Medium frequency transformers (MFTs) have emerged as a key enabling technology in medium voltage (MV) power electronics systems, supporting applications such as solid state transformers (SST), power electronic traction transformers (PETT), medium voltage direct current (MVDC) power distribution. This review provides a comprehensive overview of high-power MFTs, focusing on their state of the art, applications, technical challenges, and current research solutions. The paper surveys key considerations including core materials, core shapes, winding technologies, winding structures, insulation considerations, dielectric materials, electric field stress grading materials, and cooling technologies. Specifically targeted at modern MV power electronics, this paper discusses the challenges and solutions in MFT insulation design and test. The topics include complex voltage stress and electric field distribution, partial discharge under medium frequency high dv/dt waveforms, and insulation test standards which are critical to enhance the performance, reliability, and development of high-power MFTs in MV applications.

Comprehensive review of the requirements of AC transmission grid codes and the role of FACTS devices in integrating renewable power plants

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Enhanced high-frequency stability and power quality in photovoltaic inverters via an optimized least mean square adaptive damping controller

A comprehensive review on the development of thermal network models for power semiconductor modules is conducted in this paper. Driven by the wide-spread applications of the power electronics in numerous applications, the development of power modules is revolutionised with higher requirements in power density, switching frequency, operational temperature and reliability, which in turn induces considerable challenges on the thermal management and modelling. As one of the most promising thermal modelling technologies, the thermal networks describe superior performance in long-term profile-based temperature estimation, excellentmulti-physics analysis capability, good hardware compatibility as well as reasonable balance between computational load and accuracy. After revisiting the theoretical basis of the thermal networks, this paper focuses on the evolvement of the thermal networks in terms of format, modelling methodology, thermal boundary condition treatment and verification methods. The state-of-the-art topologies of the representative thermal networks are compared in detail, with a chronology of the thermal networks development being summarised. In addition, the typical application scenarios of the thermal networks and their advantages compared with other technologies are summarised, accompanied by a number of engineering implementation examples. What's more, the future development opportunities and challenges of the thermal networks are discussed, making this paper an all-around reference for researchers and engineers in the power module thermal modelling.

Single-crystal diamond (SCD) has excellent mechanical, thermal and physical properties that rank among the highest of all substances. It has widespread applications in diverse fields such as cutting tools, optical windows, spintronics and power electronics. However, SCD is difficult to machine via mechanical processes due to its extremely high hardness. In recent years, laser processing has attracted attention since it enables non-contact efficient machining regardless of material hardness. In this study, femtosecond pulsed laser, which is considered to be capable of high-precision machining with low thermal effect, was used for micromachining of SCD, and the effects of laser irradiation parameters on the surface texture, roughness and profile geometry, as well as subsurface material structural changes, were investigated. It was found that even under femtosecond pulses thermal effect existed and that using a higher laser scanning speed was helpful to eliminate thermal cracks formation. The laser machined surfaces were covered with nanoscale periodical surface ripples with a surface roughness in the level of 0.10 μm Ra. Raman analysis of the SCD surface after laser machining showed that amorphous carbon and nanocrystalline graphite existed as a composite material. The graphitization depended on the laser fluence and hatching width and was particularly pronounced at low fluence and small hatching width. It was also demonstrated that acid cleaning could remove most of the graphite layer and reduce surface roughness. As samples, a few three-dimensional micro cavities with flat bottoms and sharp corners were created. This study demonstrates that femtosecond pulsed laser is a suitable method for machining micro three-dimensional shapes in SCD while it is important to optimize the laser machining conditions to avoid thermal damage and improve the surface quality. • Femtosecond pulsed laser micromachining characteristics of single-crystal diamond were investigated. • By increasing laser scanning speed, crack generation due to thermal effect was suppressed. • Flat surfaces with surface roughness of 0.10 μm Ra were successfully obtained under suitable conditions. • A laser-machined surface was covered with a layer of amorphous carbon and nanocrystalline graphite. • The graphite layer thickness and surface roughness depended on the overlap of the laser beam wings.

SiC/Si heterostructure exhibits significant potential for applications in advanced electronics and optoelectronic devices, owing to its exceptional thermal and electrical properties. A direct bonding method utilizing plasma to activate surface in a vacuum environment has been developed for SiC/Si heterogeneous integration. The necessity of a vacuum environment reduces the efficiency of plasma-activated bonding (PAB), thus limiting its application in heterogeneous integration. In this study, we investigated the high-efficiency and low-temperature direct bonding of SiC/Si via atmospheric inductively coupled plasma (ICP). The mechanism of ICP activation and direct bonding of SiC/Si pairs was investigated. After less than 5 s of high-energy Ar ICP irradiation, the intrinsic bonds of surface were disrupted, leading to surface activation. The activated surface readily adsorbed hydroxyl groups from a humid environment, resulting in a superhydrophilic (contact angle <3°) surface. This facilitated a dehydration condensation reaction at low temperature (≤250 °C), enabling direct bonding. The original surface conditions were maintained during the activation process and met the requirements for direct bonding. Additionally, ICP irradiation effectively eliminated surface contaminants and enhanced the quality of direct bonding. Using optimized process parameters, the bonding efficiency is more than 99% and the maximum bonding strength is more than 6 MPa. The results of transmission electron microscopy (TEM) demonstrated that the bonding interface was dense and low-defect. Thus, ICP irradiation can efficiently activate the surface and improve the bonding quality without a vacuum environment. The direct bonding of SiC/Si heterostructures through ICP irradiation holds significant potential for the fabrication of high-performance power electronics and micro/nanofluidic devices.

This paper presents a novel Work Function Modulated Fin-channel Schottky Barrier Diode (WFM-Fin-SBD) with optimized electrical characteristics. The architecture incorporates Ti as the Schottky metal on the Fin top surface to reduce the turn-on voltage (Von) and mitigate forward conduction loss. Simultaneously, Ni is selectively deposited on the Fin sidewalls and trenches bottom. This configuration ensures effective carrier depletion within the fin channel under both zero-bias and reverse-bias conditions, thereby suppressing reverse leakage current and enhancing the breakdown voltage. As a result, the WFM-Fin-SBD achieves superior performance metrics, including a low Von of 0.35 V, a specific on-resistance (Ron,sp) of 6.25 mΩ•cm², and a current density of 598 A/cm² at 6 V. Under reverse bias, the enhanced depletion effect at the Ni/Ga2O3 interface effectively pinches off the conductive channel, which suppresses the leakage current and enables a breakdown voltage of -241 V, which is approximately 5.5 times that of conventional Ti-SBDs. Furthermore, frequency-dependent conductance measurements reveal that the interface trap density (Dit) of the WFM-Fin-SBD is situated between those of the Ti-SBD and the Ni-SBD. The slightly elevated Dit compared to the Ni-SBD is mainly attributed to the presence of Ti at the Fin top. Meanwhile, TCAD simulations elucidate the underlying physical mechanisms. The proposed WFM-Fin-SBD demonstrates superior performance, positioning it as a promising candidate for high-efficiency power electronics.

To accommodate the advancement of next-generation power electronics toward high-level integration and miniaturization, it is imperative to develop polymer dielectrics that maintain superior energy storage performance under thermal extremes. Traditional aromatic polymers (e.g., polyetherimide, PEI) suffer from severe charge delocalization at elevated temperatures due to inherent charge-transfer complexes (CTCs) within their conjugated structures, resulting in sharply increased leakage current and deteriorated energy storage performance. In this work, we propose a molecular conformation engineering strategy that incorporates sterically hindered, twisted fluorine-substituted fluorene moieties into the PEI backbone, successfully decoupling the trade-off between thermal stability and high-efficiency energy storage. Integrating experimental characterization with theoretical calculations reveals the underlying mechanism by which molecular conformation engineering regulates macroscopic electrical performance from a multiscale perspective: the inherent rigidity of the fluorene skeleton provides a robust molecular scaffold that ensures thermomechanical reliability at elevated temperatures. Meanwhile, the non-coplanar, twisted geometry disrupts long-range π-π stacking, thereby hindering intermolecular charge transfer and synergizing with strategic fluorination to reduce leakage current through reinforced electron localization. Consequently, the optimized PEI-FFDA achieves a superior discharge energy density of 3.44 J cm-3 at 200 °C (10 Hz), approximately 2.5 times that of pristine PEI (1.38 J cm-3), with exceptional reliability over 105 cycles. This work establishes an effective molecular design paradigm for intrinsically robust high-temperature dielectrics.

The crystallinity of cubic silicon carbide (3C-SiC) epilayers is improved through the use of a novel wafer bonding and regrowth technique resulting in a reduction in planar defects. The process involves the epitaxial growth of a 3–6 µm thick 3C-SiC seed on silicon (Si), which is polished and bonded to a new handle wafer before the original substrate and defective interface region of the 3C-SiC epilayer are removed. Further epitaxial growth on this Bonded Switchback template results in higher quality 3C-SiC epilayers through the reduction in crystal mosaicity, stacking fault defects, and elimination of interface voids. The process could be applied to 3C-SiC grown on both on- and off-axis substrates, and the form of the new handle has no impact on the growth process, enabling this technology to be applied to sapphire or hexagonal 4H-SiC substrates. The use of such substrates would overcome the thermal budget limitations of Si substrates for 3C-SiC heteroepitaxy and ion implantation. Bonded Switchback can improve material quality for applications in power electronics, as well as see the heterogeneous integration of 3C-SiC into other device structures, potentially leading to a new range of hybrid 3C-SiC/Si devices without the high density of defects observed at the interface between these two materials.

Electrical isolation is critical to ensure safety and minimize electromagnetic interference (EMI), yet existing methods struggle to simultaneously transmit power and signals through a unified channel. Here we demonstrate a mechanically-isolated gate driver based on microwave-frequency surface acoustic wave (SAW) device on lithium niobate that achieves galvanic isolation of 2.75 kV with ultralow isolation capacitance (0.032 pF) over 1.25 mm mechanical propagation length, delivering 13.4 V open-circuit voltage and 44.4 mA short-circuit current. We demonstrate isolated gate driving for a gallium nitride (GaN) high-electron-mobility transistor, achieving a turn-on time of 108.8 ns and validate its operation in a buck converter. In addition, our SAW device operates over an ultrawide temperature range from 0.5 K (-272.6 °C) to 544 K (271 °C). The microwave-frequency SAW devices offer inherent EMI immunity and potential for heterogeneous integration on multiple semiconductor platforms, enabling compact, high-performance isolated power and signal transmission in advanced power electronics.

Abstract Understanding and controlling molecular radio-frequency (RF) plasma characteristics is essential for low-temperature plasma applications, including air-breathing propulsion, plasma-assisted energy systems, and materials processing. Pulsed operation is often preferred for enhanced control but involves inherently transient, multi-scale processes that require robust and coupled modeling approaches. This study develops a closely coupled unsteady electron-Boltzmann and plasma-kinetics framework to investigate the strongly non-equilibrium behavior of pulsed RF molecular plasmas. The framework consistently couples electron, vibrational, and chemical processes, incorporating additional electron loss mechanisms from flow and wall interactions. A detailed state-to-state model for hydrogen (H 2 ), resolving 15 vibrational levels, is implemented to construct a comprehensive plasma kinetics database. Results highlight the limitations of conventional steady-state solvers in capturing transient phenomena. Analysis of the electron energy distribution function (EEDF) under pulsed operation revealed strong non-Maxwellian features during both the pulse-on and pulse-off phases. Furthermore, phase-resolved power absorption demonstrated electron momentum-induced negative power absorption within the RF cycle, indicating complex heating dynamics. The H 2 vibrational distribution exhibited a highly non-Maxwellian profile that, while appearing two-temperature-like, does not conform to a simple two-temperature model due to the gradual population transition between the two energy regions and the stronger depletion of the H 2 (v = 14). Quantification of electron power losses indicated that in steady pulsed operation, ionization from vibrationally excited H 2 (v ≥ 1) and atomic H contributed as significantly to electron production as non-dissociative ionization from ground-state . Additionally, the power gain from super-elastic collisions was significant during the afterglow phase.

Scaling wide-band-gap semiconductors to the ultrathin limit offers a transformative pathway for power electronics, with gallium nitride (GaN) representing a cornerstone material in this class. However, the operational resilience and functional tunability of its two-dimensional form (g-GaN) remain underexplored. This work shifts the focus from idealized systems to the complex materials behavior under realistic conditions, investigating how the synergistic effects of point vacancy defects, strain, and external electric fields govern its electronic, magnetic, and sensing landscapes. We demonstrate that these factors are not merely perturbations but are fundamental to modulating the material response. Our first-principles calculations suggest that g-GaN maintains electronic stability under intense electric fields; notably, gallium vacancies are predicted to further extend the theoretical stability limit. While in-plane tension preserves the band gap evolution under an electric field, in-plane compression facilitates low-field metallization. Using nitrogen monoxide (NO) adsorption as a prototype, we find that the interaction is defect-modulated and potentially tunable by electric fields. Analysis of adsorption energetics and diffusion barriers suggests that the gallium vacancy may act as a thermodynamic trap for NO. Targeted hybrid-functional (HSE06) validation confirms the reliability of observed adsorption trends and theoretical metallization thresholds while revealing that precise electronic-exchange treatment is critical for capturing the magnetic ground state of nitrogen vacancies. By systematically examining the geometry, energetics, band structure, density of states, magnetic response, and charge transfer, this study clarifies the interplay between defects and external electric fields, providing insights into theoretical upper bounds for property tuning and semiconductor device engineering.

This research presents the design and implementation of a high-frequency transformer winding machine driven by a DC motor with a Buck-Boost converter to produce transformers accurately and efficiently. The system integrates a DC motor controlled by a Buck-Boost converter, which regulates the voltage to the motor, ensuring stable performance during the winding process. The winding machine design includes a stable mechanical platform, a V-belt mechanism for power transmission, and optocoupler sensors for real-time monitoring of winding turns. The Buck-Boost converter stabilizes voltage fluctuations, allowing smooth motor operation under various input conditions, thereby improving machine efficiency and reliability. Experimental tests on the rectifier, Buck-Boost converter, and DC motor demonstrate high efficiency and stable performance, with minimal deviation between calculated and experimental results. Test results show that this machine can perform precise winding across different duty cycles, with optimal speed control and stable operation. Compared to existing transformer winding machines using induction motors or stepper motors, this system offers better control, faster winding speeds, and greater adaptability to different production conditions. The developed machine significantly contributes to industries such as transformer manufacturing and power electronics, with increased productivity, reduced production costs, and improved transformer quality, especially in high-frequency applications such as renewable energy systems and electric vehicle charging.

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Surface driving stress-assisted rapid and large area silver porous sheet bonding for power electronics packaging

Abstract Space power electronics systems are regarded as a critical emerging application domain for wide-bandgap GaN power devices, where space radiation involving high-energy heavy ions (e.g., krypton, Kr) remains the biggest threat, leading to susceptible AlGaN/GaN heterojunctions. This work comprehensively investigates the stress and strain in heterojunction as well as latent tracks and defect characteristics in devices under Kr irradiation fluences up to 1 × 1010 ion/cm2. Through Raman spectroscopy, the enhanced compressive stress in AlGaN is revealed and quantitatively correlated with strain-induced polarization effects, thus providing insights into the dominant mechanism underlying heterojunction performance degradation, and establishing a numerical derivation involving stress, strain and polarization effects under irradiation. Meanwhile, the discontinuities and sub-nanometer diameter latent tracks revealed in the transmission electron microscopy experiments suggest that the device suffers minimal damage under heavy ion Kr irradiation, primarily due to the low electronic energy loss. Furthermore, the intensity ratio between yellow luminescence and near band edge signals in the 80 K low-temperature photoluminescence spectrum increases evidently with elevating irradiation fluence, indicating an increase in defects caused by irradiation. These findings offer critical insights into the behavior and performance of AlGaN/GaN power devices in satellite applications under Kr ion irradiation conditions.

Lifetime prognostics for power module bonding wires under non-constant stress conditions: A compound stochastic process degradation modeling approach

Abstract Gallium oxide is an emerging ultra-wide bandgap semiconductor with great potential for power electronics, yet large-sized crystal growth remains limited by defect and yield challenges. In this study, a coupled thermal-mechanical finite element model is established to analyze the thermal field optimization and stress distribution in gallium oxide crystals grown by the vertical Bridgman method. The results show that multi-heater configurations reduce power consumption and enhance radial uniformity in the furnace, but radiative shielding by the intermediate insulation layer increases axial gradients, confirming radiation as the dominant heat transfer mechanism. Stress analysis reveals that stress concentrates mainly in the shoulder and constant-diameter growth region due to crystal-crucible thermal expansion mismatch, and larger shoulder angles effectively alleviate stress in the process of single crystal growth. These findings highlight the importance of coordinated heater-insulation optimization and crucible design, providing guidance for high-quality and large-diameter gallium oxide crystal growth.

Fracture of Ag sintering bonding interface on active metal brazing substrates for power electronics