Power Conversion Applications Research Articles

Design of radial turbomachinery for supercritical CO2 systems using theoretical and numerical CFD methodologies

In high temperature waste heat to power conversion applications, bottoming thermodynamic cycles using carbon dioxide in supercritical phase (sCO2) have recently become a promising developing technology that could outperform conventional Organic Rankine Cycle systems in terms of efficiency and compactness. Moreover, carbon dioxide is a fluid chemically stable, reliable, low-cost, non-toxic, non-flammable and readily available. Supercritical CO2 power generation systems have been investigated by scientists and engineers mostly for large scale applications. However, when the electrical target output power is lower (50-100 kW), there are additional challenges on the turbomachinery design that need to be addressed. In the current research work, with reference to simple regenerative cycle architecture, the design of small scale sCO2 radial compressor and turbine are firstly addressed through the similarity approach. Further to this study, numerical CFD simulations are performed to optimize the 3D design of the impellers and of the stators. In particular, steady state RANS simulations using the mixing plane approach are carried out taking into account real gas properties for CO2.

Low grade thermal recovery based on trilateral flash cycles using recent pure fluids and mixtures

The current work presents a thermodynamic analysis of a Trilateral Flash Cycle (TFC) system for low grade heat to power conversion applications. Novel aspects of the research are the usage of rotary positive displacement expanders as prime movers of the TFC system as well as the reference to working fluids and their mixtures at the state of the art. In particular, the role of a correct built-in volume ratio of the expander with respect to the pressure ratio of the thermodynamic cycle is emphasized. In fact, a mismatching of these two quantities would lead to an isochoric expansion process which, in turn, might negatively affect the overall power recovery. With reference to a transcritical CO2 stream at 100°C as heat source for the TFC system, parametric and screening studies were carried out using different expander built-in volume ratios and working fluids respectively. Among the fluids analyzed, results showed that pure substances such as R1234ze(E) and propane would provide a greater specific work but, on the other hand, would require built-in volume ratios (8 and 14) that are beyond the capabilities of rotary positive displacement expanders (5). The addition of CO2 to the afore mentioned working fluids would ease the mismatching issue but, at the same time, would reduce the specific power output. Regarding the built-in volume ratio analysis, it was found that optimal values change in accordance to the working fluid and refer to an expansion process with a slight isochoric phase.

Au-Free AlGaN/GaN MIS-HEMTs With Embedded Current Sensing Structure for Power Switching Applications

AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) have become a promising candidate for use in efficient power conversion applications. In order to realize converter circuit control function and overcurrent protection of device itself, we have designed, fabricated, and experimentally-measured the Au-free AlGaN/GaN MIS-HEMTs with embedded current sensing structure. A floating ohmic current sensing electrode is inserted between source and gate electrode of which the sensing voltage signal can represent the drain current. We have achieved stable current sensing ratios at various operating conditions including quasi-static, transient state, and under high temperature. The proposed structure is highly useful in monolithic power integrated circuit on CMOS-compatible AlGaN/GaN technologies.

Advanced power cycler with intelligent monitoring strategy of IGBT module under test

Power cycling (PC) test is one of the important test methods to assess the reliability performance of power device modules related to packaging technology, in respect to temperature stress. In this paper, an advanced power cycler with a real-time VCE_ON and VF measurement circuit for the IGBT and diode, which for the wear-out condition monitoring are presented. This advanced power cycler allows to perform power cycling test cost-effectively under conditions close to real power converter applications. In addition, an intelligent monitoring strategy for the separation of package-related wear-out failure mechanisms has been proposed. By means of the proposed method, the wear-out failure mechanisms of an IGBT module can be separated without any additional efforts during the power cycling tests. The validity and effectiveness of the proposed monitoring strategy are also verified by experiments.

Multicolor emission from intermediate band semiconductor ZnO1\u2212xSex

Photoluminescence and photomodulated reflectivity measurements of ZnOSe alloys are used to demonstrate a splitting of the valence band due to the band anticrossing interaction between localized Se states and the extended valence band states of the host ZnO matrix. A strong multiband emission associated with optical transitions from the conduction band to lower E− and upper E+ valence subbands has been observed at room temperature. The composition dependence of the optical transition energies is well explained by the electronic band structure calculated using the kp method combined with the band anticrossing model. The observation of the multiband emission is possible because of relatively long recombination lifetimes. Longer than 1 ns lifetimes for holes photoexcited to the lower valence subband offer a potential of using the alloy as an intermediate band semiconductor for solar power conversion applications.

PCB Embedded Semiconductors for Low-Voltage Power Electronic Applications

Power conversion applications in the low voltage (LV) range (≤ 1.2 kV)—such as three-phase inverters—are required to operate at higher efficiencies, higher ambient temperatures, increasingly smaller form factor, and higher power density. Up to now, most research has focused on voltages up to 650 V for printed circuit board (PCB) embedded power electronics. This research evaluates a novel three-phase invertor module based on six insulated gate bipolar transistors and six diodes rated to 1.2 kV and 25 A each. This unique module is compared to the Semikron MiniSKiiP 23AC126V1. This paper considers some key details of the PCB embedding assembly process, a comparative switching performance assessment, measurement of thermal resistance, comparative lifetime, and electric insulation. First, a detailed outline of the package is presented including the top- and bottom-side metallization and the copper interconnect technology. The switching performances of both modules are compared for turn-ON and turn-OFF currents for a waveform at 600 V and 25 A at 150 °C. A finite-element-method thermal simulation demonstrates up to 44% lower thermal resistance for the PCB embedded package than that of the traditional wire-bonded direct bonded copper (DBC) package for an identical applied current and cooling condition. Furthermore, both packages are active power cycled to failure with the PCB embedded package demonstrating superior lifetime to the traditional DBC module. Finally, the maximum breakdown limit and the onset of partial discharge with the embedded PCB module are reported for both aged and non-aged conditions. The overall findings identify the promising application of PCB embedded power electronics for LV power conversion.

Condition Parameter Estimation for Photovoltaic Buck Converters Based on Adaptive Model Observers

DC–DC power converters such as buck converters are susceptible to degradation and failure due to operating under conditions of electrical stress and variable power sources in power conversion applications, such as electric vehicles and renewable energy. Some key components such as electrolytic capacitors degrade over time due to evaporation of the electrolyte. In this paper, a model-observer based scheme is proposed to monitor the states of Buck converters and to estimate their component parameters, such as capacitance and inductance. First, a diagnosis observer is proposed, and the generated residual vectors are applied for fault detection and isolation. Second, component condition parameters, such as capacitance and inductance are reconstructed using another novel observer with adaptive feedback law. Additionally, the observer structures and their theoretical performance are analyzed and proven. In contrast to existing reliability approaches applied in buck converters, the proposed scheme performs online-estimation for key parameters. Finally, buck converters in conventional dc–dc step-down and photovoltaic applications are investigated to test and validate the effectiveness of the proposed scheme in both simulation and laboratory experiments. Results demonstrate the feasibility, performance, and superiority of the proposed component parameter estimation scheme.

Design, fabrication and application of organic power converters: Driving light-emitting electrochemical cells from the AC mains

The design, fabrication and operation of a range of functional power converter circuits, based on diode-configured organic field-effect transistors as the rectifying unit and capable of transforming a high AC input voltage to a selectable DC voltage, are presented. The converter functionality is demonstrated by selecting and tuning its constituents so that it can effectively drive a low-voltage organic electronic device, a light-emitting electrochemical cell (LEC), when connected to high-voltage AC mains. It is established that the preferred converter circuit for this task comprises an organic full-wave rectifier and a regulation resistor but is void of a smoothing capacitor, and that such a circuit connected to the AC mains (230 V, 50 Hz) successfully can drive an LEC to bright luminance (360 cd m−2) and high efficiency (6.4 cd A−1).

Technology and Reliability of Normally-Off GaN HEMTs with p-Type Gate

GaN-based transistors with p-GaN gate are commonly accepted as promising devices for application in power converters, thanks to the positive and stable threshold voltage, the low on-resistance and the high breakdown field. This paper reviews the most recent results on the technology and reliability of these devices by presenting original data. The first part of the paper describes the technological issues related to the development of a p-GaN gate, and the most promising solutions for minimizing the gate leakage current. In the second part of the paper, we describe the most relevant mechanisms that limit the dynamic performance and the reliability of GaN-based normally-off transistors. More specifically, we discuss the following aspects: (i) the trapping effects specific for the p-GaN gate; (ii) the time-dependent breakdown of the p-GaN gate during positive gate stress and the related physics of failure; (iii) the stability of the electrical parameters during operation at high drain voltages. The results presented within this paper provide information on the current status of the performance and reliability of GaN-based E-mode transistors, and on the related technological issues.

Partial discharges in twisted-pair magnet wires subject to multilevel PWM pulses

Researchers and designers of insulation systems are facing new challenges due to the ubiquitous presence of power electronics devices in power conversion applications across various voltage levels. The stresses applied to insulation systems by power electronic devices require new approaches for evaluating the intensity and frequency content of working electric field strength and degradation processes. Typically these parameters are assessed through the analysis of partial discharges. Multilevel converters are increasingly being implemented in industrial applications due to the advantages they offer for example in efficiency, harmonic distortion and filtering strategies. This paper presents an analysis of partial discharges in a magnet twisted-pair wires subjected to high voltage pulse-width-modulated pulses obtained from a multilevel converter. A comparison between the number of levels and modulation frequencies on partial discharge inception and intensity are presented. As an assessment criterion, the voltage-time product above partial discharge inception level was compared.

Distributed Control and Redundant Technique to Achieve Superior Reliability for Fully Modular Input-Series-Output-Parallel Inverter System

Input-series-output-parallel (ISOP) inverter system is very suitable for high input voltage and large output current power conversion applications. One of many merits of this assembly system lies in that its characteristic of multimodule series–parallel combination can significantly improve the reliability of the operation. To address this point, redundancy should be realized for the whole system. However, the existing methods for the ISOP inverter system all belong to centralized control, which restricts the modularity of the system. From the above perspective, this paper proposes a new scheme to achieve both power balance and distributed configuration according to the conception of compound control. Also, the relationship of control loops is analyzed and the design procedure of them is given. Based on the fully modular system actualized by the distributed control, the hot-swap technique is then raised to get a redundant system with superior reliability. Here, the way of bypassing other than cutting off is adopted to fulfill withdrawal of the faulty module from the system due to series connection at the input terminal. In addition, the detailed timing sequence of system operation is provided to ensure the smooth transition during the hot-plugging transient. Finally, a three-module prototype is built and the experimental results validate the effectiveness of the presented strategy.

Research on Calibration Method of 7kV Single Voltage Pulse for IGBT Models Test System

Power IGBT module is widely used in high power conversion applications in various fields because of the advantages of high voltage and high current applications. In order to avoid the influence of temperature rise to the device measurement, pulse test method is widely used to test its static and dynamic parameters, So there are single high pulse voltage source and single pulse high current source in power IGBT modules test instruments. For now in the metrology field, how to calibrate its Single High Pulse Voltage source is a difficult problem. Now the amplitude of single high pulse voltage source is above 7kV with the pulse width as 50 microseconds. Based on detailed research on the test principles of the power IGBT modules test instruments, the pulse high voltage divider and the data acquisition unit are used to setup a calibration device for Single High Pulse Voltage source to calibrate the amplitude of Single High Pulse Voltage source. Resistors are usedto develop the pulse high voltage divider, and did carefully research on the pulse response time, voltage dispersion, voltage regulation of the pulse high voltage divider. Also the data acquisition unit with 20MHz bandwidth and 100MS/s acquisition rate is evaluated through tests including vertical accuracy test, bandwidth test, rise time test, comparison test and etc.. Through above relevant test results, with the comprehensive consideration of the influence of each measurement uncertainty component, It have finished that the evaluation of the uncertainties of the amplitude measurement for Single High Pulse Voltage source. The uncertainty of measurement is better than 2% with the coverage factor as 2.

Novel Frequency Swapping Technique for Conducted Electromagnetic Interference Suppression in Power Converter Applications

Quasi-resonant flyback (QRF) converters have been widely applied as the main circuit topology in power converters because of their low cost and high efficiency. Conventional QRF converters tend to generate higher average conducted electromagnetic interference (EMI) in the low-frequency domain due to the switching noise generated by power switches, resulting in the fact they can exceed the EMI standards of the European Standard 55022 Class-B emission requirements. The presented paper develops a novel frequency swapping control method that spreads spectral energy to reduce the amplitude of sub-harmonics, thereby lowering average conducted EMI in the low-frequency domain. The proposed method is implemented in a control chip, which requires no extra circuit components and adds zero cost. The proposed control method is verified using a 24 W QRF converter. Experimental results reveals that conducted EMI has been reduced by approximately 13.24 dBμV at 498 kHz compared with a control method without the novel frequency swapping technique. Thus, the proposed method can effectively improve the flyback system to easily meet the CISPR 22/EN55022 standards.

Design of power integrated circuits in full AlGaN/GaN MIS‐HEMT configuration for power conversion

In order to examine the feasibility of full wide‐bandgap GaN‐based converters in aerospace power conversion applications, this paper proposes a monolithic DC–DC buck converter design with integrated high‐side gate driver, over‐current protection, and pulse‐width‐modulation feedback control circuits based on full AlGaN/GaN MIS‐HEMT configuration. After model calibration of the DC and transient behaviors with fabricated normally‐ON and normally‐OFF AlGaN/GaN MIS‐HEMT devices, the DC–DC buck converter is simulated. The circuit converts the input 100 V down to an adjustable range at 1 MHz switching frequency. The over‐current protection function can properly protect the converter at a preset over‐current threshold. The converter can respond to the load current and line input voltage fluctuations due to the integrated feedback control. These results illustrate the performance of proposed all‐GaN DC–DC power converter design.

Comparison of Impedance-Source Networks for Two and Multilevel Buck–Boost Inverter Applications

Impedance-source networks are an increasingly popular solution in power converter applications, especially in single-stage buck-boost power conversion to avoid additional front-end dc-dc power converters. In the survey papers published, no analytical comparisons of different topologies have been described, which makes it difficult to choose the best option. Thus, the aim of this paper is to present a comprehensive analytical comparison of the impedance-source-based buck-boost inverters in terms of passive component count and semiconductor stress. Based on the waveform of the input current, i.e., with or without a transformer, and with or without inductor coupling, the impedance-source converters are classified. The main criterion in our comprehensive comparison is the energy stored in the passive elements, which is considered both under constant and predefined high frequency current ripple in the inductors and the voltage ripple across the capacitors. Two-level and multilevel solutions are described. The conclusions provide a “one-stop” information source and a selection guide of impedance-source-based buck-boost inverters for different applications.

Magnetic Multilayer Fabrication Technology with Selective Activation of SU-8 Films

Developing magnetic multilayers are essential for reducing the core eddy current losses in the integrated power magnetic components (inductors/transformers). PVD based processes are typically used to achieve the multilayers with thin dielectric spacers. However, those processes are costly, and can be difficult to integrate. It is evident that cost effective alternative is needed. In recent years, electrochemical processes have been investigated to address these issues. One such method would be to successive metallization of insulating photoresists acting as spacer layer (such as SU-8) with soft magnetic films (such as Ni-Fe-Co alloys). This paper describes an experimental procedure to fabricate magnetic multilayers with a thin variant of SU-8 2 (<1.5 μm) as inter-layers for integrated micro-inductors/transformers for power conversion applications.

Three‐winding transformer based asymmetrical dual active bridge converter

A three-winding transformer based asymmetrical dual active bridge (TWT-ADAB) isolated dc–dc converter is proposed in this study. The converter comprises of three stages: primary, isolation, and secondary. The primary stage consists of an H-bridge converter, the isolation stage consists of a three-winding transformer with one primary and two secondary windings, and the secondary stage consists of a bridge converter having three legs and six active switches. The load current is shared equally by the secondary windings of the transformer resulting in reduced currents through switches of the secondary converter. Therefore, the turn-off losses of these switches get reduced. Further, zero voltage switching turn-on can be achieved for all switches of the TWT-ADAB converter to reduce the switching losses. A detailed analysis of the modelling and control of the proposed TWT-ADAB converter is presented. The theoretical analysis is verified using the simulation and experimental results. These demonstrated results show that the proposed TWT-ADAB converter offers wider ranges of the output voltages and powers and higher efficiency as compared with the existing dual bridge isolated dc–dc converters. Therefore, it can be an eminent structure for high-power isolated dc–dc power conversion applications.

High step‐up/step‐down non‐isolated BDC with built‐in DC‐transformer for energy storage systems

A non-isolated bidirectional DC-DC converter (BDC) is proposed for high step-up/step-down bidirectional power conversion applications. A DC-transformer is integrated into a conventional non-isolated buck-boost BDC to achieve high-voltage conversion ratio and wide voltage/power range regulation simultaneously. The switches are shared by the buck-boost BDC and the DC-transformer. Voltage stresses of switches are reduced by connecting the input and output of the DC-transformer in-series. The duty cycles of switches are used to make sure that the voltages on the two sides of the DC-transformer are matched. As a result, the DC-transformer always operates under the highest-efficiency condition. The phase-shift angle between the switching bridges is employed to achieve the power flow regulation. Soft-switching of all the switches is achieved by adopting the pulse-width modulation plus phase-shift control strategy. Operation principles, characteristics and design considerations of the proposed BDC are analysed in detail. Experimental results from a 48 V/400 V-1 kW prototype verify the effectiveness of the proposed converter. The efficiency is demonstrated to peak at 96.6% for both the step-up and step-down modes.

High-Temperature SiC CMOS Comparator and op-amp for Protection Circuits in Voltage Regulators and Switch-Mode Converters

This paper describes a high temperature voltage comparator and an operational amplifier (op-amp) in a 1.2- $\mu \text{m}$ silicon carbide (SiC) CMOS process. These circuits are used as building blocks for designing a high-temperature SiC low-side over current protection circuit. The over current protection circuit is used in the protection circuitry of a SiC FET gate driver in power converter applications. The op-amp and the comparator have been tested at 400 °C and 550 °C temperature, respectively. The op-amp has an input common-mode range of 0–11.2 V, a dc gain of 60 dB, a unity gain bandwidth of 2.3 MHz, and a phase margin of 48° at 400 °C. The comparator has a rise time and a fall time of 38 and 24 ns, respectively, at 550 °C. The over current protection circuit, implemented with these analog building blocks, is designed to sense a voltage across a sense resistor up to 0.5 V.

Practical Design of Dual Active Bridge Converter as Isolated Bi-directional Power Interface for Solid State Transformer Applications

As a bi-directional isolated interface for solid state transformer applications, practical design considerations of a dual active bridge (DAB) converter are proposed by means of a detailed mathematical model based on the converter’s steady state operations. The DAB converter is useful in isolated bi-directional power conversion applications due to high performance, high efficiency, and bi-directional control manner. However, design considerations should be taken into account to overcome the disadvantages of DAB converter, such as low efficiency caused by restricted soft switching capability at light load conditions and high circulating current at heavy load conditions. The practical design considerations of the converter’s power stage will be discussed to maximize soft switching capability and to minimize the conduction losses. In addition, to reduce the current stress of the power devices during a cold start sequence, an effective soft start algorithm will be proposed. Experimental results with a 3.3 kW prototype DAB converter will be given in order to validate the effectiveness of the proposed design considerations.