The state of health（SOH） estimation for sodium-ion batteries is crucial for their safe and efficient applications, which is also a key to large-scale energy storage implementations. However, sodium-ion batteries exhibit usage-induced degradation with unclear mechanisms and are sensitive to operating conditions and environmental factors, posing a challenge to the accurate SOH estimation. In this paper, a data-driven SOH estimation method for sodium-ion batteries is proposed. The charging data is correlated with capacity degradation, and variance filtering, grey relational analysis and recursive feature elimination are integrated for feature selection. In addition, four machine learning methods including multiple linear regression, support vector machine, Gaussian process regression and error back propagation neural network are applied to formulate the corresponding estimation methods. Test results reveal that the root mean square errors for the four methods are all less than 1.6%, with Gaussian process regression showing an error rate below 0.8%, indicating a precise SOH estimation for sodium-ion batteries.

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%.

The insulated-gate bipolar transistors（IGBTs） have been widely applied in the modern power electronics technology, and the paralleling of IGBTs has become an economical and feasible method in some working scenarios where one single device cannot meet the design requirements. The paralleling of IGBT modules can simplify the circuit structure, increase the converter output power, and improve the power density of devices. During the operation of IGBTs in parallel, the current imbalance, which may be caused by the difference in IGBTs' characteristics in a static or dynamic mode, the inconsistency of junction temperature, the asymmetry of a drive circuit or power loop, as well as the aging or failure of IGBTs due to long-term use, will affect the system's reliability and stability. The research hotspots of parallel-operating IGBTs at home and abroad are investigated. The principle and influence of static and dynamic current imbalance are summarized, and the difference in the current-sharing control principles is analyzed. The performance characteristics of current-sharing control are summarized and compared from the aspects of power loop current-sharing control and drive circuit current-sharing control. Furthermore, the development of current-sharing technologies for parallel-operating IGBTs in the future is also prospected.

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.

A Super-Boost converter can greatly reduce the mass and volume of power supply and improve the cor-responding power density by replacing the traditional charging and discharging module, so it has a broad application prospect in space power system. However, due to the existence of multiple power components and the reverse flow char-acteristics of inductance current, its power supply mode and output ripple voltage are more complex than those of the traditional Boost converter. To provide a theoretical guidance for the analysis and design of the Super-Boost converter, its power supply mode and output ripple voltage are studied. It is found that there exists continuous conduction mode, pseudo continuous conduction mode and pseudo discontinuous conduction mode in both inductor L₁ and L₂. The analyti-cal mathematical models of critical inductance and output ripple voltage in each operation mode are established, the re-lationship between peak current and inductance is discussed, and the minimum capacitance and minimum inductance that meet the design requirements are obtained. On this basis, a design method for the converter parameters is given, and experimental results verify the theoretical analysis.

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.

Aimed at the particularity of an isolated DC/DC converter when it is applied in a space radiation environment, the isolated magnetic feedback circuit is usually used to improve the feedback accuracy and stability. The working principles for several commonly used amplitude modulation magnetic feedback circuits are introduced, and a forward-flyback amplitude modulation bidirectional magnetic feedback circuit is introduced and optimized to solve the problem of the need for an additional secondary power supply voltage. Under the premise of realizing the same function, a two-winding transformer is used to replace the three-winding transformer, which effectively reduces the volume of the magnetic core and simplifies the circuit structure. The working principle for the circuit and the design method for a pulse sampling circuit and a magnetic feedback transformer are analyzed in detail. In addition, based on the magnetic feedback circuit, a DC/DC converter prototype with 100 W output was built. Simulation and experimental results show the effectiveness of the proposed isolated magnetic feedback circuit, providing a theoretical guidance for engineering design.

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.

Silicon carbide（SiC） is a promising wide-bandgap semiconductor material owing to its excellent electrical and thermal characteristics. Power metal-oxide-semiconductor field-effect transistors（MOSFETs） based on SiC are suitable for high-power fields, and their high-temperature gate oxide reliability is one of the most concerned characteristics. In this paper, the high-temperature gate oxide reliability of self-developed SiC MOSFETs is compared with that of the foreign SiC MOSFETs of the same specification by positive and negative high-temperature gate bias（HTGB） tests. The negative HTGB test results show that the deviation of threshold voltage of self-developed SiC MOSFETs is almost equal to that of the foreign SiC MOSFETs, and the maximum discrepancy between them is about 4.52%. However, the positive HTGB test results show that the deviation of threshold voltage of self-developed SiC MOSFETs is smaller than that of the foreign SiC MOSFETs, with a maximum discrepancy of 11%. The reason for the better performance of self-developed devices is that an appropriate amount of nitrogen is added to the SiC/SiO₂ interface, which can passivate interface defects and reduce the generation of fast interface states, so that the total interface state density is minimized.

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.

In this paper, the modulation scheme for a dual active bridge（DAB） bidirectional DC/DC converter is stud-ied. The main advantage of the DAB converter is that it has characteristics such as symmetrical structure, bidirectional power flow capability, wide soft switching range and flexible control capability. The simplest way to control this topology is to control the direction and magnitude of power transmission by adjusting the phase shift angle between the primary and secondary bridges. However, when the input or output voltage of the converter varies widely, a large amount of reac-tive power will be generated under light load conditions. Meanwhile, the zero voltage switching（ZVS） operation of part of switches cannot be maintained, which directly leads to a low conversion efficiency. Therefore, to improve the efficiency of the DAB converter, a hybrid phase shift modulation（PSM） scheme is proposed, which can reduce the inductor root-mean-square（RMS） current and extend the soft switching range on the basis of keeping the control simple, thereby improving the performance of the converter. First, by making the controllable variables in the extended phase shift（EPS）, dual phase shift（DPS） and triple phase shift（TPS） modulation schemes equal, four different PSM schemes are obtained. Then, the steady-state characteristics of these modulation schemes are compared and analyzed, including their transmission power capacity, inductor current level and soft switching performance. On this basis, a hybrid PSM scheme is formulated. Fi-nally, an experimental platform was built to verify the effectiveness and correctness of the proposed modulation scheme.

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.

Aimed at the problems of fast loss and high capacity configuration of battery energy storage equipment in microgrid, an optimal configuration model of battery energy storage capacity of microgrid considering life loss is established in this paper. In addition, a cost calculation method for the battery energy storage life loss based on fixed daily cycle times is also proposed. This method combines the piecewise linearization idea and the scenario analysis method, and it can effectively extend the lifetime by optimizing the discharging depth and daily cycle times of battery energy storage. Moreover, considering the uncertainties in wind power output and load power, a two-stage robust optimization model is introduced, which is further solved by the column-and-constraint generation algorithm. Finally, the effectiveness of the novel model under different uncertainties and different unit prices of battery energy storage is verified by numerical examples.

The development of power modules towards high temperature, high power and high density raises higher requirements for the packaging structures of modules. Compared with the traditional wire-bond structure, the double-sided structure has attracted more and more attention owing to its characteristics such as high heat dissipation capacity and low parasitic inductance. However, the mismatch of thermal expansion coefficient between materials used in the double-sided structure makes the structure suffer tremendous thermo-mechanical stress, thus reducing the reliability of power module. Therefore, to develop double-sided bi-directional modules with low thermo-mechanical stress, the effects of chip layouts on the heat dissipation performance of modules and the parasitic inductance were analyzed by simulations at first. Then, a flexible buffering spacer with low Young's modulus is proposed accordingly. The feasibility of reducing the thermo-mechanical stress and improving the reliability of the module was preliminarily proved by simulation and experimental results.

The phenomenon of dynamic avalanche occurring during the IGBT turn-off process is one of the important reasons for its failure. To study the dynamic avalanche failure mechanism of IGBT, the Silvaco software was used to simulate and analyze this mechanism. Through the simulation and analysis of the breakdown mechanism, current density distribution and temperature distribution of dynamic avalanche, it is concluded that dynamic avalanche can generate moving current filaments and dead filaments which are either moving slowly or fixed. However, the failure of the device is caused by the dead filaments formed by dynamic avalanche. The dead filaments will lead to a sharp increase of local temperature in the IGBT, and the IGBT will eventually fail because the local temperature is too high to burn the device. On this basis, the causes of dead filaments are analyzed, and specific measures to prevent the dynamic avalanche failure of IGBT are also put forward.

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.

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.

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.

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.

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.

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.

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.

The conventional Buck-type power factor correction（PFC） converter has a disadvantage of high harmonic current, which limits its applications. A Buck-type single-switch integrated PFC converter is proposed and analyzed. This converter is composed of a Buck-type PFC converter and a Buck-Boost PFC converter, and they are integrated by only one active switch, thereby simplifying the control. Under the constant on time（COT） control, the dead zone of input current in the Buck-type PFC converter is eliminated. With advantages of Buck and Buck-Boost converters, this converter can achieve a high power factor and a high efficiency in universal-input applications. The circuit structure, working principle, steady-state characteristics and design considerations of the proposed converter are introduced and analyzed. Finally, a 56 W prototype was built, and experimental results verified the analysis results.

The circuit topology of a novel three-phase quasi-Z source AC-AC converter is proposed, and the basic working principle and structure of the circuit are analyzed. In addition, the relationship between input voltage and output voltage is also derived. This circuit topology is controlled by a pulse width modulation method, which can achieve the effect of changing the output voltage. MATLAB/Simulink is used to build a simulation model, and the simulation results are analyzed. Finally, an experimental circuit was built on the basis of the simulation model, and experimental results verified the feasibility of the proposed circuit topology and the correctness of circuit analysis.

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.

As the number of charge and discharge cycles of a lithium-ion battery increases, its state-of-health（SOH） will degrade to some degree accordingly. Aimed at this problem, a method for estimating the SOH of lithium-ion battery based on an improved multi-objective Cuckoo search（IMOCS）-BP neural network is designed, which adaptively changes the update probability and search step size of the Cuckoo search（CS） algorithm while avoiding the algorithm from falling into the local optimum, thereby solving the problems of slow convergence speed and low solution accuracy in the CS algorithm. The IMOCS algorithm is combined with BP neural network to conduct a global search in the node space, reduce the influence of initial values of weight and threshold on BP neural network, and realize the parameter optimization. Through Matlab simulations, it is verified that the SOH estimation algorithm based on IMOCS-BP neural network has a low error and a strong performance, thus realizing an accurate SOH prediction of lithium-ion battery.

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.

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.

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.

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.