UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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Item GaN-based High-Frequency Isolated Single-Stage AC-DC Converters for More Electric Aircrafts(2021) Singh, Akshay; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There has been an increased focus on the electrification of aircrafts, with the objective of improving overall system efficiency, weight and reliability. Power electronics is a key enabling technology for this transition, and there is a greater emphasis on the design of lightweight and efficient power electronic interfaces. Traditionally, the generation of the low-voltage 28 V DC bus from the high voltage variable frequency output of the turbine generators has been performed using passive diode-bridge rectifier systems, known as transformer rectifier units (TRU). Compared to active power electronic interfaces, TRUs have higher weight, lower efficiency, and inferior voltage regulation. This work proposes an active power electronic converter architecture to replace the TRU, referred to as the Regulated Transformer Rectifier Unit (RTRU). The proposed RTRU converter topology and control are specifically formulated to harness the advantages of wide-bandgap Gallium Nitride (GaN) power transistors. The system comprises three modular single-stage high step-down isolated AC-DC converters based on the Dual Active Bridge (DAB) circuit. The modular design allows for improved failure-tolerant operation, resulting in increased overall reliability which is critical for aircraft applications. The proposed DAB AC-DC converter achieves the functions of power factor correction and isolated voltage step-down with soft switching in a single power stage, thus eliminating the bulky intermediate DC-link capacitor typically associated with two-stage converter topologies. Furthermore, the three-phase converter architecture allows for automatic pulsating power cancellation at the output DC port. In the first part of this work, the suitability of the single-stage converter topology for a modular RTRU architecture is established through a comprehensive analytical comparison framework that considers the volume and efficiency tradeoffs for all passive components, including heat sinks. On the modeling aspect, the steady-state operation of the DAB AC-DC converter can show a high dependence on the circuit non-idealities and on the transient nature of the consistently changing phase shifts necessary to achieve AC-DC operation. These aspects are not fully captured using traditional modeling approaches derived for the DC-DC DAB converter. To address these issues, an improved unified modeling approach is presented – comprising of hybrid frequency and time-domain analyses that encompass the transient nature of the AC-DC converter while providing the advantages of highly generalized steady-state frequency-domain analysis. The proposed modeling approach demonstrates a significant reduction in modeling inaccuracies, which in turn lead to more accurate tracking of optimal operating points (i.e. higher efficiency) and improved power quality. In the second part of this work, the low passive component requirement of the single-stage topology is harnessed to develop a power-dense converter design featuring a compact, high-efficiency planar integrated magnetic structure with adjustable leakage inductance. The detailed modeling, design, and optimization of the planar magnetics are presented, with a special focus on unique PCB winding layouts to achieve low AC resistance in high step-down high-current applications. Moreover, the use of paralleled GaN transistors in high current applications presents several challenges with regards to current sharing and conduction loss optimization, which are addressed by a new design approach presented in this work, that leads to optimized layouts with low parasitics. Lastly, a holistic design process is formulated to analytically estimate the differential-mode (DM) conducted emissions in a DAB AC-DC converter, which is then coupled with a multi-objective EMI filter optimization algorithm to minimize the DM EMI filter weight and converter losses. Through these improvements, the developed hardware prototype achieves a 40% higher power density than the existing state-of-the-art. In the third part of this work, the proposed modeling approach is combined with a numerical optimization routine is proposed to find the optimal-conduction-loss modulation trajectories. A hybrid closed-loop control method with offline-generated feedforward lookup tables is subsequently realized for optimal loss tracking over the entire operation range, while satisfying the stringent transient operating requirements for airborne equipment. The implementation of the closed-loop control for the multi-phase modular RTRU with variable input frequency and variable switching frequency is carried out on a single microcontroller with parallelized execution. Finally, to verify the modeling, design, optimizations, and control methods, a 5 kW 230 V – 28 V fully-GaN based RTRU is developed as a hardware proof-of-concept, which achieves a peak efficiency of 96.8% and a power density of 1.2 kW/L, and satisfies the power quality and transient requirements for airborne equipment.Item ADVANCED PACKAGING AND THERMAL MANAGEMENT OF DC-DC CONVERTERS AND NOVEL CORRELATIONS FOR MANIFOLD MICROCHANNEL HEATSINKS(2021) Yuruker, Sevket Umut; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An advanced packaging configuration of a dual-active-bridge 10 kW DC-DC converter module is introduced in this dissertation. Through utilization of novel heatsinks for the power switches and the transformer assembly, ~20 kW/Lit converter volumetric power density based on numerical and experimental analysis is obtained. Through a unique placement of the high power/high frequency SiC switches on the printed circuit board, many beneficial features such as double-sided cooling, complete elimination of wirebonding, and circumvention of the need for TIM layers between the switches and the heatsinks, and multi functioning heatsinks as electrical busbars is achieved. A Vertically Enhanced Manifold Microchannel System (VEMMS) cooler is developed to address the thermal challenges of a pair of power switches, simultaneously. Both air and liquid cooled versions of VEMMS cooler is presented, thermal resistances of 1.1 K/W and 0.3 K/W for the air and liquid cooled versions, respectively, at reasonable flow rates and pressure drops was obtained. Besides the power switches, thermal management of the transformer assembly is accomplished via Combined Core and Coil (C3) Coolers, where both the magnetic core and coils are liquid cooled simultaneously with electrically insulating but thermally conductive 3D printed Alumina heatsinks, where thermal resistances as low as 0.3 K/W for the magnetic core and 0.09 K/W for the transformer windings is experimentally demonstrated. Furthermore, a system level model was built to investigate the effect of various components in the cooling loop on each other, and what are the limiting factors to prevent a possible thermal runaway failure. Lastly, using a metamodeling approach, closed form pressure drop and heat transfer correlations are developed for thermo-fluidic performance prediction of manifold microchannel heatsinks. Due to complexity and vastness of design variables present in manifold microchannel systems, adequate CFD analysis and optimization require significant computational power. Through utilization of the developed correlations, orders of magnitude reduction in computational time (from days to milliseconds) in prediction of pressure drop and heat transfer coefficient is demonstrated. Extensive mesh independence and residual convergence algorithms are developed to increase accuracy of the created database. Between the correlation and mesh independent CFD results, a mean error of 3.9% and max error of 24% for Nusselt number, and a mean error of 4.6% and max error of 37% for Poiseuille Number predictions are achieved.Item A GALLIUM NITRIDE INTEGRATED ONBOARD CHARGER(2020) Zou, Shenli; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Compared to Silicon metal–oxide–semiconductor field-effect transistors (MOSFETs), Gallium Nitride (GaN) devices have a significant reduction in gate charge, output capacitance, and zero reverse recovery charge, enabling higher switching frequency operation and efficient power conversion. GaN devices are gaining momentum in power electronic systems such as electric vehicle (EV) charging system, due to their promises to significantly enhance the power density and efficiency. In this dissertation, a GaN-based integrated onboard charger (OBC) and auxiliary power module (APM) is proposed for EVs to ensure high efficiency, high frequency, high power density, and capability of bidirectional operation. The high switching frequency operation enabled by the GaN devices and the integration of OBC and APM bring many unique challenges, which are addressed in this dissertation. An important challenge is the optimal design of high-frequency magnetics for a high-frequency GaN-based power electronic interface. Another challenge is to achieve power flow management among three active ports while minimizing the circulating power. Furthermore, the impact of circuit layout parasitics could significantly deteriorate the system interface, due to the sensitivity of GaN device switching characteristics. In this work, the aforementioned challenges have been addressed. First, a comprehensive analysis of the front-end AC-DC power factor correction stage is presented, covering a detailed magnetic modeling technique to address the high-frequency magnetics challenge. Second, the modeling and control of a three-port DC-DC converter, interfacing the AC-DC stage, high-voltage traction battery and low-voltage battery, are discussed to address the power flow challenge. Advanced control methodologies are developed to realize power flow management while maintaining minimum circulating power and soft switching. Furthermore, a new three-winding high-frequency transformer design with improved power density and efficiency is achieved using a genetic-algorithm-based optimization approach. Finally, a GaN-based integrated charger prototype is developed to validate the proposed theoretical hypothesis. The experimental results showed that the GaN-based charging system has the capability of achieving simultaneous charging (G2B) of both HV and LV batteries with a peak efficiency of 95%.Item INTEGRATED INDUCTIVE AND CONDUCTIVE CHARGING SYSTEM FOR ELECTRIC VEHICLES(2019) Uma Sankar, Arun Sankar Uma; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The global electric vehicle (EV) market acceleration is facilitated by supporting policies deployed by governments and cities to reap multiple benefits in the fields of transport decarbonization, air pollution reduction, energy efficiency, and security. Currently, conductive chargers are a customary method of storing electric energy into the storage elements present onboard of an EV which is inadequate in supporting complete autonomy. The thriving inclination towards the design of autonomous vehicles has shaped wireless charging as an attractive solution in favor of complete autonomy. As long as the wireless charging infrastructure, as well as interoperability standards, are not completely developed, wired and wireless chargers have to co-exist onboard the vehicles for user convenience. Incorporation of an entire parallel wireless charging system on-board an EV, either during manufacturing or after-market increases size, weight, or cost while declining the electric range of the vehicle. The current requisite for multiple on-board charging options motivate the necessity for a solution for efficiently integrating wired and wireless charging systems. In this Ph.D. research, we propose multiple charging architectures capable of integrating inductive and conductive charging systems. The proposed architectures merge the output rectifying stage of an inductive charging system to the existing on-board charger eliminating the additional weight and volume associated with a wireless charger. Since the proposed system involves multiple power conversion stages, a system level study is carried out to select feasible topologies capable of maximizing the efficiency of an integrated system. Additionally, an extended harmonic approximation (EHA) technique is introduced to increase the accuracy of a resonant converter model facilitating the optimized design parameter selection of an inductive charging system. Furthermore, a novel analog synchronous rectification circuit is proposed and designed to enable active rectification maximizing power transfer efficiency. For proof of concept verification, a laboratory prototype of a 3.3kW Silicon Carbide (SiC) based integrated wireless charger is developed that can be interfaced to a variable input voltage (85-265 Vrms) 50/60Hz AC grid. According to the experimental measurements, the charger draws an input current with a total harmonic distortion of 1.3% while achieving an overall efficiency of 92.77% at rated output power.Item An Integrated Single-phase On-board Charger(2019) Lu, Jiangheng; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With the growing demand for transportation electrification, plug-in electric vehicles (PEVs), and plug-in hybrid electric vehicles (PHEVs), cumulatively called electric vehicles (EVs) are drawing more and more attention. The on-board charger (OBC), which is the power electronics interface between the power grid and the high voltage traction battery, is an important part for charging EVs. Besides the OBC, every EV is equipped with another separate power unit called the auxiliary power module (APM) to charge the low voltage (LV) auxiliary battery, which supplies all the electronics on car including audio, air conditioner, lights and controllers. The main target of this work is a novel way to integrate both units together to achieve a charger design that is not only capable of bi-directional operation with high efficiency, but also higher gravimetric and volumetric power density, as compared with those of the existing OBCs and APMs combined. To achieve this target, following contributions are made: (i) a three-port integrated DC/DC converter, which combines OBC and APM together through an innovative integration method; (ii) an innovative zero-crossing current spike compensation for interleaved totem pole power factor correction (PFC) and (iii) a new phase-shift based control strategy to achieve a regulated power flow management with minimum circulating losses.Item REGULATED TRANSFORMER RECTIFIER UNIT FOR MORE ELECTRIC AIRCRAFTS(2018) MALLIK, AYAN; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The impending trends in the global demand of more-electric-aircrafts with higher efficiency, high power density, and high degree of compactness has opened up numerous opportunities in front of avionic industries to develop innovative power electronic interfaces. Traditionally, passive diode-bridge based transformer rectifier units (TRU) have been used to generate a DC voltage supply from variable frequency and variable voltage AC power out of the turbo generators. These topologies suffer from bulky and heavy low-frequency transformer size, lack of DC-link voltage regulation flexibility, high degree of harmonic contents in the input currents, and additional cooling arrangement requirements. This PhD research proposes an alternative approach to replace TRUs by actively controlled Regulated Transformer Rectifier Units (RTRUs) employing the advantages of emerging wide band gap (WBG) semiconductor technology. The proposed RTRU utilizing Silicon Carbide (SiC) power devices is composed of a three-phase active boost power factor correction (PFC) rectifier followed by an isolated phase-shifted full bridge (PSFB) DC-DC converter. Various innovative control algorithms for wide-range input frequency operation, ultra-compact EMI filter design methodology, DC link capacitor reduction approach and novel start-up schemes are proposed in order to improve power quality and transient dynamics and to enhance power density of the integrated converter system. Furthermore, a variable switching frequency control algorithm of PSFB DC-DC converter has been proposed for tracking maximum conversion efficiency at all feasible operating conditions. In addition, an innovative methodology engaging multi-objective optimization for designing electromagnetic interference (EMI) filter stage with minimized volume subjected to the reactive power constraints is analyzed and validated experimentally. For proof-of-concept verifications, three different conversion stages i.e. EMI filter, three-phase boost PFC and PSFB converter are individually developed and tested with upto 6kW (continuous) / 10kW (peak) power rating, which can interface a variable input voltage (190V-240V AC RMS) variable frequency (360Hz – 800Hz) three-phase AC excitation source, emulating the airplane turbo generator and provide an AC RMS voltage of 190V to 260V. According to the experimental measurements, total harmonic distortion (THD) as low as 4.3% and an output voltage ripple of ±1% are achieved at rated output power. The proposed SiC based RTRU prototype is ~8% more efficient and ~50% lighter than state-of-the art TRU technologies.Item Novel Materials and Structures for Wide and Ultra-Wide Bandgap Semiconductor Switches(2018) Shahin, David Issa; Christou, Aristos; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Semiconductor power switches are necessary for the deployment of next-generation electrical systems, including renewable energy generators, electric vehicle drivetrains, and high-power communications systems. Current silicon-based technologies are limited by insufficient blocking voltages due to bandgap limitations and processing-induced defects, undesirably high on-state resistances due to gate charge trapping at poorly understood dielectric/semiconductor interfaces, and limited reliability due to electrical and thermal failure under aggressive operating conditions. As such, new materials and device architectures are required to achieve previously unattained power, efficiency, and reliability. This dissertation identifies and investigates material candidates and demonstrates their incorporation into new device architectures for power switches. Wide bandgap (WBG) semiconductors such as GaN, and ultra-wide bandgap (UWBG) semiconductors such as Ga2O3 and diamond are employed to address the previously stated limitations. Gate charge trapping in these systems is addressed through use of high-k dielectrics not previously employed for WBG and UWBG switches. ZrO2 and HfO2 dielectrics are presented as candidates for dielectric and interface charge tuning on GaN and Ga2O3, thereby allowing the possibility of threshold voltage manipulation and normally-off behavior in WBG and UWBG switches. Fabrication technologies for WBG and UWBG switches are also reported. Normally-on and -off AlGaN/GaN MOS-HEMTs with threshold voltages between -3 to +4 V are demonstrated through a combination of ZrO2 dielectric selection and AlGaN recess etching. Design and processing for normally-off vertical GaN MOSFETs are also developed, with emphasis on critical challenges in fabricating these devices. Additionally, the fabrication and stability of hydrogen-terminated diamond switches with Al2O3 surface transfer dopants are reported. Finally, new materials and processes for improved electrical and thermal stability in power switches are demonstrated. TiN is presented as a reliable gate electrode for AlGaN/GaN HEMTs, imparting superior resistance to reverse gate bias electrical stress and temperatures up to 800 °C that otherwise destroyed conventional Ni/Au-gated HEMTs. A novel process for plasma-free selective area etching of nanocrystalline diamond heat spreading films is also presented, which promises to avoid plasma damage to the underlying semiconductor and enables etching of diamond films along features inaccessible to a typical plasma-based process.Item Methods and Models for Assessing Solder Interconnect Reliability of Control Boards in Power Electronic Systems(2013) Squiller, David; McCluskey, Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past 20 years, power electronic systems have been increasingly required to operate in harsh environments including automotive, deep-well drilling and aerospace applications. In parallel, the higher power densities and miniaturization of the power switching module result in elevated stress levels on the control circuitry. The objective of this study was to develop methods and models for assessing the interconnect reliability of components used in the control circuitry for power electronic systems. Physics-of-Failure modeling and a series of thermal and reliability simulations were conducted on a 2.2 kW variable-frequency drive to evaluate the susceptibility of system level and component level failure mechanisms. Assessment methods consisted of developing CalcePWA simulation models of the primary sub-assemblies and constructing a power cycling apparatus to perform accelerated testing of the drive.