Browsing by Author "Lai, Jih-Sheng"

Now showing 1 - 20 of 137
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    Accelerated commutation for passive clamp isolated boost converters
    (United States Patent and Trademark Office, 2005-04-05)
    An efficient and cost effective bidirectional DC/DC converter reduces switch voltage stress via accelerated commutation allowing use of a low-cost passive clamp circuit in boost mode. The converter includes a primary circuit, transformer and secondary circuit. The primary circuit takes the form of a “full bridge converter,” a “push-pull converter,” or an “L-type converter.”. The primary circuit may include a dissipator such as a snubber circuit or small buck converter. A secondary side of the transformer is momentarily shorted by the secondary circuit by, for example, turning on at least two switches in the secondary circuit simultaneously for a minimal calibratable period when a pair of primary circuit controllers turn off to protect the primary circuit switches from voltage spikes during switching conditions.
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    Adaptive Control of a Step-Up Full-Bridge DC-DC Converter for Variable Low Input Voltage Applications
    Pepa, Elton (Virginia Tech, 2004-02-06)
    This thesis shows the implementation of a novel control scheme DC-DC converter. The converter is a phase-shifted full-bridge PWM converter that is designed to operate as a front stage of a power conversion system where the input is a variable low voltage high current source. The converter is designed to step-up the low voltage input to an acceptable level that can be inverted to a 120/240 VAC 60Hz voltage for residential power. A DSP based adaptive control model is developed, taking into account line variations introduced by the input source while providing very good load dynamics for the converter in both discontinuous and continuous conduction modes. The adaptive controller is implemented using two voltage sensors that read the input and the output voltages of the converter. The controller's bandwidth is comparable to current mode control, without the need for an expensive current sensor, yet providing the noise immunity seen in voltage mode controllers. The intended input source was a fuel cell but in its absence a DC supply is utilized instead. The system is simulated for both discontinuous and continuous conduction modes and implemented and demonstrated for the continuous conduction mode. The test results are shown to match the simulation results very closely.
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    Advanced High-Frequency Electronic Ballasting Techniques for Gas Discharge Lamps
    Tao, Fengfeng (Virginia Tech, 2001-12-19)
    Small size, light weight, high efficacy, longer lifetime and controllable output are the main advantages of high-frequency electronic ballasts for gas discharge lamps. However, power line quality and electromagnetic interference (EMI) issues arise when a simple peak rectifying circuit is used. To suppress harmonic currents and improve power factor, input-current-shaping (ICS) or power-factor-correction (PFC) techniques are necessary. This dissertation addresses advanced high-frequency electronic ballasting techniques by using a single-stage PFC approach. The proposed techniques include single-stage boost-derived PFC electronic ballasts with voltage-divider-rectifier front ends, single-stage PFC electronic ballasts with wide range dimming controls, single-stage charge-pump PFC electronic ballasts with lamp voltage feedback, and self-oscillating single-stage PFC electronic ballasts. Single-stage boost-derived PFC electronic ballasts with voltage-divider-rectifier front ends are developed to solve the problem imposed by the high boost conversion ratio required by commonly used boost-derived PFC electronic ballast. Two circuit implementations are proposed, analyzed and verified by experimental results. Due to the interaction between the PFC stage and the inverter stage, extremely high bus-voltage stress may exist during dimming operation. To reduce the bus voltage and achieve a wide-range dimming control, a novel PFC electronic ballast with asymmetrical duty-ratio control is proposed. Experimental results show that wide stable dimming operation is achieved with constant switching frequency. Charge-pump (CP) PFC techniques utilize a high-frequency current source (CS) or voltage source (VS) or both to charge and discharge the so-called charge-pump capacitor in order to achieve PFC. The bulky DCM boost inductor is eliminated so that this family of PFC circuits has the potential for low cost and small size. A family of CPPFC electronic ballasts is investigated. A novel VSCS-CPPFC electronic ballast with lamp-voltage feedback is proposed to reduce the bus-voltage stress. This family of CPPFC electronic ballasts are implemented and evaluated, and verified by experimental results. To further reduce the cost and size, a self-oscillating technique is applied to the CPPFC electronic ballast. Novel winding voltage modulation and current injection concepts are proposed to modulate the switching frequency. Experimental results show that the self-oscillating CS-CPPFC electronic ballast with current injection offers a more cost-effective solution for non-dimming electronic ballast applications.
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    Advanced Integrated Single-Stage Power Factor Correction Techniques
    Zhang, Jindong (Virginia Tech, 2001-03-15)
    This dissertation presents the in-depth study and innovative solutions of the advanced integrated single-stage power-factor-correction (S2PFC) techniques, which target at the low- to medium-level power supplies, for wide range of applications, from power adapters and computers to various communication equipment. To limit the undesirable power converter input-current-harmonic's impact on the power line and other electronics equipment, stringent current harmonic regulations such as IEC 61000-3-2 have already been enforced. The S2PFC techniques have been proposed and intensively studied, in order to comply these regulations with minimal additional component count and cost. This dissertation provides a systematic study of the S2PFC input-current-shaping (ICS) mechanism, circuit topology generalization and variation, bulk capacitor voltage stress and switch current stress, converter design and optimization, and evaluation of the state-of-the-art S2PFC techniques with universal-line input. Besides, this presentation also presents the development of novel S2PFC techniques with a voltage-doubler-rectifier front end to both improve the performance and reduce the cost of S2PFC converters for (international voltage range) universal-line applications. The calculation and experimental results show that the proposed techniques offer a more cost-effective and efficient solution than industries' current practice, with universal-line input and converter power level up to 600 W. Finally, further improved technique is also presented with reduced filter inductor size and increased power density.
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    Advanced Semiconductor Device and Topology for High Power Current Source Converter
    Xu, Zhenxue (Virginia Tech, 2003-12-02)
    This dissertation presents the analysis and development of an innovative semiconductor device and topology for the high power current source converter (CSC). The CSC is very attractive in high power applications due to its lower output dv/dt, easy regeneration capability and implicit short-circuit protection. Traditionally, either a symmetrical gate turn-off (GTO) thyritor or an asymmetrical GTO in series with a diode is used as the power switch in the CSC. Since the GTO has a lower switching speed and requires a complicated gate driver, the symmetrical GTO based CSC usually has low dynamic response speed and low efficiency. To achieve high power rating, fast dynamic response speed and low harmonics, an advanced semiconductor device and topology are needed for the CSC. Based on symmetrical GTO and power MOSFET technologies, a symmetrical emitter turn-off (ETO) thyristor is developed that shows superior switching performance, high power rating and reverse voltage blocking capability. The on-state characteristics, forced turn-on characteristics, forced turn-off characteristics and the load-commutated characteristics are studied. Test results show that although the load-commutation loss is high, the developed symmetrical ETO is suitable for use in high power CSC due to its low conduction loss, fast switching speed and reverse voltage blocking capability. The snubberless turn-on capability is preferred for a semiconductor device in a power conversion system, and can be achieved for devices with forward biased safe operation area (FBSOA). The FBSOA of the ETO is investigated and experimentally demonstrated. The ETO device has excellent FBSOA due to the negative feedback provided by the emitter switch. However, the FBSOA for a large area ETO is poor. A new ETO concept is therefore proposed for future development in order to demonstrate the FBSOA over a large area device. To improve the turn-on performance of the large area ETO, a novel concept, named the transistor-mode turn-on, is proposed and studied. During the transistor-mode turn-on process, the ETO behaves like a transistor instead of a thyristor. Without a snubber, the transistor-mode turn-on for the ETO is hard to achieve. Through the selection of a proper gate drive and di/dt snubber, the transistor-mode turn-on can be implemented, and the turn-on performance for the ETO can be dramatically improved. To increase the power rating of the CSC without degrading the utilization of power semiconductor devices, a novel multilevel CSC, named the parallel-cell multilevel CSC, is proposed. Based on a six-switch CSC cell, the parallel-cell multilevel CSC has the advantages of high power rating, low harmonics, fast dynamic response and modularity. Therefore, it is very suitable for high power applications. The power stage design, modeling, control and switching modulation scheme for a parallel-cell multilevel CSC based static var compensator (STATCOM) are analyzed and verified through simulation.
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    Analysis and Design of a DCM SEPIC PFC with Adjustable Output Voltage
    Chen, Rui (Virginia Tech, 2015-03-31)
    Power Factor Correction rectifiers are widely adopted as the first stage in most grid-tied power conversion systems. Among all PFC converts for single phase system, Boost PFC is the most popular one due to simplicity of structure and high performance. Although the efficiency of Boost PFC keeps increasing with the evolution of semiconductor technology, the intrinsic feature of high output voltage may result cumbersome system structure with multiple power conversion stages and even diminished system efficiency. This disadvantage is aggravated especially in systems where resonant converters are selected as second stage. Especially for domestic induction cooker application, step-down PFC with wide range output regulation capability would be a reasonable solution, Conventional induction cooker is composed by input filter, diode-bridge rectifier, and full bridge or half bridge series resonant circuit (SRC). High frequency magnetic field is induced through the switching action to heat the pan. The power level is usually controlled through pulse frequency modulation (PFM). In such configuration, first, a bulky input differential filter is required to filter out the high frequency operating current in SRC. Second, as the output power decreases, the operating point of SRC is moved away from the optimum point, which would result large amount circulating energy. Third, when the pan is made of well conducting and non-ferromagnetic material such as aluminum, due to the heating resistance become much smaller and peak output voltage of the switching bridge equals to the peak voltage of the grid, operating the SRC at the series resonant frequency can result excessive current flowing through the switch and the heating coil. Thus for pan with smaller heating resistance, even at maximum power, the operating frequency is pushed further away from the series resonant point, which also results efficiency loss. To address these potential issues, a PFC circuit features continuous conducting input current, high power factor, step-down capability and wide range output regulation would be preferred. The Analysis and design work is present in this article for a non-isolated hard switching DCM SEPIC PFC. Due to DCM operation of SPEIC converter, wide adjustable step-down output voltage, continuous conduction of input current and elimination of reverse recovery loss can be achieved at same time. The thesis begins with circuit operation analysis for both DC-DC and PFC operation. Based on averaged switching model, small signal model and corresponding transfer functions are derived. Especially, the impact from small intermediate capacitor on steady state value are discussed. With the concept of ripple steering, theoretic analysis is applied to SEPIC converter with two coupled inductors. The results indicate if the coupling coefficient is well designed, the equivalent input inductance can be multiple times larger than the self-inductance. Because of this, while maintaining input current ripple same, the two inductors of SEPIC can be implemented with two smaller coupled inductors. Thus both the total volume of inductors and the total number of windings can be reduced, and the power density and efficiency can be improved. Based on magnetic reluctance model, a corresponding winding scheme to control the coupling coefficient between two coupled inductors is analyzed. Also the impact of coupled inductors on the small signal transfer function is discussed. For the voltage follower control scheme of DCM PFC, single loop controller and notch filter design are discussed. With properly designed notch filter or the PR controller in another word, the closed loop bandwidth can be increased; simple PI controller is sufficient to achieve high power factor; THD of the input current can be greatly reduced. Finally, to validate the analysis and design procedure, a 1 kW prototype is built. With 120 Vrms AC input, 60V to 100V output, experimental results demonstrate unity power factor, wide output voltage regulation can be achieved within a single stage, and the 1 kW efficiency is around 93%.
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    Analysis and Design of Phase Lock Loop Based Islanding Detection Methods
    Martin, Daniel (Virginia Tech, 2011-05-04)
    As distributed generation penetrates the electric power grid at higher power levels, grid interface issues with distributed generation must be addressed. The current power system consists of central power generators, while the future power system will include many more distributed resources. The centralized power generation system is controlled by utility operators, but many distributed resources will not be controlled by utility operators. Distributed generation must use smart control techniques for high reliability and ideal grid interface. This thesis discusses the grid interface issue of anti-islanding. An electric island occurs when a circuit breaker in the electric power system trips. The distributed resource should disconnect from the electric grid for safety reasons. This thesis will give an overview of the possible methods. Each method will be analyzed using the ability to detect under the non-detection zone and the economic feasibility of the method. This thesis proposes two addition cases for analysis that exist in the electric power system: the effect of multiple methods in parallel in the non-detection zone and the possibility of a false trip caused by a load step. Multiple methods in parallel are possible because the islanding detection method is patentable, so each grid interface inverter company is likely to implement a different islanding detection method. The load step represents a load change when a load is switched on.
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    Analysis and Evaluation of Soft-switching Inverter Techniques in Electric Vehicle Applications
    Dong, Wei (Virginia Tech, 2003-04-22)
    This dissertation presents the systematic analysis and the critical assessment of the AC side soft-switching inverters in electric vehicle (EV) applications. Although numerous soft-switching inverter techniques were claimed to improve the inverter performance, compared with the conventional hard-switching inverter, there is the lack of comprehensive investigations of analyzing and evaluating the performance of soft-switching inverters. Starting with an efficiency comparison of a variety of the soft-switching inverters using analytical calculation, the dissertation first reveals the effects of the auxiliary circuit's operation and control on the loss reduction. Three types of soft-switching inverters realizing the zero-voltage-transition (ZVT) or zero-current-transition (ZCT) operation are identified to achieve high efficiency operation. Then one hard-switching inverter and the chosen soft-switching inverters are designed and implemented with the 55 kW power rating for the small duty EV application. The experimental evaluations on the dynamometer provide the accurate description of the performance of the soft-switching inverters in terms of the loss reductions, the electromagnetic interference (EMI) noise, the total harmonic distortion (THD) and the control complexity. An analysis of the harmonic distortion caused by short pulses is presented and a space vector modulation scheme is proposed to alleviate the effect. To effectively analyze the soft-switching inverters' performance, a simulation based electrical modeling methodology is developed. Not only it extends the EMI noise analysis to the higher frequency region, but also predicts the stress and the switching losses accurately. Three major modeling tasks are accomplished. First, to address the issues of complicated existing scheme, a new parameter extraction scheme is proposed to establish the physics-based IGBT model. Second, the impedance based measurement method is developed to derive the internal parasitic parameters of the half-bridge modules. Third, the finite element analysis software is used to develop the model for the laminated bus bar including the coupling effects of different phases. Experimental results from the single-leg operation and the three-phase inverter operation verify the effectiveness of the presented systematic electrical modeling approach. With the analytical tools verified by the testing results, the performance analysis is further extended to different power ratings and different bus voltage designs.
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    Analysis of Direct-Soldered Power Module / Heat Sink Thermal Interface for Electric Vehicle Applications
    Kim, Junhyung (Virginia Tech, 2001-04-27)
    Reducing the thermal impedance between power module and heat sink is important for high-power density, low-cost inverter applications. Mounting a power module by directly soldering it onto a heat sink can significantly reduce the thermal impedance at the module / heat sink interface, as compared to the conventional method of bolting the two together with a thermal grease or some other interface materials in between. However, a soldered interface typically contains a large number of voids, which results in local hot spots. This thesis describes approaches taken to reduce voids in the solder layer through surface treatment, solder paste selection, and adjustment in solder-reflow conditions. A 15MHz scanning acoustic microscope (SAM), a non-destructive inspection tool, was used to determine the void content at the module / heat sink interface. The experimental results show that a significant reduction in thermal resistance can be achieved by reducing the void content at the soldered module / heat sink interface. Moreover, a comparison of the thermal resistances in cases using the worst soldering, which contains the largest voided area, ThermstrateTM and thermal grease are presented. Thermal performances of the modules are studied by simulation with Flotherm.
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    Analysis of Inductor-Coupled Zero-Voltage-Transition Converters
    Choi, Jae-Young (Virginia Tech, 2001-07-24)
    As is the case for DC-DC converters, multi-phase converters require both high-quality power control and high power-density. Although a higher switching frequency not only improves the quality of the converter output but also decreases the size of the converter, it increases switching losses and electromagnetic interference (EMI) noise. Since the soft-switching topologies reduce the switching losses of the converter main switches, the topologies make converters partially independent from the switching frequency. However, the conventional soft-switching topologies have already proposed most of the possible ways to improve converter performance. In addition, the trends of the newly generated power devices reduce the advantages of soft-switching topologies. This critical situation surrounding soft-switching topologies gives research motivations: What features of soft-switching topologies facilitate their practical applications? Given this motivation, the dissertation discusses two aspects = simplifying auxiliary circuits and accounting for the effects of soft-switching operations on the converter control. Engineers working with medium- and high-power multi-phase converters require simplified soft-switching topologies that have the same level of performance as the conventional soft-switching topologies. This demand is the impetus behind one of the research objectives = simplifying the auxiliary circuits of Zero-Voltage-Transition (ZVT) inverters. Simplifying the auxiliary circuits results in both a smaller number of and lower cost for auxiliary components, without any negative impact on performance. This dissertation proposes two major concepts for the simplification - the Single-Switch Single-Leg (S3L) ZVT cell and the Phase-Lock (PL) concept. Throughout an effort to eliminate circulating currents of inductor-coupled (IC) ZVT converters, the S3L ZVT cell is developed. The proposed cell allows a single auxiliary switch to achieve zero-voltage conditions for both the top and bottom main switches, and it achieves the same level of performance as the conventional ZVT cell, as well. This proposal makes IC ZVT topologies more attractive to multi-phase converter applications. Because all of the top main switches generally have identical sequences for zero-voltage turn-on commutations, one auxiliary switch might handle the commutations of all of the top main switches. This possibility introduces the PL concept, which allows the two auxiliary switches to provide a zero-voltage condition for any main switch commutation. In order to compensate for restrictions of this concept, a modified space-vector modulation (SVM) scheme also is introduced. A soft-switching topology changes the duty ratios of the converter, which affects the controllability of the converter. Therefore, this dissertation selects resolution of this issue as one of the research objectives. This dissertation derives the generalized timing equations of ZVT operations, and the generalized equations formulize the effect of ZVT operation on both duty ratios and DC current. Moreover, the effect of SVM schemes is also investigated. An average model of the ZVT converter is developed using both the timing analysis and the investigation of SVM schemes, and small-signal analysis using the average model predicts the steady-state characteristics of the converter.
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    Analysis of Performance Characteristics of Electric Vehicle Traction Drive in Low Speed/Low Torque Range
    Kouns, Heath (Virginia Tech, 2001-12-06)
    In a world with a growing population there is a trend toward higher and higher energy usage. Because of the cost involved in producing extra energy, there is a need for more efficient usage of the energy that is already available. The issue of efficiency rings home especially clear with electric motors. Although induction motors are used in many different applications, the motors used in electric vehicles must be able to generate a large starting torque as well as operate over a wide speed range. This work analyzes the restrictions placed on the motor and inverter drive system. It also looks at the best method for controlling the drive in order to achieve the highest efficiency out of the drive. While other works have shown methods of achieve high efficiency out of the motor, it is the assertion of this work that the efficiency of the total drive is more important. It is to that end that this work analyzes the performance of an induction motor under low torque and speed where a traction drive utilizes the most energy.
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    Analysis, simulation and modeling of three-level VSIs
    Cosan, Muhammet (Virginia Tech, 1997-11-05)
    Analysis of three-phase, three-level VSIs is done for high-power high-voltage applications. Complete Space Vector Modulation (SVM) algorithm is developed for a three-phase, three-level converter. Special attention is given to minimization of output ripple and voltage balance of the dc-link input capacitors. Verification of the proposed SVM algorithm is done by computer simulation. Comprehensive small-signal modeling of the three-level converter with a resistive load is developed the first time. Steady-state solutions reveal that the voltage across dc-link input capacitors is constant at the half of the dc-link voltage.
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    Application of High-Power Snubberless Semiconductor Switches in High-Frequency PWM Converters
    Motto, Kevin (Virginia Tech, 2000-11-21)
    For many years, power electronics in the high-power area was performed with extremely slow semiconductor switches. These switches, including the thyristor and the Gate Turn-Off (GTO) thyristor, had the capacity to handle very high voltages and currents but lacked the ability to perform high frequency switching. Low-power converters, such as computer power supplies and low horsepower motor drives, have employed high-frequency switching for years and have benefited from very nice output waveforms, good control dynamic performance, and many other advantages compared to low frequency switching. Recent improvements in high-power semiconductor technology has brought switching performance similar to that of the low-power MOSFETs and IGBTs to the high-power area through the advancement of the IGBT's ratings to create the High Voltage IGBT (HVIGBT) and the development of new GTO-derived devices including the Integrated Gate Commutated Thyristor (IGCT) and the Emitter Turn-Off (ETO) thyristor. These new devices all feature high switching speed and the capability to turn off without the requirement for a turn-off snubber. With these new device technologies the high-power field of power electronics can realize dramatic improvements in the performance of systems for utility applications and motor drives. However, with these high-speed switches come new issues relating to noise, protection, performance of diodes, and thermal management in high-frequency applications. This thesis addresses the application of these new devices, especially the ETO and the IGCT. Examples of each device technology (IGBT, IGCT, and ETO) have been characterized in both their switching performance and conduction loss. The tests performed show how these new devices may be applied to various applications. The switching loss, especially related to turn-off, is the dominant factor in the power dissipation of the high-power switches, so knowledge of these characteristics are very important in the system design. To demonstrate the operation of the ETO, two power converters were constructed. The first was a 100 kW DC/DC converter, which demonstrated the operation of the ETO in a typical building block configuration, the half-bridge. The second system, a 1 MegaVolt-Amp (MVA) three-phase inverter, demonstrated the ETO in an application where the switching frequency and power level were both high. The test results demonstrate the expected characteristics of the high-frequency converters. The development of the ETO's gate driver is described. During the inverter testing, a new failure mode was found involving a parasitic diode within the ETO. This failure mode was analyzed and solutions were proposed. One of the proposed solutions was implemented and there were no more failures of this type. Another possible failure mode regarding a circulating current in an IGCT-based system is also analyzed. Soft-switching techniques can help reduce the switching loss in power semiconductor switches. Several topologies were considered for application in the high-power area, and one was selected for further investigation. A prototype Zero Current Transition (ZCT) circuit was developed using an IGCT as the main switch. The turn-off loss was reduced dramatically through the tested ZCT circuit, and the diode recovery was also alleviated.
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    Back to Back Active Power Filter for Multi-Generator Power Architecture with Reduced dc-link Capacitor
    Kim, Jong Wan (Virginia Tech, 2020-01-30)
    Multi-pulse converters have been widely used for a multi-megawatt scale power generating system to comply with harmonic regulations. Among all types of multi-pulse converters, a 12-pulse converter is the most widely used due to the simple structure, which consists of a delta-delta and a delta-wye phase-shift transformer pair and it effectively mitigates undesirable harmonics from the nonlinear load. In the early 2000s, a shunt type passive front-end for a shipboard power system was proposed. By shunting the two gensets with 30° phase angle difference, a single phase-shift transformer effectively eliminates 5th and 7th harmonics. It achieves a significant size and weight reduction compared to a 12-pulse converter while keep the comparable harmonic cancellation performance. Recently, a hybrid type front-end was proposed. On top of the passive front-end, 3 phase active power filter was added and an additional harmonic cancellation was achieved to further eliminate 11th and 13th harmonics. However, the performance of both the passive and hybrid type front-end are highly dependent on the size of the line reactor in ac mains. A back to back active power filter is proposed in this dissertation to replace the phase-shift transformer in the multi-generator power architecture. The proposed front-end does not include phase-shift transformer and the size and the weight of the overall front-end can be significantly reduced. Due to the active harmonic compensation, the back to back front-end achieves better power quality and the line reactor dependency is improved. The number of required dc-link capacitors is reduced by half by introducing a back to back configuration and the capacitor size is reduced by adjusting the phase angle difference of genset to cancel out the most significant voltage harmonics in the shared dc-link bus. The overview of the existing shunt type front-end is provided and the concept of back to back active power filter is validated by simulation and prototype hardware. The comparison between existing front-end and the proposed front-end is provided to highlight the superior performance of back to back active front-end. The dc-link bus current and voltage ripple analysis is provided to explain the dc-link ripple reduction mechanism.
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    Bidirectional DC-DC Power Converter Design Optimization, Modeling and Control
    Zhang, Junhong (Virginia Tech, 2008-01-30)
    In order to increase the power density, the discontinuous conducting mode (DCM) and small inductance is adopted for high power bidirectional dc-dc converter. The DCM related current ripple is minimized with multiphase interleaved operation. The turn-off loss caused by the DCM induced high peak current is reduced by snubber capacitor. The energy stored in the capacitor needs to be discharged before device is turned on. A complementary gating signal control scheme is employed to turn on the non-active switch helping discharge the capacitor and diverting the current into the anti-paralleled diode of the active switch. This realizes the zero voltage resonant transition (ZVRT) of main switches. This scheme also eliminates the parasitic ringing in inductor current. This work proposes an inductance and snubber capacitor optimization methodology. The inductor volume index and the inductor valley current are suggested as the optimization method for small volume and the realization of ZVRT. The proposed capacitance optimization method is based on a series of experiments for minimum overall switching loss. According to the suggested design optimization, a high power density hardware prototype is constructed and tested. The experimental results are provided, and the proposed design approach is verified. In this dissertation, a general-purposed power stage model is proposed based on complementary gating signal control scheme and derived with space-state averaging method. The model features a third-order system, from which a second-order model with resistive load on one side can be derived and a first-order model with a voltage source on both sides can be derived. This model sets up a basis for the unified controller design and optimization. The Δ-type model of coupled inductor is introduced and simplified to provide a more clearly physical meaning for design and dynamic analysis. These models have been validated by the Simplis ac analysis simulation. For power flow control, a unified controller concept is proposed based on the derived general-purposed power stage model. The proposed unified controller enables smooth bidirectional current flow. Controller is implemented with digital signal processing (DSP) for experimental verification. The inductor current is selected as feedback signal in resistive load, and the output current is selected as feedback signal in battery load. Load step and power flow step control tests are conducted for resistive load and battery load separately. The results indicate that the selected sensing signal can produce an accurate and fast enough feedback signal. Experimental results show that the transition between charging and discharging is very smooth, and there is no overshoot or undershoot transient. It presents a seamless transition for bidirectional current flow. The smooth transition should be attributed to the use of the complementary gating signal control scheme and the proposed unified controller. System simulations are made, and the results are provided. The test results have a good agreement with system simulation results, and the unified controller performs as expected.
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    Cascade Dual-Buck Inverters for Renewable Energy and Distributed Generation
    Sun, Pengwei (Virginia Tech, 2012-03-28)
    Renewable energy and distributed generation are getting more and more popular, including photovoltaic modules (PV), wind turbines, and fuel cells. The renewable energy sources need the power electronics interface to the utility grid because of different characteristics between the sources and the grid. No matter what renewable energy source is utilized, inverters are essential in the microgrid system. Thanks to flexible modular design, transformerless connection, extended voltage and power output, less maintenance and higher fault tolerance, the cascade inverters are good candidates for utility interface of various renewable energy sources. This dissertation proposes a new type of cascade inverters based on dual-buck topology and phase-shift control scheme. Compared to traditional cascade inverters, they have enhanced system reliability thanks to no shoot-through problems and lower switching loss with the help of using power MOSFETs. With phase-shift control, it theoretically eliminates the inherent current zero-crossing distortion of the single-unit dual-buck type inverter. In addition, phase-shift control can greatly reduce the ripple current or cut down the size of passive components by increasing the equivalent switching frequency. An asymmetrical half-cycle unipolar (AHCU) PWM technique is proposed for dual-buck full-bridge inverter. The proposed approach is to cut down the switching loss of power MOSFETs by half. At the same time, this AHCU PWM leads to current ripple reduction, and thus reducing ripple-related loss in filter components. Therefore, the proposed PWM strategy results in significant efficiency improvement. Additionally, the AHCU PWM also compensates for the zero-crossing distortion problem of dual-buck full-bridge inverter. Several PWM techniques are analyzed and compared, including bipolar PWM, unipolar PWM and phase-shifted PWM, when applied to the proposed cascade dual-buck full-bridge inverter. It has been found out that a PWM combination technique with the use of two out of the three PWMs leads to better performance in terms of less output current ripple and harmonics, no zero-crossing distortion, and higher efficiency. A grid-tie control system is proposed for cascade dual-buck inverter with both active and reactive power flow capability in a wide range under two types of renewable energy and distributed generation sources. Fuel cell power conditioning system (PCS) is Type I system with active power command generated by balance of plant (BOP) of each unit; and photovoltaic or wind PCS is Type II system with active power harvested by each front-end unit through maximum power point tracking (MPPT). Reactive power command is generated by distributed generation (DG) control site for both systems. Selective harmonic proportional resonant (PR) controller and admittance compensation controller are first introduced to cascade inverter grid-tie control to achieve better steady-state and dynamic performances.
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    CCM Totem Pole Bridgeless PFC with Ultra Fast IGBT
    Zhou, Bo (Virginia Tech, 2014-12-09)
    The totem pole PFC suffers from the Mosfet body diode reverse recovery issue which limits this topology adopted in the CCM high power condition. As the ultra-fast IGBT which is capable of providing 100 kHz switching frequency is available in the market, it is possible to apply the totem pole PFC in CCM high power condition. The thesis provides a method by implementing the ultra-fast IGBT and SiC diode to replace the MOSFET in this topology. To verify the method, a universal CCM totem pole PFC is designed and tested. The design adopts the ADP1048 programmable digital PFC controller by adding external logic gate for totem-pole PFC. ADP1048 greatly simplifies the design process and satisfies the design requirements. The experiment results verify that the totem-pole PFC can be applied into CCM high power condition by using the method. The DC output voltage is well regulated. The power factor is higher than 0.98 when the load is above 400W. The measured efficiency can achieve up to 96.8% at low line and 98.2% at high line condition with switching frequency 80 kHz.
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    Characterization and modeling of silicon and silicon carbide power devices
    Yang, Nanying (Virginia Tech, 2010-11-05)
    Power devices play key roles in the power electronics applications. In order for the power electronics designers to fully utilize the performance advantages of power devices, compact power device models are needed in the circuit simulator (Saber, P-spice, etc.). Therefore, it is very important to get accurate device models. However, there are many challenges due to the development of new power devices with new internal structure and new semiconductor materials (SiC, GaN, etc.). In this dissertation, enhanced power diode model is presented with an improvement in the reverse blocking region. In the current power diode model in the Saber circuit simulator, an empirical approach was used to describe the low-bias reverse blocking region by introducing an effect called "conduction loss," a parameter that causes a linear relationship between the device voltage and current at low bias voltages with no physics meaning. Furthermore, this term is not sufficient to accurately describe the changes to the device characteristics as the junction temperature is varied. In the enhanced model, an analytical temperature dependent model for the reverse blocking characteristics has been developed for Schottky/JBS diodes by including the thermionic-emission mechanism in the low-bias range. The newly derived model equations have been implemented in Saber circuit simulator using MAST language. An automated parameter extraction software package developed for constructing silicon (Si) and silicon carbide (SiC) power diode models, which is called DIode Model Parameter extrACtion Tools (DIMPACT). This software tool extracts the data necessary to establish a library of power diode component models and provides a method for quantitatively comparing between different types of devices and establishing performance metrics for device development. This dissertation also presents a new Saber-compatible approach for modeling the inter-electrode capacitances of the Si CoolMOSTM transistor. This new approach accurately describes all three inter-electrode capacitances (i.e., gate-drain, gate-source, and drain-source capacitances) for the full operating range of the device. The model is derived using the actual charge distribution within the device rather than assuming a lumped charge or one-dimensional charge distribution. The comparison between the simulated data with the measured results validates the accuracy of the new physical model.
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    Comparison and Design of High Efficiency Microinverters for Photovoltaic Applications
    Dominic, Jason (Virginia Tech, 2014-11-14)
    With the decrease in availability of non-renewable energy sources coupled with the increase in the amount of energy required for the operation of personal electronic devices there has been an increased focus on developing systems that take advantage of renewable energy sources. Renewal energy sources such as photovoltaic (PV) panels have become more popular due to recent developments in PV panel manufacturing that decreases material costs and improves energy harvesting efficiency. Since PV sources are DC sources power conversion stages have to be used in order to interface this power to the existing electrical utility system. The structure of large scale PV systems usually consists of several PV panels connected in series to achieve a high input source voltage that can be fed into a high power centralized DC-AC inverter. The drawback to this approach is that when the PV panels are subjected to less than ideal conditions. If a single PV panel is subjected to drastically less solar irradiation during cloud conditions, then its output power will drop dramatically. Since this panel is series connected with the other PV panels, their current output is also dragged low decreasing the power output of the system. Algorithms that have the power converter operate at different input conditions allow the system to operate at a maximum power point (MPP), however this only allows the system to operate at a higher power point and not the true MPP. To get around this limitation a new PV system implementation was created by giving each panel its own DC-AC power conversion system. This configuration gives each panel the ability to operate at its own MPP increasing the total system energy harvest. Another advantage of the single panel DC-AC microinverter power conversion stage is that the outputs are parallel connected to the utility grid easily allowing the ability to expand the system without having to shut down the entire system. The most prevalent implementation of the microinverter consists of a single power converter that uses the PV low voltage DC and outputs high voltage AC. In order to ensure that the double line AC ripple does not propagate to the PV panel a large bank of electrolytic capacitors are used to buffer the ripple. There is concern that the electrolytic capacitor will degrade over time and affect the system efficiency. To get around having to use electrolytic capacitors a two stage microinverter has been proposed. The two stage microinverter consists of a DC-DC converter that steps up the low DC voltage of the PV panel to high voltage DC and the second stage is a DC-AC inverter that takes the high voltage DC and converts it to high voltage AC. There is a capacitor that connects the two power converter stages called the DC link capacitor which can buffer the double line energy ripple without using electrolytic capacitors. This thesis focuses on the review of several DC-AC inverter topologies suitable for use in PV microinverter systems. Operation capabilities such as common mode noise and efficiency are compared. The main focus of the review is to determine the optimal DC-AC inverter using the performance metrics of cost, efficiency and common mode performance. A 250 W prototype is built for each inverter topology to verify its performance and operation.
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    Control Design and Analysis of an Advanced Induction Motor Electric Vehicle Drive
    Herwald, Marc A. (Virginia Tech, 1999-04-29)
    This thesis is about the development and performance enhancement of an induction motor electric vehicle drive system. The fundamental operation of the induction motor drive hardware and control software are introduced, and the different modulation techniques tested are described. A software simulation package is developed to assist in the control design and analysis of the drive system. Next, to establish the efficiency gains obtained by using space vector modulation in the improved drive system, an inverter with hysteresis current control is compared to the same inverter with space vector modulation in steady state and on separate driving profiles. A method for determining induction motor harmonic losses is introduced and is based on obtaining the phase current harmonics from sampled induction motor stator phase currents obtained. Using a semi-empirical loss model, the induction motor losses are compared between different pulse width modulation control strategies throughout the torque versus speed operating region. Next, several issues related to the robustness of the control design are addressed. To obtain good performance in the actual vehicle, a new method for driveline resonance compensation is developed and proven to work well through simulation and experiment. Lastly, this thesis discusses the development of a new method to compensate for the gain and phase error obtained in the feedback of the d-axis and q-axis stator flux linkages. Improved accuracy of the measured stator flux linkages will be shown to improve the field oriented controller by obtaining a more accurate measurement of the feedback electromagnetic torque.