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The detection of rolling-element bearing fault can be accomplished by monitoring and interpreting a variety of signals, including the vibration, the acoustic noise, and the stator current. The existence of a bearing fault as well as its specific fault type can be readily determined by performing frequency spectral analysis on the monitored signals with various signal processing techniques. However, this traditional approach, despite being simple and intuitive, is not able to identify the severity of a bearing fault in a quantitative manner. Moreover, it is often times tedious and time-consuming to apply this approach to electric machines with different power ratings, as the bearing fault threshold values need to be manually calibrated for each motor running at every possible speed and carrying any possible load. This article, thus, proposes a quantitative approach to estimate the bearing fault severity based on the airgap displacement profile, which is reconstructed from the mutual inductance variation profile estimated from a quantitative electrical model that takes the stator current as input. In addition, the accuracy of the developed electrical model and the estimated bearing fault severity are validated by the simulation and experimental results, and the explicit airgap variation profile is reconstructed with the superposition of multiple Fourier series terms estimated from the stator current via the proposed scheme. The proposed method offers a quantitative and universal bearing fault indicator for induction machines with any power ratings and operating under any speed and load conditions.

This paper implements a physically derived compact model of current conduction and gate leakage in AlGaN/GaN high-electron mobility transistors (HEMTs). The drain-source current conduction through the device is described using the surface potential based virtual-source model applicable for scaled gate length devices. The gate leakage model includes contributions from thermal emission (TE), trap-assisted tunneling (TAT), Poole Frenkel (PF) emission, and Fowler-Nordheim (FN) tunneling. The full I-V model is applied to fabricated AlGaN/GaN HEMTs with SiN passivation and excellent agreement of the model against measured data is demonstrated over a broad bias and temperature range from 298 K to 573 K.

Some recent developments in 2D and 3D GaN devices and their improved performance parameters such as efficiency, fT, linearity, power density and switching speed are briefly outlined. Most of the cases briefed are for applications in RF with one example for power electronics and another for GaN integrated circuit.

Millimeter-wave (mm-wave) applications such as the satellite communication (Sat-com) system and the fifth-generation (5G) mobile communication system have attracted a great deal of attention. In the mm-wave applications, a GaN device which can obtain high power at mm-wave band is considered as one of the promising device for power amplifiers (PA) as shown in Fig. 1 [1] . In order to realize the attractive GaN PA, a GaN device model with high accuracy at mm-wave band is required for design of GaN PA. However, there are still some problems to obtain a large-signal model with high accuracy at mm-wave band. One of the problems is trapping effects under large-signal operation. Modeling of trapping effects on drain current and trans-conductance has been already reported in [2] .This paper presents a GaN device model including trapping effects on non-linear capacitance and a Ka-band high efficiency GaN Doherty PA designed by using the proposed model.

This paper presents a general space vector modulation (SVM) method for the modular multilevel converter (MMC). Compared with earlier modulation methods, the proposed SVM method not only utilizes the maximum level number (i.e., $2n+ 1$ , where $n$ is the number of submodules in the upper or lower arm of each phase) of output phase voltages, but also leads to an optimized control performance in terms of capacitor voltage balancing, circulating current suppression, and common-mode voltage reduction. The maximum level number is achieved by introducing a new equivalent circuit of the MMC, and the optimized control is obtained by selecting the optimal redundant switching states. Since the computational burden of the SVM scheme is independent of the voltage level number, the proposed method is well suited to the MMC with any number of submodules. Simulation and experimental results are presented to validate the proposed method.

Using the transimissivity between two thermal reservoirs and the generalized Planck distributions, we describe the devices that use radiative energy transfer between thermal reservoirs in a unified formalism. Four types of devices are distinguished. For power generators that use the temperature difference between reservoirs, photovoltaic (PV) and thermoradiative (TR) devices respectively use the low-temperature photovoltaic cell and high-temperature thermoradiative cell to generate electricity. For active cooling, the electroluminescent (EL) cooling devices apply a forward bias voltage on the object we want to cool, whereas the negative EL cooling devices apply a reverse bias voltage to the heat sink. The relationship among these four devices is explicated. The performance of the negative EL cooling is analyzed, both in the Shockley-Queisser (blackbody spectrum and radiative recombination) framework and the near-field enhancement. The “impedance match” condition derived for PV systems is applied to the negative EL devices. One advantageous feature of the negative EL cooling is that it does not apply the voltage to the target object which we want to cool, and the near-field enhancement can apply to various target materials that support the surface resonant modes.

Contiguous/noncontiguous carrier aggregation (CA) is one of the key features from 4G systems, which is expected to be evolved within 5G technologies. Thus, there is a need for the development of flexible, agile, and reconfigurable radio transceivers with a native support for the integration of multiple bands and multiple standards. All-digital radio-frequency (RF) transmitters have demonstrated promising potential to the design of next-generation RF transceivers. However, the simultaneous multiband transmission is still one of the key limitations of current approaches. To address this problem, this paper presents a fully digital and parallel architecture that enables the real-time design of agile and concurrent triple-band transmission. The proposed architecture is suitable for both contiguous and noncontiguous CA scenarios and considerably surpasses the state of the art in terms of frequency agility, maximum spacing between bands, and aggregated bandwidth. To enhance the system performance, an extension to a multilevel architecture based on the analog combination of pulsed waveforms is also demonstrated. Both architectures (two and seven levels) were implemented in a field-programmable gate array. Measurement results in terms of signal-to-noise ratio, error-vector magnitude, and adjacent-channel power ratio are presented and discussed. In Implementation-I, the two-level architecture presents a frequency agility from 0.1 to 2.5 GHz (with a frequency resolution of 4.88 MHz) with an aggregated bandwidth of 56.26 MHz. In Implementation-II, the seven-level design presents a frequency agility from 0.1 to 2 GHz (with a frequency resolution of 3.906 MHz) with an aggregated bandwidth of 112.5 MHz.

The ever-increasing data rate and number of connections for mobile communication offer exciting user experiences in our everyday lives. Currently, the wireless communication frontier is shifting from the current fourth generation (4G) to the forthcoming fifth generation (5G). We expect much of society to go through a revolutionary change with the advent of the 5G era-a change that will involve not only the telecommunication industry but also a wide range of vertical sectors, including automobiles, robotics, health care, factory automation, agriculture, and education.

GaN technology is not only gaining traction in power and RF electronics but is also rapidly expanding into other application areas including digital and quantum computing electronics. This paper provides a glimpse of future GaN device technologies and advanced modeling approaches that can push the boundaries of these applications in terms of performance and reliability. While GaN power devices have recently been commercialized in the 15–900 V classes, new GaN devices are greatly desirable to explore both higher-voltage and ultra-low-voltage power applications. Moving into the RF domain, ultra-high frequency GaN devices are being used to implement digitized power amplifier circuits, and further advances using the hardware–software co-design approach can be expected. On the horizon is the GaN CMOS technology, a key missing piece to realize the full-GaN platform with integrated digital, power, and RF electronics technologies. Although currently a challenge, high-performance p-type GaN technology will be crucial to realize high-performance GaN CMOS circuits. Due to its excellent transport characteristics and ability to generate free carriers via polarization doping, GaN is expected to be an important technology for ultra-low temperature and quantum computing electronics. Finally, given the increasing cost of hardware prototyping of new devices and circuits, the use of high-fidelity device models and data-driven modeling approaches for technology-circuit co-design are projected to be the trends of the future. In this regard, physically inspired, mathematically robust, less computationally taxing, and predictive modeling approaches are indispensable. With all these and future efforts, we envision GaN to become the next Si for electronics.

Thermoradiative (TR) technology was recently proposed as a new mechanism to efficiently convert low-temperature waste heat into electric energy. It has been shown that near-field coupling between a TR cell and a heat sink is an effective way in boosting the performance of the TR device. The enhancement becomes more significant when surface resonant modes are supported by the heat sink. Surface plasmon polaritons on metal surfaces, and surface phonon polaritons on Lorentz materials, are two major types of materials investigated in the near-field enhancement. However, to significantly boost the performance, the resonant mode energy needs to match the bandgap energy of the TR cell, which is often difficult to realize with natural material selections. Typical metals have plasma frequency much higher than the bandgap of TR cell. In this paper, we show that new resonant modes with energy close to the bandgap of TR cell can be created with nanostructured gratings on the heat sink. This enables use to build the heat sink using commonly available metallic materials, whose typical surface resonant frequency is much higher than the TR bandgap.

We examine the superconducting solution in the Hubbard–Holstein model using Dynamical Mean Field Theory. The Holstein term introduces the site-independent Boson fields coupling to local electron density, and has two competing influences on superconductivity: The Boson field mediates the effective electron-electron attraction, which is essential for the S-wave electron pairing; the same coupling to the Boson fields also induces the polaron effect, which makes the system less metallic and thus suppresses superconductivity. The Hubbard term introduces an energy penalty U when two electrons occupy the same site, which is expected to suppress superconductivity. By solving the Hubbard–Holstein model using Dynamical Mean Field theory, we find that the Hubbard U can be beneficial to superconductivity under some circumstances. In particular, we demonstrate that when the Boson energy Ω is small, a weak local repulsion actually stabilizes the S-wave superconducting state. This behavior can be understood as an interplay between superconductivity, the polaron effect, and the on-site repulsion: As the polaron effect is strong and suppresses superconductivity in the small Ω regime, the weak on-site repulsion reduces the polaron effect and effectively enhances superconductivity. Our calculation elucidates the role of local repulsion in the conventional S-wave superconductors.

In recent years, digital beamforming (DBF) is gaining more attention for next generation directional communication systems such as 5G, thanks to its flexibility for beamstearing compared with that of both analog- and hybrid-beamforming. However, conventional digital beamforming requires the use of independent Digital to Analog converter (DAC) and RF chain for each antenna element, which poses significant challenges in cost, power and heat management. In this work, we report for the first time a novel low-cost solution with reduced hardware complexity in DBF. Digital beamformed multiple baseband signal is designed to feed different antenna elements through a single DAC. Coding schemes were implemented in digital baseband domain, while decoding was done in analog RF domain. The decoded signals are tested before connected with antenna for proof-of-concept. The demonstrated technique provides cost and size reduction of the array system, while keeping the integrity of the signals and beam pattern.

The characteristic frequencies of different types of bearing faults can be calculated by a well-defined frequency-based model that depends on the motor speed, the bearing geometry and the specific location of a defect inside a bearing. Therefore, the existence of a bearing fault as well as its specific fault type can be readily determined by performing frequency spectral analyses on the monitored signals. However, this traditional approach, despite being simple and intuitive, is not able to identify the severity of a bearing fault in a quantitatively manner. Moreover, it is often tedious and time-consuming to apply this approach to electric machines with different power ratings, as the bearing fault threshold values need to be manually calibrated for each motor running at every possible speed and carrying any possible load. This paper thus proposes a quantitative approach to estimate a bearing fault severity based on the air gap displacement profile, which is reconstructed from the mutual inductance variation profile estimated from a novel electrical model that only takes the stator current as input. In addition, the accuracy of the electrical model and the estimated bearing fault severity are validated by simulation results. The proposed method can be used to monitor bearing faults in induction machines with any power ratings that operate under any speeds and loads, and a bearing fault alarm will be triggered if the fault severity exceeds a universal threshold value.

This work explores the use of p-diamond back-barriers (BBs) and cap layers to enhance the performance of GaN-based high electron mobility transistors (HEMTs). Diamond can offer a heavily doped p-type layer, which is complementary to GaN electronics. Self-consistent electrothermal simulations reveal that the use of p-diamond BBs and cap layers can increase the breakdown voltage of GaN-based HEMTs by fourfold, at the same time that they enhance the 2-D-electron-gas confinement and reduce short channel effects. These results highlight that p-diamond layers can improve the performance of GaN HEMTs for high-power and high-frequency applications beyond the thermal improvements pursued until now.

This paper proposes a simplified space vector modulation (SVM) scheme for multilevel converters. Compared with earlier SVM methods, the proposed scheme simplifies the detection of the nearest three vectors and the generation of switching sequences, and therefore is computationally more efficient. Particularly, for the first time, the proposed scheme achieves the same easy implementation as phase-voltage modulation techniques. Another superior characteristic of the proposed scheme over earlier methods is its potential for multiphase multilevel applications. The proposed scheme also offers the following significant advantages: 1) independence of the level number of the converter; 2) more degrees of freedom, i.e., redundant switching sequences and adjustable duty cycles, to optimize the switching patterns; and 3) no need for lookup tables or coordinate transformations. These advantages make the proposed scheme well suited to large level-number applications, such as modular multilevel converters and high voltage direct current systems. Simulation and experimental results verify this new concept.

Long-range wireless power transmission (WPT) is implemented with the beamforming using the phased array transmitter technology, which has been extensively applied in the radar systems. The cost of a conventional phased array transmitter module scales up in proportion to the number of antenna elements, as the massive number of transmit channels results in the increasing complexity of hardware and feeding antenna elements. Besides, the conventional phase-shifting transmitter architecture has lower DC to RF power conversion efficiency due to the insertion loss of power combining network at microwave frequency. In this paper, the concept of spatial power combining transmitter is utilized, and the upconversion circuit is greatly simplified to an injection locked oscillator. Our WPT system is implemented with the technology of oscillator array antenna at 2.4 GHz, which converts DC power to RF power and radiates into the air directly. The feedback voltage controlled oscillator (VCO) is implemented as the microwave source using a off-the-shelf bandpass filter, and the external signal is injected to the oscillator via a microstrip coupler. The oscillator core shows the DC-to-RF conversion efficiency of 45.87% with the injected power of 0 dBm at 2.4 GHz. Then the digital phase shifter is used to phase shifting the injected signal to extend the beam coverage. From the link budget analysis, the overall DC-to-DC efficiency of our highly-integrated system shows 1.5 times (0.22%) of the conventional phased array (0.15%) when the separation between the transmit array and the receive horn antenna is 1.2 meters. Therefore, as an modularized array, the proposed system demonstrates the promising capability of upscaling to an efficient massive array with greatly reduced bill-of-materials (BOM).

This paper reports a new architecture of power amplifiers (PA), for which machine learning is applied in real-time to adaptively optimize PA performance. For varying input stimuli such as carrier frequency, bandwidth and power level, developed algorithms can intelligently optimize parameters including bias voltages, input signal phases and power splitting ratios based on a user-defined cost function. Our demonstrator of a wideband GaN Digital Doherty PA achieves significant performance enhancement from 3.0-3.8 GHz, in particular, at high backoff power with approximately 3dB more Gain and 20% higher efficiency compared with analog counterpart. To the authors' best knowledge, this is the first reported work of model-free machine learning applied for Doherty PA control. It explores a new area of RF PA optimization, in which accurate analytical models and tedious manual tuning can be avoided.

This paper presents the first real-time Field-Programmable Gate Array (FPGA)-based All-Digital Transmitter architecture with a usable bandwidth of 1.25 GHz. The proposed architecture was implemented and embedded into an FPGA, and the results surpass the reported state-of-the-art. Measurement results in terms of Signal-to-Noise Ratio (SNR) and Error-Vector Magnitude (EVM) are presented and discussed. Specifically, modulated signals of 1.25 GHz of bandwidth were successfully transmitted with 30.51 dB of SNR and 2.23% of EVM.

Pulse-width modulation (PWM) has recently become very popular power encoding method in all-digital transmitter (ADT) architectures, which employ switch-mode power amplifiers (SMPA) for high power efficiency. Main drawback of PWM is high level of in-band harmonic distortion when digitally implemented, which limits its usage in transceivers realized for software defined radio. Furthermore, upconversion to RF frequency in such ADT systems is commonly done in digital domain. This introduces additional design difficulties, since significant amount of out-of-band harmonic noise, originating from PWM power encoding process, gets folded back into the input signal frequency band, further increasing the total in-band distortion. This paper presents a novel method for simultaneous multilevel power encoding and upconversion to RF frequency, by the means of a ΔΣM-PWM hybrid architecture. It fully exploits time-domain characterization of multilevel carrier-based digital PWM, to both provide optimal parameter selection for ΔΣM (which gives truly aliasing-free digital PWM), and to utilize harmonic nature of the PWM output for upconversion to digital RF frequency. Experiments with an arbitrary waveform generator are used to demonstrate effectiveness of the proposed novel power encoding scheme. The novel algorithm uses 3-level digital PWM and offers in-band SNR of more than −40dB.

All-digital outphasing transmitter architecture using multidimensional power coding (MDPC) is proposed for noncontiguous concurrent multiband transmission with a high power efficiency. MDPC transforms multiband digital baseband signals into multibit low-resolution digital signals that drive switching-mode PAs. A prototype digital outphasing transmitter consists of two 1-GHz bandwidth GaN Class-D PAs and a Chireix power combiner. The two GaN PAs are driven by bipolar radio frequency (RF) pulse-width modulation (PWM) signals, which are transformed from a concurrent dual-band LTE signal by MDPC. The dual-band LTE signal with 15-MHz aggregate channel bandwidth at 240 and 500 MHz frequency band is transmitted with $-$ 30 and $-$ 37 dBc out-of-band emissions, respectively. Digital outphasing achieves more than two times higher coding efficiency than conventional concurrent dual-band digital transmitters with the same PAs in Class-S operation. Measured power coding efficiencies of 35.4% and 47.1% are observed with outphasing bipolar and 3-level RF PWM signals respectively, which are encoded from the dual-band LTE signal.

Design approach of wideband envelope termination (ET) for high efficiency RF power amplifiers (PAs) is discussed, considering power loss subject to design constraints including envelope bandwidth, inductor quality factor, and transistor output impedance. Analysis shows that the optimum ET topology depends on the design constraints. For LTE-advanced transmission with 80 MHz channel bandwidth, a novel wideband ET is designed using an AC-terminated third-order LC filter (LCF-ET), providing 3 dB envelope impedance bandwidth of 150 MHz. Measured results demonstrate 1.75 dB envelope power loss and −39.2 dB normalised root-mean-square error with the LTE envelope signals. For the LTE signal with 10.3 dB peak-to-average power ratio, the average power efficiency of an ideal Class-A RF PA with the LCF-ET prototype is calculated as 33.4% from the measured 1.75 dB envelope power loss, which exceeds 4.7% and 24.0% average power efficiency of conventional Class-A and Class-B PAs without ET.