Small Power Density Research Articles

Ultra-Thin Ceramic Substrates for Improved Heatsinking for microLEDs.

MicroLEDs provide unrivaled luminance and operating lifetime, which has led to significant activity using devices for display and non-display applications. The small size and high power density of microLEDs, however, causes increased adverse heating effects which can limit performance. A new generation of electrically insulating high thermal conductivity materials, such as alumina, has been proposed to mitigate these thermal effects when used as a substrate as an alternative to glass. This strategy then could be used as a method of passive heatsinking to improve the overall performance of the microLED. In this work, a newly available material, an 80 micron thick alumina ceramic substrate, is shown to yield a 30 % improvement on average in the maximum current drive over a glass substrate.

Single-Switch Non-Isolated Resonant DC-DC Converter for Single-Input Dual-Output Applications

This paper describes a new configuration of Cuk and SEPIC (Single-Ended Primary Converter) ZVS-QR (zero-voltage switching quasi-resonant) combination DC-DC converter for bipolar output with a single switch. The proposed topology employs a single ground-referenced power switch, which simplifies the gate drive design with a single L-C resonant network and provides a bipolar output voltage with good regulation, acceptable efficiency and a step-down/up conversion ratio. This configuration provides dual-output voltage by switching the power switch to zero voltage, which is an interesting alternative for many applications where small size, light weight and high power density are very important aspects. In order to verify its performance, a SEPIC–Cuk Combination ZVS-QR prototype with a cost-effective commercial resonant controller was designed and tested. The experimental results show that the proposed combined topology is suitable for Single-Input Dual-Output (SIDO) applications.

Performance degradation analysis and fabrication guidance of μ-TEG from material to device

Micro thermoelectric generator (μ-TEG) attracts more and more attention due to its small size and high power density. Many two-dimensional thermoelectric materials with high performance have facilitated the development of μ-TEG. However, the performance of μ-TEG fabricated by these great thermoelectric materials is significantly degraded due to size effect, interfacial effects (include contact effect and boundary effect) and structure effect. To accurately assess the performance degradation degree from material to μ-TEG and guide the device fabrication, an experiment-verified mathematical model considering interfacial and size effects is proposed. Firstly, the phonon/electron temperature distribution in thermoelectric leg of μ-TEG is analyzed to investigate the device-level thermoelectric properties of material. Then based on the device-level thermoelectric properties, the actual power generation performance model of μ-TEG is established to conduct the influence analysis of these effects (boundary, size, contact and structure effects) on material and device. Finally, the thermoelectric leg thickness (Hte) is optimized to realize optimal power generation. The study results reveal that boundary and size effects weaken the device-level thermoelectric properties, and the reduction trend is more obvious when Hte is smaller, especially when Hte ≤ 20 μm. The decrease from the material intrinsic figure of merit ((ZT)m) to the device figure of merit ((ZT)D) is owing to the boundary effect, structure effect and contact effect, and the dominant factor of this decrease changes from structure effect (Hte＜7 μm)to contact effect (Hte ≥ 7 μm) as Hte increases, which points to a main optimization direction for (ZT)D for different Hte. As for contact effect, the electrical contact resistivity (re,c) has a more significant impact on weakening the performance of μ-TEG than thermal contact resistivity (rk,c), and their optimization goals are explored (re,c ≤ 5.1 × 10−12 Ω·m2, rk,c ≤ 9.3 × 10−8 K·m2/W). At given electrical and thermal contact resistivity, there exists an optimal Hte for achieving the optimal power generation (Popt) and a large range of Hte for achieving 95%Popt, and the optimal Hte increases with increasing electrical and thermal contact resistivity. This study can reduce the processing difficulty and save time and economic costs of μ-TEG fabrication, which can avoid the blind fabrication of μ-TEG.

Dead-Time Optimization and Magnetizing Current Design for a Current-Fed Dual Active Bridge DC–DC Converter to Secure Full Load Range ZVS in Wide Voltage Range

Current-fed dual active bridge (CF-DAB) converters are widely used because of the small input current ripple and high power density. This paper proposes the dead-time optimization methods and the magnetizing current design for a CF-DAB converter under the pulse-width plus phase-shift (PPS) control to achieve full load range zero-voltage-switching (ZVS) in a wide voltage range. In the ZVS analysis, the influence of junction capacitance and dead time is considered, including the dead-time effect on leakage inductance current, which can effectively explain that the CF-DAB converter under PPS control is difficult to achieve ZVS at light load in practice. Based on the analysis, it is first revealed that the CF-DAB converter under PPS control is not sufficient to achieve ZVS only by adjusting the dead time. Based on the new ZVS model and analysis of the dead-time effect, the magnetizing inductance design and dead-time optimization without adding extra control degrees of freedom and auxiliary components are proposed to achieve the full load range ZVS and improve the overall efficiency of the converter in a wide voltage range. Besides, to obtain the low root-mean-square (RMS) current, the design of DC inductance and leakage inductance are also investigated. Finally, a 1 kW prototype is built to verify the effectiveness of the proposed solution.

Structure Design of Multi-DOF Motor with Stator Permanent Magnet based on Flux Switching

This paper aims at the problems of no core in the stator coil of a compact multi-degree-of-freedom (M-DOF) motor, i.e., small output torque and low power density. In order to increase the output torque, a stator permanent magnet spherical motor based on flux switching is proposed. Firstly, the principle of flux switching is introduced. The basic structure of the new motor is preliminarily designed, and the motion mechanism is analysed for spin motion and tilt motion respectively. Then the minimum model of a novel motor is established in ANSYS by using the virtual displacement method. The key parameters such as the ratio of stator pole tooth to permanent magnet size, different stator pole tooth end structures, different pole tooth surface shapes, different pole sizes, permanent magnet thickness, different magnetisation methods and different ampere-turn numbers are optimised within a reasonable parameter range. The simulation results are analysed and discussed, and the optimal results for each key parameter are thus given. Finally, considering the arrangement of the stator and rotor poles, the rationality of the three-DOF motion was verified by means of two energising strategies, spin motion and tilt motion, verified the reasonableness of the above optimal parameters and thus determined the structural parameters of the new spherical motor to accomplish the three-DOF motion and the maximum torque range. The simulation results show that the electromagnetic torque varies approximately linearly over a range of ampere-turns for the same position of the rotor pole teeth. It provides an opportunity for the M-DOF motor design of a high torque output and power density.

Optimal design of VHF resonant boost converter considering the nonlinear characteristic of parasitic capacitance

SummaryVery‐high‐frequency (VHF) resonant converters have the advantages of small volume, light weight, and high power density. However, under the operating frequency of tens of megahertz, the nonlinear characteristics of the power semiconductor devices intrinsic capacitances will affect the analysis accuracy of VHF converters and increase the difficulty of parameter design of these converters. Thus, this paper proposes a method to obtain the equivalent capacitances of power semiconductor devices. Taking the VHF resonant boost converter as an example, the process of obtaining the equivalent capacitances of power semiconductor devices is elaborated first. And then a prototype with operating frequency 17.30 MHz, input voltage 3–6 V, and output voltage 9–18 V is built to verify the feasibility and effectiveness of the proposed method.

Efficient van der Waals layered gallium telluride-based passive photodetectors for low-power-density sensing of visible light

This work demonstrates the capability of a van der Waals layered GaTe single crystal to detect a wide range of wavelengths at small power density (0.05 W m−2) while being operated at a low bias voltage of 0.2 V.

Polygon-Retrofitted Integrated Modular Motor Drive for Switched Reluctance Machines

Thanks to the absence of rotor windings and permanent magnets, switched reluctance motors have a small weight construction. The integration of a modular power converter in a switched reluctance motor leads to a highly compact and fault-tolerant motor drive with small weight and high power density. In this article, a polygon-retrofitted integration topology is proposed for switched reluctance motors. Both the switched reluctance motor and the power converter are cooled with one shared cooling circuit. The power converter is designed and implemented using silicon carbide technology for its low losses and small thermal resistance. The proposed integration topology is extensively studied using multiphysics modeling and experimental measurements on a fully integrated rotating setup. A highly compact and power dense drive of 3.1 kW/L is obtained.

Thermal Network Modeling of High Frequency Insulated Core Transformers

The high frequency insulated core transformer (HF-ICT) accelerator is expected to replace the traditional insulated core transformer (ICT) for its small size and high power density in the field of irradiation below 1 MeV. As a key component of the accelerator, the ICT high-voltage power supply is used to feed the accelerator tube. The segmented core structure of the ICT will lead to the different magnetic flux of each core section. The nonuniform of power loss and structure results in the uneven temperature distribution of the HF-ICT. In this regard, a thermal model with highly accurate temperature prediction on the HF-ICT parts is required. A thermal resistance network based on the HF-ICT is proposed, considering the thermal coupling of the ambient, cores and windings. The multiphysics coupling analysis is carried out by COMSOL Electromagnetic Thermal Module and Nonisothermal Flow Module. Instead of linear materials, the measured B-P curve and B-H curve are applied for a more realistic simulation model. Finally, the thermal model demonstrates acceptable accuracy and small computational expense within the simulation results, which verifies the effectiveness of the proposed model.

Thermal Analysis of Dual-Axis-Direction Hybrid Excitation Generator for Electric Vehicles

Electric vehicle (EV) generators have characteristics of small volume and high power density, which leads to high temperature rise. The temperature rise will directly influence the service life and reliability of the generator, so thermal analysis of EV generators is necessary in machine design. This paper carries out thermal analysis of a dual-axis-direction hybrid excitation generator (DHEG) used in EVs. The lumped parameter thermal network (LPTN) method is used to build the three-dimensional thermal model of DHEG. Compared with finite element analysis, LPTN can significantly reduce the simulation time while ensuring a good accuracy. Then, the influence of heat dissipation rib on DHEG temperature rise is studied. The results prove that adding heat dissipation rib on the enclosure surface can effectively reduce the maximum temperature of DHEG.

Thermal Analysis of Axial-Flux Permanent Magnet Motors for Vehicles Based on Fast Two-Way Magneto-Thermal Coupling

Axial-flux permanent magnet motor (AFPMM) have small size and high power density. It has a good application prospect in the field of new energy vehicle driving. In this paper, based on a 56 kW AFPMM, the magnetic circuit characteristics are calculated by the split loop method, considering the influence of pulse width modulated (PWM) power supply. The loss is taken as the heat source, combined with the motor structure characteristics and cooling conditions, the lumped-parameter thermal network model of the motor is established to solve the steady-state and transient temperature distribution of each structure. By this way, fast and accurate thermal calculation of the motor is realized in design stage. The accuracy of the lumped-parameter thermal network model is verified by experiment. At the same time, the effects of spliting the permanent magnet (PM) into pieces, flow rate of cooling water, and loss distribution on temperature rise are analyzed. This research work provides an effective fast thermal calculation method for AFPMM and provides a reference basis for the design of similar motors. It has important value of theoretical significance and engineering practical.

Experimental study on temperature and performance of an open-cathode PEMFC stack under thermal radiation environment

The open-cathode proton exchange membrane fuel cell stack is an air-cooled stack, and has the characteristics of small footprint and high power density because of its unique configuration, combining oxidizer and coolant via axial fan. Depending on the demands of the application, proton exchange membrane fuel cells inevitably operate in harsh environments, such as high pressure, low or high temperature, etc. In this study, the performance of a 10-cell open-cathode PEMFC stack under thermal radiation was evaluated. The variations in cathode air temperature and relative humidity, stack temperature, output power and single-cell voltage under three different radiant heat fluxes were obtained by changing the load current and cathode air flow rate. The experimental results show that, even if the thermal radiation is beneficial to improve the stack temperature, the stack performance is not improved in all tested cases. Conversely, the degradation of stack power and failure of the stack continuous operation can be observed, especially for higher radiant heat flux, higher load current and lower air velocity. For the highest radiant heat flux in current study, the temperature of the first single cell closest to the radiant heat source exceeds the maximum allowable operating temperature of 65 °C due to the insufficient capacity of air cooling and the heating effect of radiant heat source. The significant drop in the first single-cell voltage results in the rapid degradation of the overall performance of the stack, and finally the stack fails to run continuously.

Low Cost Solvothermal Processed CTS QDs (0D)-Based Visible-NIR Photoconductor

This paper reports a photoconductive type (Ag/Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (CTS) QDs/Ag) Vis-NIR photodetector having small channel spacing ( ~ 63.3 μm) on glass substrate. The low cost solvothermal processed CTS QDs have been used as carrier transporter as well as broad light absorber. The optical parameters of the proposed structure for small illumination power density have been calculated for broad range light illumination wavelengths (650-1100 nm). The responsivity of device was found to be ~37 (mA/W) for visible (730 nm) and ~67 (mA/W) for NIR (940 nm) illumination wavelengths. The sensitivity of the device was observed to be ~2.28 and ~8.2 for 730 and 940 nm illumination wavelengths, respectively, at 5V. The time response of the device have been measured under Vis-NIR illumination and found to be ~0.96 s (rise time) and ~1.29 s (fall time).

Load simulation analysis of a new erection system based on the gas-assisted drive erecting scheme

To improve the traditional erecting process with lower velocity and smaller power density, a new erection system is designed based on the hybrid drive of gas and electric cylinder, witch could erect quickly without increasing the power of the whole machine. According to the prescribed rotational motion and the rotational differential equation of the erection mechanism, the computational models of the related characteristic parameters in erection process were set up. Taking some parameters of a missile as boundary conditions, the models were simulated in MATLAB, and the parameters were obtained graphically. The results can offer references for the design of a new erection mechanism based on pyrotechnics.

Three-Dimensional Coherent Bragg Imaging of Rotating Nanoparticles.

Bragg coherent diffraction imaging is a powerful strain imaging tool, often limited by beam-induced sample instability for small particles and high power densities. Here, we devise and validate an adapted diffraction volume assembly algorithm, capable of recovering three-dimensional datasets from particles undergoing uncontrolled and unknown rotations. We apply the method to gold nanoparticles which rotate under the influence of a focused coherent x-ray beam, retrieving their three-dimensional shapes and strain fields. The results show that the sample instability problem can be overcome, enabling the use of fourth generation synchrotron sources for Bragg coherent diffraction imaging to their full potential.

Dual role of macrophytes in constructed wetland-microbial fuel cells using pyrrhotite as cathode material: A comparative assessment

In the recent years many studies have shown that wetland plants play beneficial roles in bioelectricity enhancement in constructed wetland-microbial fuel cell (CW-MFC) because of the exudation of root oxygen and root exudates. In this study, the long-term roles of plants on the bioelectricity generation and contaminant removal were investigated in multi-anode (Anode1 and Anode2) and single cathode CW-MFCs. The electrode distances were 20 cm between Anode1-cathode and 10 cm between Anode2-cathode, respectively. Additionally, the employment of natural conductive pyrrhotite mineral as cathode material was firstly investigated in CW-MFC system. A cathode potential of −98 ± 52 mV to −175 ± 60 mV was achieved in the unplanted (CW-MFC 1), and planted CW-MFCs with Iris pseudacorus (CW-MFC 2), Lythrum salicaria (CW-MFC 3), and Phragmites australis (CW-MFC 4). The maximum power densities of Anode1-cathode and Anode2-cathode were 8.23 and 15.29 mW/m2 in CW-MFC 1, 8.51 and 1.67 mW/m2 in CW-MFC 2, 5.67 and 3.15 mW/m2 in CW-MFC 3, and 7.59 and 14.71 mW/m2 in CW-MFC 4, respectively. Interestingly, smaller power density was observed at Anode2-cathode, which has shorter electrode distance than Anode1-cathode in both CW-MFC 2 and CW-MFC 3, which indicates the negative role of oxygen released from the flourished plant roots at Anode2 micro-environment in power production. Therefore, recovering power from commercial CW-MFCs with flourished plants will be a challenge. The contradiction between keeping short electrode distance and avoiding the interference from plant roots to maintain anaerobic anode may be solved by the proposed modular CW-MFCs.

Highly Sensitive UV Photodiode Composed of β-Polyfluorene/YZnO Nanorod Organic-Inorganic Hybrid Heterostructure.

The highly sensitive ultra-violet (UV) photodiode was demonstrated on the organic-inorganic hybrid heterostructure of β-phase p-type polyfluorene (PFO)/n-type yttrium-doped zinc oxide nanorods (YZO-NRs). The device was fabricated through a simple fabrication technique of β-phase PFO coating onto YZO-NRs that had been directly grown on graphene by the hydrothermal synthesis method. Under UV illumination (λ = 365 nm), the device clearly showed excellent photoresponse characteristics (e.g., high quantum efficiency ~690%, high photodetectivity ~3.34 × 1012 cm·Hz1/2·W−1, and fast response time ~0.17 s). Furthermore, the ratio of the photo current-to-dark current exceeds 103 even under UV illumination with a small optical power density of 0.6 mW/cm2. We attribute such superb photoresponse characteristics to both Y incorporation into YZO-NRs and conformation of β-phase PFO. Namely, Y dopants could effectively reduce surface states at YZO-NRs, and β-phase PFO might increase the photocarrier conductivity in PFO. The results suggest that the β-phase p-PFO/n-YZO-NR hybrid heterostructure holds promise for high-performance UV photodetectors.

Bi2Te3 photoconductive detector under weak light

Bi2Te3 − xSex photoconductive detectors are attractive due to their linear photo-response (LPR), potential in ultrabroadband photo-detection, and compatibility with CMOS technology. However, as a fundamental issue for photo-detectors, the weak-light photo-response of Bi2Te3 − xSex detectors was covered by the bolometric response, the photo-thermoelectric effect, and the 300 K background black-body radiation from the optical window. Here, using a window-less system, we systematically measure the weak-light photo-response of a Bi2Te3 photoconductive detector. The bolometric response and photo-thermoelectric effect are carefully excluded. It shows that LPR can persist to a small light power density of P &amp;lt; 10−8 W/cm2. Meanwhile, the response is maximized at ∼80 K temperature. We suggest that the LPR is rooted in the unique carrier recombination configuration of Bi2Te3, which can result in a constant lifetime for light-generated carriers. The concerned trap level is manifested by a negative photo-response under reduced light intensity. The signature of surface state related signal is presented as a photo-response with the maximum at ∼10 K temperature, which is fragile under increased light intensity. Although the window-less photo-response measurement is at its early stage, it provides a perspective which is different from that of the traditional optical-window based measurement.

Sensorless Control for PMSM Based on Multi-Innovation Two-Stage Extended Kalman Filter Algorithm

Permanent magnet synchronous motors (PMSM) have been widely used in robotics, CNC machine tools, and aerospace, aerospace and marine fields due to its small size, high efficiency and high power density, and good operating performance. With the development of sensorless control technology, sensorless control has been widely used in many special areas. This paper is aiming at the sensorless control of permanent magnet synchronous motor, the sensorless control system of permanent magnet synchronous motor based on Multi-innovation Two-Stage Extended Kalman Filter (MI-TSEKF) algorithm is studied and implemented.

Spectrum Sensing: Analog (or Partially Analog) CMOS Real-Time Spectrum Sensing Techniques

Congestion of the radio spectrum, caused by licensed primary users and an increasing demand for new devices, has created a need for alternative methods of radio spectrum use in the future. Licensed primary users can be inactive for a specified time, leaving their allocated frequency band unoccupied so that secondary users can then use it. The U.S. Federal Communications Commission (FCC) has allowed unlicensed users to operate under specified circumstances for specific frequency bands. Secondary users cannot interfere with primary users at any time, whether by detecting the presence of primary signals and avoiding the occupied bands or by having a very small transmission power density compared to primary users. Hence, they appear as noise in primary receivers.