effectiveness of the proposed algorithm was verified on a simulation platform and an experimental platform, respectively.

An evaluation platform for the CM noise suppression characteristics of high-frequency transformer was established, which was suitable for batch applications in engineering. The conduction mechanism of common-mode（CM） noise in a transformer was analyzed, and the conduction characteristics of CM noise along the coupling path in the transformer was investigated for evaluating the transformer’s capability of suppressing the CM noise. First, a function generator was used to generate a high-frequency voltage pulsation signal, which was assigned on the primary winding of the transformer to simulate the transmission characteristics of CM noise. Then, an oscilloscope was used to capture the voltage drop generated by the CM signal on the sampling resistor to judge the suppression effect of the transformer on the CM noise, so as to analyze the influence of the sampling resistor selection on the evaluation results. The effectiveness of the proposed evaluation method for the CM noise suppression characteristics of transformer was verified by comparing the evaluation results with the test results of conducted electromagnetic interference spectrum.

Owing to their merits including continuous input and output current, high efficiency and high power density, non-isolated Superboost converters are widely applied in spacecraft power systems. However, the switching loss of the device will increase in a scenario with a high step-up ratio, resulting in a decrease in the converter efficiency. To solve this problem, a zero-voltage switching pulse-width modulation (ZVS-PWM) Superboost converter with low voltage stress is proposed. By introducing a resonant tank, the main switch can be turned on or off under ZVS, and the auxiliary switch can be turned on under zero current switching and turned off under ZVS. Besides, all the diodes are operating under soft-switching. As a result, the switching loss is reduced effectively, and the converter efficiency is improved without increasing the voltage and current stress of the main power device. The operation principle, soft-switching conditions and device stress are analyzed in detail, and the state-space averaging approach is used to estimate the steady-state and dynamic characteristics of the proposed converter. In addition, its feasibility was verified by a prototype with 100 kHz and 400 W.

Aimed at the problems of a traditional dual-active-bridge (DAB) converter such as large switching loss, large circulating current, narrow range of load variation and low operating efficiency, a novel power control method for DAB converter based on variable inductance and phase shift (PS) angle is proposed, in which the variable inductance and PS angle are taken as the main control parameters to improve the operating efficiency of the DAB converter in a wider range of load variations. In addition, the transfer function of the DAB converter is linearized to improve the practicality and convenience of the controller. Because the device saturation is controllable in the proposed method, the core size can be reduced to optimize the converter size. Finally, the effectiveness and superiority of the proposed method were verified by experiments. Resultsshow that under the condition of maximum PS angle, the inductance variation significantly affected the power transmission of the DAB converter, and its overall operating efficiency was about 5% higher than that of the traditional DAB converter under both the light and heavy load conditions.

The influence of ash deposition on the output characteristics of photovoltaic（PV） modules and the deposition rule of pollution particles on the modules’ surface are studied, which are helpful for formulating an ash removal scheme and improving the efficiency of photoelectric conversion. The PV array on the roof of a fan hall in North China Electric Power University is taken as the research object, and an artificial ash distribution experiment was carried out to explore the influence of different ash deposition amounts on the output power, current and voltage of modules. To determine the influence of one single factor on the deposition of pollution particles, a numerical model of particle deposition is established using COMSOL under the same conditions as those in the natural deposition test of PV modules, and the influences of wind speed, humidity, particle size and pollution concentration on the deposition of pollution particles on the surface of PV modules are simulated and analyzed. Test results show that ash deposition has little influence on the working voltage, but a great influence on the output power and working current. When the ash deposition density is 5.07 g/m², the changing rates of output power, current and voltage are 8.71%, 6.48% and 0.40%, respectively. Simulation results show that when other conditions are the same, the deposition amount of particles decreases first and then increases with the growing wind speed and particle size, and the minimum value is reached at the wind speed of 3 m/s and particle size of 15 μm. Under the same conditions, the particle deposition amount increases with the growing humidity and pollution concentration.