High Power Electronics Research Articles

Analyzing the Effectiveness of Single Active Bridge DC-DC Converter under Transient and Load Variation

Over the last couple of years, the use of DC-DC converters is being widely used in several applications. The need for a reliable and robust converter for application is very important because of the social impact that it can have on the losses in high-power electronics applications and output efficiency. This paper gives a comprehensive review of single-active-bridge (SAB) converters. The detail modelling and performance evaluation of SAB topology with PI controllers are also presented in this article. The performance of SAB topology is analyzed under transient and load variations. Due to the nonlinear behavior of SAB topology, the traditional PI controllers have not minimized transient disturbances. The major problem with the usage of conventional controllers and traditional configurations for the SAB converter are the ripples, which are generated in output voltage and current. These ripples severely affect the performance of the SAB converter. An effort has been made to analyze and contribute to the importance of a robust controller to mitigate the converter dynamics. This article also gives an in-depth dynamic analysis of SAB converters, which were controlled using PI controllers and failed to perform satisfactorily under non-linearities.

Properties of ScAlMgO4 as Substrate for Nitride Semiconductors

SCAM has been expected to be a suitable substrate for GaN blue-light-emitting-diodes (LEDs) and high-power high electron mobility transistors (HEMTs) because of its lower lattice mismatch to GaN than that of the widely used sapphire. Considering both potential device applications, the crystal lattice and optical properties of SCAM substrates were investigated on selected high quality samples. As lattice parameters, the thermal expansion coefficient as well as the lattice constant were extrapolated from room temperature to 2000 °C by using a high temperature X-ray diffraction (XRD) system with the heating unit on a sample stage. The thermal conductance, which is also important for growing bulk SCAM crystals and the operation of devices on the SCAM substrate, was measured. Raman scattering measurements were carried out to better understand crystal lattice characteristics. It was clearly confirmed that prepared SCAM crystals were of high quality. Similar to sapphire, SCAM has the high transparency over the wide wavelength range from ultraviolet to mid-infrared. The refractive index, important for the design of any optical devices, was measured. From these results, it can be said that SCAM is a suitable substrate for nitride devices, especially LEDs and solar cells.

Enhanced thermal performance from liquid metal in copper/graphite filled elastomer

With the increasing integration level of modern electronics, thermal management becomes an urgent issue for guaranteeing the work efficiency and lifespan of electronics. On the basis of intrinsic high thermal conductivity nature, highly ordered graphite and copper stripes are densely aligned in the silicone gel pads in vertical (VCuGr) and oblique (@15°CuGr) directions to couple the high thermal conductivity and mechanical softness. The wetting nature of liquid metal (LM) on the chemically treated Cu surface is utilized to form a LM layer on the two surfaces of thermal pads. The obtained LM-pad TIMs possessed ultrahigh through-plane thermal conductivity (VCuGr: 71.4 W/(m K), @15°CuGr: 62.5 W/(m K)) under the normal packaging pressure. The thermal resistance decreased from 0.69 cm2 K/W to 0.25 cm2 K/W with the surface modification with LM. Theoretical simulation and practical thermal dissipation test results further demonstrate the excellent thermal management capability of these composites in high-power electronics.

Micro pin fins with topologically optimized configurations enhance flow boiling heat transfer in manifold microchannel heat sinks

The manifold microchannel heat sink (MMCHS) is a promising design for heat dissipation in high power electronics. However, the configuration of the microchannels in MMCHS is generally rectangular straight channels, which would limit the further improvement of heat transfer performance. Topology optimization has been proven to be an effective approach to design single-phase heat sinks, which only depends on the iterative algorithm. In this work, we first design three single-phase heat transfer structures (2D) by a density-based topology optimization method, and then systematically investigate the two-phase flow and heat transfer characteristics of MMCHS (3D configurations that are converted based on 2D designs). The thermohydraulic performances of three topologically optimized MMCHS obtained by varying the constrained pressure drop (named TOC-I, TOC-II and TOC-III for simplicity) and the baseline rectangular MMCHS are compared. The results show that the flow paths in the TOCs exhibit root-like configurations and the number of pin fins increases with the increase in constrained pressure drop (i.e., TOC-III has the most complex flow path). Due to the differences in velocity distribution and channel width among various configurations, churn flow and confined bubbly flow are observed in BC and TOCs, respectively. Compared to BC, the maximum average heat transfer coefficients of TOC-I, TOC-II and TOC-III are increased by 40%, 62% and 87%, respectively. The TOC-I has the lowest pressure drop due to the minimum number of pin fins and the relatively large channel width. The maximum values of Figure of Merit (FOM), a parameter usually employed to analyze the comprehensive performance of MMCHS, for TOC-I, TOC-II and TOC-III are 1.72, 1.77 and 1.8 at Qb= 1000 W/cm2, respectively. Moreover, TOC-I and TOC-III have the best comprehensive performance at 300 W/cm2≤ Qb ≤ 800 W/cm2 and 900 W/cm2 ≤ Qb ≤ 1000 W/cm2, respectively.

Flow and thermal analysis of the transition scheme between micro-channel and micro-jet cooling solution

With the rapid development of high-power electronics, the thermal management of these emerging devices is challenging due to the danger of ultra high heat flux and uneven temperature distribution. In the past decade, certain heat-dissipation technologies based on micro-channel and micro-jet cooling have utilized geometrically identical heat sinks; however, their corresponding flow and thermal characteristics are quite dissimilar including temperature gradients, pressure drop and heat transfer coefficients only due to the different ways the coolant enters the test module. To characterize the enhanced heat-transfer mechanism driving this dissimilitude, a novel transition scheme between them was designed, featuring a gap on the channel fins top to provide extra flow area, and investigated experimentally and numerically. In this paper, by controlling the volumetric flow ratio of the channel and jet, including 1:1, 1:3 and 3:1, the flow and thermal characteristics of the transition scheme could be interchangeably transformed between the two schemes. Moreover, the effect of the vortex generated by the jet impingement was investigated, which was verified as an important factor causing the distinctions. The new results revealed that the vortex in the stagnation zone enhanced the local heat-transfer coefficient corresponding to the channel bottom and promoted temperature uniformity; however, it caused the thermal boundary layer on the fins top to thicken. Although the vortex was favorable for the forced convection in the acceleration zone, the vortex intersections formed by adjacent jets introduced a greater pressure-drop penalty and flow blockage, which were not conducive to enhancing the heat transfer. Subsequently, the Taguchi optimization method was employed to analyze effects of the aspect ratio, gap and jet diameters on flow and thermal characteristics. Accordingly, the optimal parameter combinations and contribution ratios were obtained. This paper systematically shows how the micro-channel is transformed into a micro-jet cooling by adjusting the flow ratio and reports an intrinsic connection between them to promote a better understanding of cooling technologies used in electronic devices.

Jet microchannel with sawtooth wall for efficient cooling of high-power electronics

Dissipation of ultrahigh heat fluxes is critical to modern electronic chips as they become ever more compact and powerful. Here, by employing a systematic numerical study, we present a highly-efficient embedded single-phase water-cooled silicon heat sink featuring a manifold with interdigitated inlets and outlets and jet microchannels with sawtooth side walls. Compared to straight channels, the sawtooth walls reduce blockage of the jet inflow and therefore the pressure drop. Meanwhile, the sawtooth profile helps suppress the large recirculation region around the jet inlet and generate small vortices near the channel walls which facilitate fluid mixing and enhance heat exchange. This simultaneous enhancement of heat transfer and reduction of pressure drop leads to some of the highest cooling efficiencies. In particular, for a 3 × 3 mm2 chip at a temperature rise of 60 K, dissipation of heat fluxes over 2100 W cm−2 is possible with a pumping power of about 1.3 W cm−2. For heat fluxes around 1000 W cm−2, the coefficient of performance can reach beyond 20,000. In addition to the high performance, our heat sink can in principle be manufactured via standard silicon microfabrication technology.

Conceptual design of hybrid target for molybdenum-99 production based on heavywater

Molybdenum-99 (99Mo) is used for preparing Technetium-99 m (99mTc), which is the most widely used isotope in nuclear medicine. In this work, a study for 99Mo production based on a high-power electron accelerator has been performed as an alternative approach to produce 99mTc. In this study, Monte Carlo MCNPX2.6 code has been used to examine a novel idea of simultaneous hybrid production of 99Mo via both photoneutron and neutron capture reactions using an electron accelerator in heavy water tank. It is expected that this conceptual design including an arrangement of metallic plates of 100Mo and 98Mo produces total activity of 97.5 Ci at the end of 20-h continuous e-beam irradiation (30 MeV, 10 mA).

Size- and temperature-dependent thermal transport across a Cu−diamond interface: Non-equilibrium molecular dynamics simulations

Cu−diamond composites (CDCs) have greatly promising applications in thermal management for high-power electronics because of their outstanding thermophysical properties. Nonetheless, many fundamental mechanisms of interfacial thermal transport for CDCs remain poorly understood at present. Here we focus on investigating the size- and temperature-dependent thermal transport across a Cu−diamond interface using non-equilibrium molecular dynamics simulations. Results show that interfacial thermal conductance (ITC) is proportional to both the system size and ambient temperature. Especially, our predicted room-temperature ITC of 41.12 MW⋅m−2⋅K−1 at an infinitely long system is close to that of the experiments after an interface optimization. Additionally, the ITC at the system with a length of 323.2 Å is increased by over 2.5 times in the temperature range of 100−500 K, up to 36.39 MW⋅ m−2⋅ K−1. Detailed analyses of interfacial disorder and its concomitant effects, related to system size and temperature, are implemented for helping understand the significant improvement of ITC. The underlying mechanism is further uncovered by the phonon density of states as well as the spectral overlap factor at interfacial Cu and diamond. This study provides an important insight into the understanding of interfacial thermal transport in CDCs and a guideline for optimizing the design of CDCs in experiments.

Dielectric Properties Characterization and Evaluation of Commercial Silicone Gels for High-Voltage High-Power Power Electronics Module Packaging

The lack of a method to select the suitable silicone gel from various commercial silicone gels for encapsulation in next-generation high-voltage high-power electronics modules has become a challenge. In this article, a comprehensive evaluation approach of three commercial silicone gels is carried out via multiple characterizations in terms of surface microstructure, main internal components, dc volume resistivity, dielectric properties, breakdown strengths under dc and ac voltages, thermogravimetric properties, and glass transition temperature. This study shows that the analytical tools set proposed in this article can be used as an efficient guideline for comparing different types of silicone gels and selecting suitable silicone gels for different applications. For example, the drawn radar chart intuitively shows that Gel 3 has the best all-around performance in both electrical and thermal properties, and Gel 1 has the highest thermal stability. The surface microstructure and internal functional groups of the three silicone gels are almost the same. The glass transition temperatures of the three silicone gels were also measured, and the thermo gravimetric analysis (TGA) showed that the three silicone gels had high thermal stability. According to the test results and analytical tools established in this article, we were able to identify silicone gels suitable for different applications. In this regard, Gel 3 is suitable for applications with higher voltage levels (10–20 kV), while Gel 1 and Gel 2 are suitable for lower voltage levels.

Al Coordination and Ga Interstitial Stability in a β-(Al0.2Ga0.8)2O3 Thin Film.

Alloying Al2O3 with Ga2O3 to form β-(AlxGa1-x)2O3 opens the door to a large number of new possibilities for the fabrication of devices with tunable properties in many high-performance applications such as optoelectronics, sensing systems, and high-power electronics. Often, the properties of these devices are impacted by defects induced during the growth process. In this work, we uncover the crystal structure of a β-(Al0.2Ga0.8)2O3/β-Ga2O3 interface grown by molecular beam epitaxy. In particular, we determine Al coordination and the stability of Al and Ga interstitials and their effect on the electronic structure of the material by means of scanning transmission electron microscopy combined with density functional theory. Al atoms can substitutionally occupy both octahedral and tetrahedral sites. The atomic structure of the β-(Al0.2Ga0.8)2O3/β-Ga2O3 interface additionally shows Al and Ga interstitials located between neighboring tetrahedrally coordinated cation sites, whose stability will depend on the number of surrounding Al atoms. The presence of Al atoms near interstitials leads to structural distortions in the lattice and creates interstitial-divacancy complexes that will eventually form deep-level states below the conduction band (Ec) at Ec -1.25 eV, Ec -1.68 eV, Ec -1.78 eV, Ec -1.83 eV, and Ec -1.86 eV for a Ga interstitial surrounded by zero, one, two, three, and four Al atoms, respectively. These findings bring new insight toward the fabrication of tunable β-(AlxGa1-x)2O3 heterostructure-based devices with controlled electronic properties.

Drop impact phenomena and spray cooling on hot nanotextured surfaces of various architectures and dynamic wettability

Spray cooling has been used to quench metal slabs during casting, cool nuclear reactors, suppress accidental fires, and remove heat from high-power density electronics. In particular, the miniaturization of electronic devices inevitably results in an increased power density or heat flux on the microelectronics surfaces and poses a threat of a thermal shutdown of such devices when cooling is insufficient. Surface nanotexturing effectively augments additional liquid-to-substrate surface area, thereby increasing cooling capability, as well as an effective heat transfer coefficient. In spray cooling, surface dynamic wettability also affects drop impact dynamics and subsequent coolant evaporation on a hot surface. Herein, we introduced various nanotextured surfaces and affected dynamic wettability using the so-called thorny-devil nanofibers, nickel nanocones, Teflon and titania nanoparticles, and zinc nanowires. The effect of these different nanoscale architectures on drop impact phenomena and subsequent evaporative cooling was investigated. These nanotextured surfaces were fabricated using various deposition methods, including electrospinning, electroplating, supersonic spraying, aerosol deposition, and chemical bath deposition. We found that the surface with greater dynamic wettability related to the hydrodynamic focusing considerably improved the heat removal capability by furthering the Leidenfrost limit and facilitating drop spreading. In particular, the thorny-devil nanofiber surface yielded the highest heat flux at all ranges of the Reynolds and Weber numbers. Spray cooling on a model electronic kit also confirmed that the thorny-devil nanofibers were most effective in cooling the surface of the model kit during multiple cycles of water spraying.

Electrically stimulated optical spectroscopy of interface defects in wide-bandgap field-effect transistors

Wide-bandgap semiconductors such as silicon carbide, gallium nitride, and diamond are inherently suitable for high power electronics for example in renewable energy applications and electric vehicles. Despite the high interest, the theoretical limit regarding device performance has not yet been reached for these materials. This is often due to charge trapping in defects at the semiconductor-insulator interface. Here we report a one-to-one correlation between electrically stimulated photon emission and the threshold voltage shift obtained from a fully processed commercial 4H-SiC metal-oxide-semiconductor field-effect power transistor. Based on this observation, we demonstrate that the emission spectrum contains valuable information on the energetic position of the charge transition levels of the responsible interface defects. We etch back the transistor from the reverse side in order to obtain optical access to the interface and record the emitted light. Our method opens up point defect characterization in fully processed transistors after device passivation and processing. This will lead to better understanding and improved processes and techniques, which will ultimately push the performance of these devices closer to the theoretical limit.

A compact jet array impingement cooling system driven by integrated piezoelectric micropump

Thermal management has become a bottleneck for high-power electronics due to high heat flux. Liquid cooling has been developed as an effective way to dissipate the heat of electronic devices. However, the existing liquid cooling systems are mostly too bulky to be applied in the miniaturized devices with very limited installation space. In this work, by integrating jet impingement cooling with piezoelectric micropump, we proposed a compact liquid cooling system that simultaneously enables high heat removal capability and tiny volume. The functions of jet array impingement cooling and coolant pumping are realized in a single component with an external dimension of 40 mm × 40 mm × 10 mm. The thermal performance of the system was investigated by experiments. The results show that the convective heat transfer coefficient achieves 20572 W·m−2·K−1 at a heat flux of 91.5 W/cm2. And the electric power consumption of the system is only 0.023 W under a heat load of 200 W (50 W/cm2). Simulations, which agree well with experiments, were further employed to study the transient heat transport process. This novel jet impingement cooling system driven by integrated piezoelectric micropump is compact and efficient, which provides a promising solution for the thermal management of high-power, miniaturized electronic devices.

A study on thermal and hydraulic performance of ultra-thin heat pipe with hybrid mesh-groove wick

High power electronics require ultra-thin heat pipe (UTHP) with more efficient heat transfer capabilities to meet thermal management challenges. And the design of the wick structure is crucial to the heat transfer performance improvement of the UTHP. At the present work, a thermo-hydraulic model is proposed for UTHP with composite mesh-grooved wick structure and the potential applications of the hybrid wick with non-full coverage by mesh is analyzed. According to the different mesh coverage areas, the wick structures are classified into three types, including evaporator covered, evaporator-adiabatic section covered, and full covered. The results show the flow characteristics and thermal performance of UTHPs is closely related to mesh coverage area and vapor core thickness. The reduction in mesh coverage area causes an expansion in vapor space, the vapor velocity and the vapor pressure drop both decreases, the mass flow rises with the higher vapor-liquid circulation efficiency. The liquid pressure drop is positively related to working fluid mass flow. Moreover, a theoretical model to predict the heat transfer limit of the heat pipe with composite wick was established and verified by experimental results with a maximum error of 3.63%.

Numerical Study of Heat and Mass Transfer in the Original Structure and Homogeneous Substitution Model for Three Dimensional Porous Metal Foam

In many applications, such as the miniaturization and cooling of high-power electronics in aerospace, a new thermal management solution is needed, and metal foam radiators may be a valuable solution. In this work, X-ray scanning was applied to obtain the original structure of the metal foam. The real structure calculation model of the metal foam was obtained through a series of modeling, and high-precision numerical simulation was built to study heat and mass transfer in the original structure and homogeneous substitution model for three-dimensional porous metal foam. The distribution of velocity, pressure and temperature field is investigated. The results show that the heat transfer characteristics increase and flow resistance decreases with an increase in the Reynolds number. The heat transfer performance and flow resistance increase with the decrease of porosity. The porous media homogenization model can be consistent with the original real calculation results of metal foam by using appropriate values of resistance coefficient and porosity. The variation of resistance coefficient and porosity with the working condition in the porous homogenization model is identified.

Cu3Sn joint based on transient liquid phase bonding of Cu@Cu6Sn5 core–shell particles

With the development of high-integration and high-power electronics, the lack of matching chip connecting materials that can withstand high temperatures has been a challenge. In this manuscript, a Cu@Cu6Sn5 core–shell bimetallic particles (approx. 1 μm in diameter) are successfully prepared and introduced as a new solder material for the packaging of power devices to obtain a Cu3Sn all-IMC solder joint. The joint consisted mainly of equiaxed Cu3Sn grains, and a small portion of columnar Cu3Sn grains. In columnar-type growth, Sn is the dominant diffusing species, which comes from the depletion of Sn in Cu6Sn5. The depleted Cu6Sn5 is transformed into columnar Cu3Sn. In equiaxed-type growth, Cu is the dominant diffusing species. Cu reacts with Cu6Sn5 to grow a Cu3Sn layer. This conclusion was confirmed by the orientation relationship. The equiaxed Cu3Sn grain nucleates at the Cu/Cu3Sn interface have an orientation relationship with the Cu substrate. Columnar Cu3Sn grains at the Cu6Sn5/Cu3Sn interface have an orientation relationship with Cu6Sn5.

Intrinsic electron mobility and lattice thermal conductivity of β-Si3N4 from first-principles

Silicon nitride based materials have emerged as the promising candidates for high-power electronics and next-generation gate dielectrics. Herein, the crucial characteristics of electron mobility and lattice thermal conductivity of β-Si3N4 are investigated from first-principles. The predicted electron mobility and averaged lattice thermal conductivity is 228.4 cm2/Vs and 325.06 W/m·K at 300 K, which demonstrates a good agreement with literature data. The electron mobility exhibits strong temperature-dependence at a low carrier concentration where the polar-optical phonon scattering dominates. For the heavy doping case, the ionized impurity scattering becomes dominant. A well-trained momentum tensor potential (MTP) with an accuracy comparable to density functional theory shows advantages in predicting thermal transport properties over a large-scale system containing thousands of atoms. The relaxation lifetimes for heat-carrying acoustic phonons are over tens of picoseconds which can explain the high thermal conductivity of β-Si3N4, but the nanoscale grain size crucially limits the thermal transport properties.

Enhancement of heat transfer on micro- and macro- structural surfaces in close-loop R410A flashing spray cooling system for heat dissipation of high-power electronics

High effective spray cooling system is urgently required for the thermal management of high-power electronics in small areas. These electronic devices need to be cooled at functional operating temperature, and it is particularly lacking in current spray cooling studies. For this purpose, a hybrid structural surface that integrates micro- roughness and macro- structure is investigated in close-loop R410A flash spray cooling system. The heat transfer performance is tremendously enhanced as the increase of roughness on flat surface until it reaches the value of 3.5 μm. Meanwhile, a novel tetrahedral fin surface is proposed and compared with modified surfaces including flat, wavy, square, and pyramid fins. It performs high heat flux of 411.2 W cm−2, a 101.5 % enhancement over the flat surface. What’s more, the hybrid structure with 3.5 μm roughness can further improve the maximum critical heat flux and heat transfer coefficient by up to 424 W cm−2 and 814 kW K−1 m−2, respectively, while keeping the surface temperature below 50 °C. Besides, correlations and energy efficiency are studied to explore the performance of structural surfaces. These findings in a wider range of roughness and macrostructure will provide insights to guide the surface modification in flash spray cooling.

Effect of AlN and AlGaN Interlayers on AlScN/GaN Heterostructures Grown by Metal–Organic Chemical Vapor Deposition

AlScN/GaN heterostructures with their high sheet carrier density (ns) in the two-dimensional electron gas (2DEG) have a high potential for high-frequency and high-power electronics. The abruptness of the heterointerface plays a key role in the 2DEG confinement, and the presence of interlayers (AlN, AlGaN) affects ns and electron mobility (μ) and determines the sheet resistance (Rsh). AlScN/GaN heterostructures suitable for high-electron mobility transistors (HEMT) with and without a nominal AlN interlayer were grown by metal–organic chemical vapor deposition (MOCVD) and characterized electrically and structurally to gain a systematic insight into the unintentional formation and control of graded AlGaN interlayers by diffusion of atoms at the heterointerface. The AlN interlayer increases ns from 2.52 × 1013 cm–2 to 3.25 × 1013 cm–2 and, as calculated by one-dimensional Schrödinger–Poisson simulations, improves the 2DEG confinement. The barrier growth temperature was varied from 900 °C to 1200 °C to investigate the effect of the thermal budget on diffusion. Growth at 900 °C reduces the thickness of the graded AlGaN interlayer and improves the 2DEG confinement, leading to Rsh of 211 Ω/sq, ns of 2.98 × 1013 cm–2, and μ of 998 cm2/(Vs).

Effects of fast and thermal neutron irradiation on Ga-polar and N-polar GaN diodes

Studies of the radiation tolerance and electrical behavior of gallium nitride (GaN) based devices are important for the next generation of high-power and high-voltage electronics that may be subjected to harsh environments such as nuclear reactor and fusion facilities, particle accelerators, and post-denotation environments. In this work, we study the behavior of Ga-polar and N-polar GaN Schottky diodes before and after exposure to fast and thermal + fast neutrons. Temperature-dependent current–voltage (I–V) and circular transmission line method (CTLM) measurements were used to study the electrical characteristics. A strong reduction in reverse leakage current and an increase in differential resistance in forward bias were observed after neutron irradiation. Thermionic emission (TE), Frenkel–Poole (FP) emission, and Fowler–Nordheim (FN) tunneling models were used to explain the forward and reverse I–V characteristics pre- and post-irradiation. The study confirms that Ga-polar and N-polar GaN Schottky diodes exhibit different electrical responses to fast and thermal neutron irradiations. The reverse bias characteristics of N-polar diodes are less affected after the fast neutron irradiation compared to Ga-polar diodes, while in the forward bias region, the electrical behavior after fast and thermal neutron irradiations is similar in Ga-polar and N-polar diodes. The results indicate that the role of orientation should be considered in the design of GaN-based radiation-tolerant electronics.