Nonlinear Analysis and Control of Interleaved Boost Converter Using Real-Time Cycle to Cycle Variable Slope Compensation

Switched-mode power converters are inherently nonlinear and piecewise smooth systems that may exhibit a series of undesirable operations that can greatly reduce the converter's efficiency and lifetime. This paper presents a nonlinear analysis technique to investigate the influence of system parameters on the stability of interleaved boost converters. In this approach, Monodromy matrix that contains all the comprehensive information of converter parameters and control loop can be employed to fully reveal and understand the inherent nonlinear dynamics of interleaved boost converters, including the interaction effect of switching operation. Thereby not only the boundary conditions but also the relationship between stability margin and the parameters given can be intuitively studied by the eigenvalues of this matrix. Furthermore, by employing the knowledge gained from this analysis, a real-Time cycle to cycle variable slope compensation method is proposed to guarantee a satisfactory performance of the converter with an extended range of stable operation. Outcomes show that systems can regain stability by applying the proposed method within a few time periods of switching cycles. The numerical and analytical results validate the theoretical analysis, and experimental results verify the effectiveness of the proposed approach

Analysis and design of a modular multilevel converter with trapezoidal modulation for medium and high voltage DC-DC transformers

Conventional dual active bridge topologies provide galvanic isolation and soft-switching over a reasonable operating range without dedicated resonant circuits. However, scaling the two-level dual active bridge to higher dc voltage levels is impeded by several challenges among which the high dv/dt stress on the coupling transformer insulation. Gating and thermal characteristics of series switch arrays add to the limitations. To avoid the use of standard bulky modular multilevel bridges, this paper analyzes an alternative modulation technique where staircase approximated trapezoidal voltage waveforms are produced; thus alleviating developed dv/dt stresses. Modular design is realized by the utilization of half-bridge chopper cells. Therefore, the analyzed converter is a modular multi-level converter operated in a new mode with no common-mode dc arm currents as well as reduced capacitor size, hence reduced cell footprint. Suitable switching patterns are developed and various design and operation aspects are studied. Soft switching characteristics will be shown to be comparable to those of the two-level dual active bridge. Experimental results from a scaled test rig validate the presented concept

A Two Stage Stand Alone Solar PV System

Stand-alone systems based on solar photovoltaic (PV) have developed as a sustainable alternative pertaining to issue of electrification in areas particularly where the grid is not present. Main challenges in scheming this kind of systems are 1) Maximum power extraction from PV array 2) Dc to ac conversion 3) Protecting battery from over discharging and overcharging and 4) Facilitating satisfactory stepping up of voltage. As many objectives are needed to be met, schemes hitherto for standalone systems need a minimum of three converter stages, resulting in significant decline in efficiency and reliability of system. To tackle this problem, a two stage stand-alone topology comprising a novel transformer coupled dual input converter (TCDIC) is connected to a normal full bridge inverter is discussed in this thesis. Discussed TCDIC can facilitate charge control of battery while tracking maximum power point and keeping proper voltage level at load terminal. Mathematical model of TCDIC through small signal based approach is also included. A suitable control strategy for TCDIC is also presented to generate pulses. The operational efficiency of this topology is verified by performing simulation studies in detail

Analysis and design of switched-capacitor DC-DC converters with discrete event models

Ph. D. Thesis.Switched-capacitor DC-DC converters (SCDDCs) play a critical role in low power integrated systems. The analysis and design processes of an SCDDC impact the performance and power efficiency of the whole system. Conventionally, researchers carry out the analysis and design processes by viewing SCDDCs as analogue circuits. Analogue attributes of an SCDDC, such as the charge flow current or the equivalent output impedance, have been studied in considerable detail for performance enhancement. However, in most existing work, less attention is paid to the analysis of discrete events (e.g. digital signal transitions) and the relationships between discrete events in SCDDCs. These discrete events and the relationships between discrete events also affect the performance of SCDDCs. Certain negative effects of SCDDCs such as leakage current are introduced by unhealthy discrete states. For example, MOS devices in an SCDDC could conduct undesirably under certain combinations of signals, resulting in reversion losses (a type of leakage in SCDDCs). However, existing work only use verbal reasoning and waveform descriptions when studying these discrete events, which may cause confusion and result in an informal design process consisting of intuitive design and backed up merely by validation based on natural language discussions and simulations. There is therefore a need for formalised methods to describe and analyse these discrete events which may facilitate systematic design techniques. This thesis presents a new method of analysing and designing SCDDCs using discrete event models. Discrete event models such as Petri nets and Signal Transition Graphs (STGs) are commonly used in asynchronous circuits to formally describe and analyse the relationships between discrete transitions. Modelling SCDDCs with discrete event models provides a formal way to describe the relations between discrete transitions in SCDDCs. These discrete event models can be used for analysis, verification and even design guidance for SCDDC design. The rich set of existing analysis methods and tools for discrete event models could be applied to SCDDCs, potentially improving the analysis and design flow for them. Moreover, since Petri nets and STGs are generally used to analyse and design asynchronous circuits, modelling and designing SCDDCs with STG models may additionally facilitate the incorporation of positive features of asynchronous circuits in SCDDCs (e.g. no clock skew). In this thesis, the relations between discrete events in SCDDCs are formally described with SC-STG (an extended STG targeting multi-voltage systems, to which SCDDCs belong), which avoids the potential confusion due to natural language and waveform descriptions. Then the concurrency and causality relations described in SC-STG model are extended to Petri nets, with which the presence of reversion losses can be formally determined and verified. Finally, based on the STG and Petri net models, a new design method for reversion-loss-free SCDDCs is proposed. In SCDDCs designed with the new method, reversion losses are entirely removed by introducing asynchronous controls, synthesised with the help of a software synthesis toolkit “Workcraft”. To demonstrate the analysis capabilities of the method, several cross-coupled voltage doublers (a type of SCDDC) are analysed and studied with discrete event models as examples in this thesis. To demonstrate the design capabilities of the method, a new reversion-loss-free cross-coupled voltage doubler is designed. The cross-coupled voltage doubler is widely used in low power integrated systems such as flash memories, LCD drivers and wireless energy harvesting systems. The proposed modelling method is potentially used in both research and industrial area of those applications for a formal and high-efficiency design proces

DC/DC converters for high voltage direct current transmission

High Voltage Direct Current (HVDC) transmission has to date mostly been used for point-to-point projects, with only a few select projects being designed from the outset to incorporate multiple terminals. Any future HVDC network is therefore likely to evolve out of this pool of HVDC connections. As technology improves, the voltage rating, at the point of commission, of the these connections increases. Interconnection therefore requires the DC equivalent of the transformer, to bridge the voltage levels and create a multi-terminal network. This thesis investigates new potential DC/DC converter topologies, which may be used for a range of HVDC applications. Simple interconnections of new and legacy HVDC links is unlikely to require a large voltage-step, but will be required to transfer a large amount of power. As the HVDC network develops it may become feasible for wind-farms and load-centres to directly connect to the DC network, rather than requiring new and dedicated links. Such a connection is called an HVDC tap and is typically rated at only a small fraction of the link's peak capacity (around 10\%). Such taps would connect a distribution voltage level to the HVDC network. DC/DC converters suitable for large-step ratios (>5:1) may find their application here. In this work DC/DC converters for both small and large step-ratios are investigated. Two approaches are taken to design such converters: first, an approach utilising existing converter topologies is investigated. As each project comes with a huge price-tag, their reliability is paramount. Naturally, technology that has already proven itself in the field can be modified more readily and quickly for deployment. Using two modular multilevel converters in a front-to-front arrangement has been found to work efficiently for large power transfers and low step-ratios. Such a system can be operated at higher than 50 Hz frequencies to reduce the volume of a number of passive components, making the set-up suitable for compact off-shore applications. This does however incur a significant penalty in losses reducing the overall converter efficiency. In the second approach DC/DC converter designs are presented, that are more experimental and would require significantly more development work before deployment. Such designs do not look to adapt existing converter topologies but rather are designed from scratch, purely for DC/DC applications. An evolution of the front-to-front arrangement is investigated in further detail. This circuit utilises medium frequency (>50 Hz) square current and voltage waveforms. The DC/DC step-ratio is achieved through a combination of the stacks of cells and a transformer. This split approach allows for high-step ratios to be achieved at similar system efficiencies as for the front-to-front arrangement. The topology has been found to be much more suitable for higher than 50 Hz operation from a losses perspective, allowing for a compact and efficient design.Open Acces

State-plane trajectories used to observe and control the behavior of a voltage step-up dc-to-dc converter

State-plane analysis techniques are employed to study the voltage step up energy storage dc-to-dc converter. Within this framework, an example converter operating under the influence of a constant on time and a constant frequency controller is examined. Qualitative insight gained through this approach is used to develop a conceptual free running control law for the voltage step up converter which can achieve steady state operation in one on/off cycle of control. Digital computer simulation data is presented to illustrate and verify the theoretical discussions presented

Analysis on Supercapacitor Assisted Low Dropout (SCALDO) Regulators

State-of-the-art electronic systems employ three fundamental techniques for DC-DC converters: (a) switch-mode power supplies (SMPS); (b) linear power supplies; (c) switched capacitor (charge pump) converters. In practical systems, these three techniques are mixed to provide a complex, but elegant, overall solution, with energy efficiency, effective PCB footprint, noise and transient performance to suit different electronic circuit blocks. Switching regulators have relatively high end-to-end efficiency, in the range of 70 to 93%, but can have issues with output noise and EMI/RFI emissions. Switched capacitor converters use a set of capacitors for energy storage and conversion. In general, linear regulators have low efficiencies in the range 30 to 60%. However, they have outstanding output characteristics such as low noise, excellent transient response to load current fluctuations, design simplicity and low cost design which are far superior to SMPS. Given the complex situation in switch-mode converters, low dropout (LDO) regulators were introduced to address the equirements of noise-sensitive and fast transient loads in portable devices. A typical commercial off-the-shelf LDO has its input voltage slightly higher than the desired regulated output for optimal efficiency. The approximate efficiency of a linear regulator, if the power consumed by the control circuits is negligible, can be expressed by the ratio of Vo/Vin. A very low frequency supercapacitor circulation technique can be combined with commercial low dropout regulator ICs to significantly increase the end-to-end efficiency by a multiplication factor in the range of 1.33 to 3, compared to the efficiency of a linear regulator circuit with the same input-output voltages. In this patented supercapacitor-assisted low dropout (SCALDO) regulator technique developed by a research team at the University of Waikato, supercapacitors are used as lossless voltage droppers, and the energy reuse occurs at very low frequencies in the range of less than ten hertz, eliminating RFI/EMI concerns. This SCALDO technique opens up a new approach to design step-down, DC-DC converters suitable for processor power supplies with very high end-to-end efficiency which is closer to the efficiencies of practical switching regulators, while maintaining the superior output specifications of a linear design. Furthermore, it is important to emphasize that the SCALDO technique is not a variation of well-known switched capacitor DC-DC converters. In this thesis, the basic SCALDO concept is further developed to achieve generalised topologies, with the relevant theory that can be applied to a converter with any input-output step-down voltage combination. For these generalised topologies, some important design parameters, such as the number of supercapacitors, switching matrix details and efficiency improvement factors, are derived to form the basis of designing SCALDO regulators. With the availability of commercial LDO ICs with output current ratings up to 10 A, and thin-prole supercapacitors with DC voltage ratings from 2.3 to 5.5 V, several practically useful, medium-current SCALDO prototypes: 12V-to-5V, 5V-to-2V, 5.5V-to-3.3V have been developed. Experimental studies were carried out on these SCALDO prototypes to quantify performance in terms of line regulation, load regulation, efficiency and transient response. In order to accurately predict the performance and associated waveforms of the individual phases (charge, discharge and transition) of the SCALDO regulator, Laplace transform-based theory for supercapacitor circulation is developed, and analytical predictions are compared with experimental measurements for a 12V-to-5V prototype. The analytical results tallied well with the practical waveforms observed in a 12V-to-5V converter, indicating that the SCALDO technique can be generalized to other versatile configurations, and confirming that the simplified assumptions used to describe the circuit elements are reasonable and justifiable. After analysing the performance of several SCALDO prototypes, some practical issues in designing SCALDO regulators have been identified. These relate to power losses and implications for future development of the SCALDO design

PSCAD Modeling and Stability Analysis of a Microgrid

As power systems are evolving, engineers are facing, and will continue to face, new challenges with respect to maintaining the system in terms of stable operation. Many different forms of generation are becoming prevalent, including; small synchronous generators, photovoltaic generation, and energy storage techniques in the form of battery and ultracapacitor systems. One of the evolutions occurring in the power system is the emergence of microgrids, small power systems capable of isolating from the major power grid in the form of islands. Microgrids use distributed generation to provide power to small communities, and they come with several advantages and disadvantages. This thesis shows the design process employed to model a microgrid, which contains a variety of distributed resources, in PSCAD, as well as investigate the transient instability of the microgrid when transitioning to islanded operation. Modeling techniques for both grid-connected and islanded operation of the microgrid are considered in this study. In addition to modeling techniques, the effectiveness of proper control of energy storage assets in a microgrid is demonstrated through the implementation and comparison between real & reactive power regulation and voltage & frequency regulation