Increased Conduction Losses Research Articles

Structural design in reduced graphene oxide (RGO) metacomposites for enhanced microwave absorption in wide temperature spectrum

High-temperature microwave absorbing materials (MAMs) and structures are increasingly appealing due to their critical role in stealth applications under harsh environments. However, the impedance mismatch caused by increased conduction loss often leads to a significant decline in electromagnetic wave absorption (EMWA) performance at elevated temperatures, which severely restricts their practical application. In this study, we propose a novel approach for efficient electromagnetic wave absorption across a wide temperature range using reduced graphene oxide (RGO)/epoxy resin (EP) metacomposites that integrate both electromagnetic parameters and metamaterial design concepts. Due to the discrete distribution of the units, electromagnetic waves can more easily penetrate the interior of materials, thereby exhibiting stable microwave absorption (MA) performance and impedance-matching characteristics suitable across a wide temperature range. Consequently, exceptional MA properties can be achieved within the temperature range from 298 to 473 K. Furthermore, by carefully controlling the structural parameters in RGO metacomposites, both the resonant frequency and effective absorption bandwidth (EAB) can be optimized based on precise manipulation of equivalent electromagnetic parameters. This study not only provides an effective approach for the rational design of MA performance but also offers novel insights into achieving super metamaterials with outstanding performance across a wide temperature spectrum.

Simultaneous Inhibition of Conduction Loss and Enhancement of Polarization Intensity of Polyetherimide Dielectrics for High-Temperature Capacitive Energy Storage.

Polymer dielectrics with excellent high-temperature capacitive energy storage performance are in urgent demand for modern power electronic devices and high-voltage electrical systems. Nevertheless, the energy storage capability usually degrades dramatically at increased temperatures, owing to the exponentially increased conduction loss. Herein, a trace of commercially available aluminum nitride (AlN) nanoparticles is incorporated into the poly(ether imide) (PEI) matrix to inhibit the conduction loss. The nanostructured AlN component with a large specific surface area can provide abundant sites for the collision of carriers. More importantly, the generated new trap energy levels can immobilize the carriers, accordingly contributing to the reduction in leakage current. From this, the discharged energy density at 150 °C of PEI composites increases by 82.13% from 2.63 J/cm3 for pristine PEI to 4.79 J/cm3 for PEI composites. This work establishes a facile approach to enhancing the high-temperature capacitive performance of polymer dielectrics.

Effect of Fluorine in Redesigning Energy-Storage Properties of High-Temperature Dielectric Polymers.

Exploration of novel polymer dielectrics exhibiting high electric-field stability and high energy density with high efficiency at elevated temperatures is urgently needed for ever-demanding energy-storage technologies. Conventional high-temperature polymers with conjugated backbone structures cannot fulfill this demand due to their deteriorated performance at elevated electric fields. Here, in search of new polymer structures, we have explored the effect of fluorine groups on the energy-storage properties of polyoxanorbornene imide polymers with simultaneous wide band gap and high glass transition temperature (Tg). The systematic synthesis of polymers with varying amounts of fluorine is carried out and characterized for the energy-storage properties. The incorporation of fluorine imparts flexibility to the polymer structure, and free-standing films can be obtained. An oxanorbornene copolymer with 25% fluorination exhibits a high breakdown strength of 700 MV/m and a discharged energy density of 6.3 J/cm3 with 90% efficiency. The incorporation of fluorine helps to increase the polymer band gap, as observed using UV-vis spectroscopy, but lowers the polymer Tg, as shown by differential scanning calorimetry. Both the displacement-electric field (D-E) hysteresis loop and high-field conduction measurements show increased conduction loss for polymers with higher fluorine content, despite their larger band gap. The presence of excess free volume may play a key role in increasing the conduction current and lowering the efficiency of polymers with high fluorine content. Such an improved understanding of the effect of fluorination on the polymer energy-storage properties, as revealed in this systematic molecular engineering study, broadens the basis of material-informatic proxies to enable a more targeted codesign of scalable and efficient polymer dielectrics.

H-Bridge Derived Topology for Dynamic On-Resistance Evaluation in Power GaN HEMTs

Gallium nitride high electron mobility transistors outperform Silicon devices due to their excellent physical properties. However, being an immature technology, it exhibits dynamic <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -state resistance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R_{\text{ds}, \text{on}}$</tex-math></inline-formula> ). This causes increased conduction loss and leads to reduced operating efficiency. To evaluate the dynamic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R_{\text{ds}, \text{on}}$</tex-math></inline-formula> of the device, a measurement circuit is required. In literature, a standardized double pulse test is widely used for the measurement of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R_{\text{ds}, \text{on}}$</tex-math></inline-formula> . However, due to the uncontrolled <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> -state time and inability to independently adjust, the test conditions result in restricted measurement. This article proposes a measurement circuit of the device in power electronics operations. The circuit uses an H-Bridge-derived topology with capacitive storage to enable forward and reverse conduction mode measurement of the device. In all the modes, the circuit allows measurement from the first switching cycle and independent control over the testing conditions by using a hysteresis current control. The proposed measurement circuit is validated using simulation and experimentation. Finally, analytical efficiency compared with the state-of-the-art measurement circuits is provided.

Enhanced High-Temperature Capacitive Performance of a Bilayer-Structured Composite Film Employing a Charge Blocking Layer

The great development potential of polymer dielectric capacitors in harsh environments urgently requires enhancing capacitive performance at high temperatures. However, the exponentially increased conduction loss at high temperature and high field results in a drastic drop in energy density and charge-discharge efficiency. Here, a bilayer-structured polyimide (PI) composite film containing a wide-band gap inorganic layer as a charge blocking layer is designed. The inorganic layer improves the charge trapping ability and regulates the charge mobility at the electrode/dielectric interface. The charge injection mechanism in the interface-optimized PI/boron nitride nanosheet (BNNS) composite films is investigated by finite element simulation, and the effect of the BNNS layer on high temperature conduction is further understood. An appropriate thickness of the charge blocking layer establishes an effective energy barrier. Therefore, the composite films exhibit significantly suppressed conduction loss and excellent capacitive performance at a high temperature. A high energy density of 4.37 J cm-3 with efficiency of 92% is obtained at 200 °C and 500 MV m-1, which is superior to reported high-temperature dielectric polymers and their composite films. This work provides a promising approach to improve the energy storage performance of polymer materials at high temperatures.

Design and Implementation of Bridgeless AC/DC PFC Sepic Converter with Valley-Fill Circuit

The continuous requirement to improve power quality has motivated the proposal of several PFC circuit topologies. The conventional AC/DC SEPIC has increased conduction loss due to the presence of bridge rectifier. In order to reduce the conduction loss and to improve the efficiency this research work deals with a bridgeless SEPIC (single ended primary inductance converter) AC/DC PFC integrated valley-fill circuit (VFC). The number of components is reduced compared to the existing topologies. The bridgeless topology results in reduced conduction loss, reduced current stress and improved system performance compared with the traditional SEPIC and the existing topologies. The circuit configuration of the suggested topology has been simulated using MATLAB/SIMULINK. The functional parameters of the projected AC/DC integrated VFC is compared with the conventional SEPIC (Con-SEPIC), con-SEPIC integrated VFC and bridgeless SEPIC in terms of passive component count, output voltage ripple, input current ripple, supply power factor, Total Harmonic Distortion. From the simulation outcomes, it is concluded that the bridgeless topology provides high power factor, reduced ripple and reduced supply current harmonics. The results are validated.

Fabrication of N−doped carbon nanotube/carbon fiber dendritic composites with abundant interfaces for electromagnetic wave absorption

Three−dimensional (3D) hierarchical nano−microstructures have shown unprecedented physicochemical properties. However, it still remains great challenge to construct well−defined architectures with 3D hierarchical feature. Herein, we develop an electrospinning technique followed by a high−temperature carbonization process to fabricate N−doped carbon fibers (NCF) with N−doped carbon nanotubes (NCNTs) vertically grown on the surface of NCF using the Fe species as catalysts. In the 3D hierarchical structures, the Fe3C nanoparticles (NPs) are embedded within NCF matrix, while the Fe NPs are encapsulated within NCNT. Due to the unique structural feature, the as−prepared hierarchical structure exhibits excellent electromagnetic wave absorption performance with an absorption bandwidth of 4.0 GHz and a minimum reflection loss of −49.56 dB with a thickness as low as 1.50 mm. Experimental and theoretical studies indicate that the enhanced electromagnetic wave absorption performance of the hierarchical structure can be explained by the increased conduction loss induced by the introduction of metallic NPs and formation of 3D conductive networks, and the enhanced polarization loss caused by the additional interfaces and defects in the hierarchical structure. This work provides an efficient way to fabricate 3D architecture for high−efficiency electromagnetic wave absorption.

Lightweight and robust Ti3C2Tx/carbon nanotubes foam with tuneable and highly efficient microwave absorption performance

In this study, a lightweight and robust Ti3C2Tx/carbon nanotubes (CNTs) foam (TCF) was fabricated using HCl-induced self-assembly, followed by vacuum freeze-drying. The electrical conductivity and mechanical elasticity of the TCF was higher than those of monolithic Ti3C2Tx foams. This was ascribed to the incorporation of CNTs into Ti3C2Tx preventing the stacking of the Ti3C2Tx nanosheets and producing a well-developed three-dimensional honeycomb-like porous network structure, which considerably improved impedance matching, promoted multiple reflection loss, increased conduction loss and polarisation loss, thus imparting remarkable microwave absorption properties to the TCF. The 1.72 and 1.92 mm thick TCF samples with absorber loadings of 4 wt%, which were obtained by immersing TCF into molten paraffin, followed by cutting it into coaxial rings, presented an optimum reflection loss of −48.8 dB and a maximum effective absorption bandwidth (EAB) of 5.44 GHz, respectively. Moreover, upon increasing the thickness of the TCF samples from 1.52 to 4.92 mm, the EAB could be regulated from 4.16 to 18 GHz, respectively. In this study, we developed a facile method for fabricating a lightweight and robust TCF, which met the ‘light, thin, broad, and strong’ criteria and presented a broad EAB and remarkable dissipation capability, for microwave absorption materials.

Noninvasive Model-Based Open-Circuit Switch Fault Detection of AC-Bypass Leg Switches in Transformerless PV Inverter

Transformerless inverters are being used for integrating photovoltaic (PV) sources into the grid. Highly efficient and reliable inverter concept (HERIC) inverter is one such topology, which is derived from an H-bridge by adding a bypass leg on the ac side using two back-to-back insulated gate bipolar transistors (IGBTs). Open-circuit fault in the bypass leg would result in conduction of current through antiparallel diodes of the main switches of the inverter, effectively making the inverter operate in bipolar mode. As a result of the fault, there would not be an issue of leakage current, and the operation would continue at the expense of increased conduction losses and dc-offset in the grid current. This article proposes an online noninvasive model-based technique to detect switch faults in the bypass leg of the HERIC inverter. The grid current at the peak of the carrier signal is predicted based on the measured grid current at the start of the zero states. By comparing the estimated and measured grid currents at the peak of the carrier signal, the faulty switch in the bypass leg is localized. Once the fault is localized, the inverter’s modulation scheme is changed to conventional bipolar mode to improve the performance of the converter during the post-fault stage. A detailed simulation study is carried out to verify the effectiveness of the proposed approach. Experimental tests on the prototype converter show that the proposed algorithm can reliably detect the open-circuit fault in switches of the bypass leg, and once a fault is detected, the algorithm would change the modulation scheme to bipolar mode to minimize the inverter loss and dc-offset in the grid current.

Polyimide films coated by magnetron sputtered boron nitride for high-temperature capacitor dielectrics

Polymers are widely used as dielectric materials in dielectric capacitors, which are the key component of various circuits in power electronic devices. The sharply increased conduction loss at elevated temperatures limits their high-temperature applications. To address this issue, here in this study boron nitride (BN)/polyimide (PI)/BN sandwich-structured films were developed by depositing BN of optimal thickness onto both sides of PI films using magnetron sputtering. Dielectric, insulating and energy-storage performance at 150 °C of the resultant composite films were investigated. The experimental results showed that a dense layer of hexagonal boron nitride (h-BN) was formed on the surface of the polyimide and the coating layer could effectively suppress the charge injection at the electrode/dielectric interfaces by improving the barrier height. Consequently, the sandwiched film of optimal BN thickness exhibited a declined leakage current of about one order of magnitude lower and an improved breakdown strength by 12% in comparison to the neat PI. The sandwiched film could discharge an energy density of 0.493 J/cm3 with charge-discharge efficiency of over 90% under an electric field of 200 MV/m at 150 °C, while the neat PI only possesses an efficiency of 72% and a much lower discharged energy density.

A Modified Higher Operational Duty Phase Shifted Full Bridge Converter for Reduced Circulation Current

Besides many advantages, the reduction in the operational duty of a traditional phase shifted full converter limits its scope in applications where a wide range of input voltage is the main requirement. Operation with low duty cycle extends freewheeling interval, which results in degraded performance such as more circulating current, increased conduction loss in power devices, narrow range of zero voltage switching and increased EMI. To overcome these drawbacks, this work suggests a modified phase shifted full bridge converter that keeps the operational duty of the converter high for a wide range of input voltage. This cuts the freewheeling interval and improves performance. The proposed converter consists of four low profile transformers having a structure of reconfigurable interconnections. There are two distinct reconfigurable operational modes, a low-gain mode and a high-gain mode, which can be adopted following the variation in line voltage. The proposed work is validated in LTspice simulation and hardware characterization for a wide range of input voltage 100-400 Vdc/ 12 Vout and up to the load power of 1.2 kW.

Experimental Evaluation and Analysis of Switching Transient's Effect on Dynamic on-Resistance in GaN HEMTs

The dynamic on -resistance in gallium nitride (GaN) devices is problematic as it can impair the converter's efficiency due to the increased conduction loss. In this paper, the hard-switching transient's effect on the dynamic on -resistance is, for the first time, evaluated experimentally on a commercial high-voltage GaN high electron mobility transistor. A new ${R_{{\rm{dyn}} - {\rm{ds}},{\rm{on}}}}$ measurement circuit with fast sensing speed is designed, and an accurate measurement of ${R_{{\rm{dyn}} - {\rm{ds}},{\rm{on}}}}$ can be realized experimentally within 49.6 ns after the device's current reaches to the load current. A double-pulse-test setup is designed to comprehensively evaluate the switching transient's effect on ${R_{{\rm{dyn}} - {\rm{ds}},{\rm{on}}}}$ under different operating conditions. From the experimental results, it is found that the turn- on and turn- off gate resistance have a significant impact on the dynamic on -resistance whereas the cross-talk effect on ${R_{{\rm{dyn}} - {\rm{ds}},{\rm{on}}}}$ is negligible. Specifically, at 400 V/25 A, more than 27% (28.2%) increase in ${R_{{\rm{dyn}} - {\rm{ds}},{\rm{on}}}}$ is observed when the external turn -on (turn- off ) gate resistance increases from 0 to 20 Ω. Detailed discussion and quantitative analysis are provided to explain the experimental results. In terms of the turn -on process, it is concluded that the ${R_{{\rm{dyn}} - {\rm{ds}},{\rm{on}}}}$ variation is mainly caused by the different numbers of generated hot electrons. For the turn -off transient, it is confirmed the variation of drain current at different dv/dt slew rate leads to the ${R_{{\rm{dyn}} - {\rm{ds}},{\rm{on}}}}$ difference.

An Improved Proposed Single Phase Transformerless Inverter with Leakage Current Elimination and Reactive Power Capability for PV Systems Application

Single-phase transformerless inverters are broadly studied in literature for residential-scale PV applications due to their great advantages in reducing system weight, cost and elevating system efficiency. The design of transformerless inverters is based on the galvanic isolation method to eliminate the generation of leakage current. Unfortunately, the use of the galvanic isolation method alone cannot achieve constant common mode voltage (CMV). Therefore, a complete elimination of leakage current cannot be achieved. In addition, modulation techniques of single-phase transformerless inverters are designed for the application of the unity power factor. Indeed, next-generation PV systems are required to support reactive power to enable connectivity to the utility grid. In this paper, a proposed single-phase transformerless inverter is modified with the clamping method to achieve constant CMV during all inverter operating modes. Furthermore, the modulation technique is modified by creating a new current path in the negative power region. As a result, a bidirectional current path is created in the negative power region to achieve reactive power generation. The simulation results show that the CMV is completely clamped at half the DC link voltage and the leakage current is almost completely eliminated. Furthermore, a reactive power generation is achieved with the modified modulation techniques. Additionally, the total harmonic distortion (THD) of the grid current with the conventional and a modified modulation technique is analyzed. The efficiency of the system is enhanced by using wide-bandgap (WBG) switching devices such as SiC MOSFET. It is observed that the efficiency of the system decreased with reactive power generation due to the bidirectional current path, which leads to increasing conduction losses.

Effect of Ti3SiC2 addition on microwave absorption property of plasma sprayed Ti3SiC2/NASICON coatings

The work attempted to develop a kind of high temperature microwave absorption coating. The Ti3SiC2/NASICON composite coatings with different Ti3SiC2 concentrations were fabricated by atmospheric plasma spraying. The effect of Ti3SiC2 addition on phase, density, microstructure, dielectric property and microwave absorption property of as-sprayed coatings was investigated. Results show that the complex permittivity increases with increasing the content of Ti3SiC2 due to the enhanced space charge polarization, decreased porosity and increased conduction loss. When the content of Ti3SiC2 increases to 30 wt%, the coating exhibits the optimal microwave absorption property with a bandwidth (below − 5 dB) of 4.01 GHz and lowest reflection loss of − 12.4 dB at 9.63 GHz in 1.4 mm thickness. It indicates that the Ti3SiC2/NASICON composite coating can be a potential candidate for microwave absorption.

A High-Efficiency MOSFET Transformerless Inverter for Nonisolated Microinverter Applications

State-of-the-art low-power-level metal-oxide-semiconductor field-effect transistor (MOSFET)-based transformerless photovoltaic (PV) inverters can achieve high efficiency by using latest super junction MOSFETs. However, these MOSFET-based inverter topologies suffer from one or more of these drawbacks: MOSFET failure risk from body diode reverse recovery, increased conduction losses due to more devices, or low magnetics utilization. By splitting the conventional MOSFET-based phase leg with an optimized inductor, this paper proposes a novel MOSFET-based phase leg configuration to minimize these drawbacks. Based on the proposed phase leg configuration, a high efficiency single-phase MOSFET transformerless inverter is presented for the PV microinverter applications. The pulsewidth modulation (PWM) modulation and circuit operation principle are then described. The common-mode and differential-mode voltage model is then presented and analyzed for circuit design. Experimental results of a 250 W hardware prototype are shown to demonstrate the merits of the proposed transformerless inverter on nonisolated two-stage PV microinverter application.

Superhydrophobic condenser surfaces for air gap membrane distillation

Superhydrophobic surfaces for enhanced condensation in air gap membrane distillation (AGMD) may provide significantly improved distillate production rates and increased thermal efficiency. While AGMD is one of the most thermally efficient membrane distillation desalination configurations, large transport resistances in the air gap limit distillate production rates. AGMD experiments were performed with combinations of untreated, hydrophobic, and superhydrophobic condensation surfaces. A nanostructured copper oxide coated condensing surface produced durable 164°±4° contact angles and jumping droplet condensation. Tests were also performed on the air gap spacer, in this case a small diameter support mesh, to judge the effects of superhydrophobic treatment and conductivity on distillate production for AGMD. A novel visualization technique was implemented to see through PVDF membranes and confirm air gap behavior. The experiments were compared with numerical modeling of AGMD film-wise condensation and flooded-gap MD. The results indicate that the introduction of superhydrophobic surfaces can result in improvements in distillate production in excess of 60% over standard AGMD. However, for high distillate production, condensation on the superhydrophobic plate transitions from a partially wetted droplet morphology to Wenzel flooded (wetting) conditions. Mildly hydrophobic condensing surfaces were found to provide moderate improvement in distillate production. Superhydrophobic support meshes made a negligible difference in distillate production, but high conductivity support meshes showed significant increases in flux at the expense of increased conduction losses. The results outline recommended superhydrophobic condensation conditions at varied feed and cold side temperatures for substantial improvement to distillate production rate for AGMD systems in a flat plate configuration.

The Cryogenic Testing and Characterisation of SiC Diodes

High-resolution cryogenic performance testing is carried out on 4H-SiC PiN and Schottky diodes. At 2K intervals from 20 to 320K, current-voltage tests are performed to extract static characteristics such as turn-on, ideality factor and barrier height from across the temperature range. We also analyse the performance of the diodes within a low current/voltage switching circuit across the same temperature range using an inductive switching setup. Both diodes suffer markedly increased conduction losses at the lower temperatures, the PiN diode losing all the benefits of conductivity modulation as dopant freezes-out, reducing its series resistance. However, minor gains in the total switching losses are expected at low temperature due to faster switching speeds.

On the Causes of Circulating Currents in PWM Drives With Open-End Winding AC Machines

Electric drives with open-end winding ac machines offer certain advantages over drives with star- or delta-connected machines. Such drives have been recently considered for some applications such as electric vehicles. Circulating currents have been experimentally observed in such open-end winding ac drives. These currents have the effect of increasing conduction losses in the stator winding resistance of the ac machine. Two major causes for these currents have been identified as device voltage drops in power semiconductor devices and dead time used for shoot-through current protection. These effects are analyzed, and a mathematical description for predicting their severity is provided. Experimental results are presented that support the theory. The theory developed should be useful for devising solutions for this problem such as the design of common-mode filters. Two methods of suppressing the circulating currents are also outlined, and their experimental results are presented.

Open-Winding Power Conversion Systems Fed by Half-Controlled Converters

An open-winding power conversion system with two conventional six-switch voltage-source converters (VSC) affords operation with a lower volt-ampere (VA) rating of each device for a given power rating, as well as a degree of fault tolerance. The disadvantages of such a configuration include a higher total device VA rating and increased conduction loss as compared to a single Y/Δ-connected VSC. In certain ac-dc applications, such as telecommunications, wind, and aerospace generator drives, it is desired to have regulated input currents and output voltage, but regenerative operation is either not required or prohibited. For such applications, an alternative open-winding power converter is proposed in this paper where half-controlled converter (HCC) is employed at each end of an open-winding structure. The resultant total switch count and total VA rating are reduced by half, compared to using full bridges. Besides, the basic advantages of an open-winding configuration, use of HCCs also guarantees immunity to dc-bus shootthrough and simplifies the gate drive circuit. The total VA rating of the proposed topology is found to be 42% less than a six-switch VSC. The operating principle, control method, and analysis with simulation and experimental results of the proposed topology in both a grid-tied rectifier application and a PM generator application are illustrated in this paper.

Feed-Forward Compensator of Operating Frequency for APWM HB Flyback Converter

A feed-forward frequency compensator for an asymmetrical pulsewidth modulated (APWM) half-bridge (HB) flyback converter is proposed to increase its power conversion efficiency. Under a variable input voltage, the APWM HB flyback converter cannot be satisfied with the zero-current switching (ZCS) condition of output rectifiers because the variation of the turn-ON duration of main switches, which regulate the output voltage, induces their improper turn-OFF duration. This means that the resonance of a primary current during the turn-OFF of the switches is out of the ZCS operation of the output rectifiers. To avoid this improper operation, the resonant network should be designed with a marginal turn-OFF duration that guarantees the ZCS. However, this makes the converter operate with poor efficiency because of increasing conduction losses. The proposed feed-forward frequency compensator can properly change the converter's operating frequency to reduce the conduction losses according to the variation of the input voltage satisfying the ZCS condition of the output rectifiers. Operation principles, steady-state analysis, and soft-switching conditions are analyzed to derive the frequency compensation algorithm of the APWM HB flyback converter. Experimental results using a 240-W prototype converter show that the proposed compensation algorithm can improve the power conversion efficiency of the APWM HB flyback converter under the input-voltage variation.