Multiphase Buck Converter with Minimum Time Control Strategy for RF Envelope Modulation

Power amplifier supplied with constant supply voltage has very low efficiency in the transmitter. A DC-DC converter in series with a linear regulator can be used to obtain voltage modulation. Since this converter should be able to change the output voltage very fast, a multiphase buck converter with a minimum time control strategy is proposed. To modulate supply voltage of the envelope amplifier, the multiphase converter works with some particular duty cycle (i/n, i=1, 2 ... n, n is the number of phase) to generate discrete output voltages, and in these duty cycles the output current ripple can be completely cancelled. The transition times for the minimum time are pre-calculated and inserted in a look-up table. The theoretical background, the system model that is necessary in order to calculate the transition times and the experimental results obtained with a 4-phase buck prototype are given

Multiphase transformer-coupled converter: two different strategies for energy conversion

The operation of a multiphase topology, ideally, without energy storage presents the advantage of achieving very high efficiency over a wide load range as well as a fast dynamic response. However, ideal no-energy storage operation also implies a limitation in the regulation capability of the topology, the output voltage can only take discrete values. These features (high efficiency and discrete regulation capability) of the proposed energy conversion strategy enable the topology as a candidate for `DC-DC transformer' applications. The advantages, drawbacks and the operating principle of this concept, implemented with a `closed chain' magnetic structure have been already presented. In this work, the minimum energy storage operation, is applied to two different magnetic structures. These magnetic structures are called `closed chain' and `pyramidal' the main advantage of the `pyramidal' coupling structure is to improve the size of the converter without increasing the operating frequency. Both magnetic structures are analyzed, compared and experimentally implemented.

Novel Inductor‐less Conversion Strategy Based on Multiphase Transformer‐Coupled Converters: Analysis, Design and Applications

There are many the requirements that modern power converters should fulfill. Most of the applications where these converters are used, demand smaller converters with high efficiency, improved power density and a fast dynamic response. For instance, loads like microprocessors demand aggressive current steps with very high slew rates (100A/mus and higher); besides, during these load steps, the supply voltage of the microprocessor should be kept within tight limits in order to ensure its correct performance. The accomplishment of these requirements is not an easy task; complex solutions like advanced topologies - such as multiphase converters- as well as advanced control strategies are often needed. Besides, it is also necessary to operate the converter at high switching frequencies and to use capacitors with high capacitance and low ESR. Improving the dynamic response of power converters does not rely only on the control strategy but also the power topology should be suited to enable a fast dynamic response. Moreover, in later years, a fast dynamic response does not only mean accomplishing fast load steps but output voltage steps are gaining importance as well. At least, two applications that require fast voltage changes can be named: Low power microprocessors. In these devices, the voltage supply is changed according to the workload and the operating frequency of the microprocessor is changed at the same time. An important reduction in voltage dependent losses can be achieved with such changes. This technique is known as Dynamic Voltage Scaling (DVS). Another application where important energy savings can be achieved by means of changing the supply voltage are Radio Frequency Power Amplifiers. For example, RF architectures based on ‘Envelope Tracking’ and ‘Envelope Elimination and Restoration’ techniques can take advantage of voltage supply modulation and accomplish important energy savings in the power amplifier. However, in order to achieve these efficiency improvements, a power converter with high efficiency and high enough bandwidth (hundreds of kHz or even tens of MHz) is necessary in order to ensure an adequate supply voltage. The main objective of this Thesis is to improve the dynamic response of DC-DC converters from the point of view of the power topology. And the term dynamic response refers both to the load steps and the voltage steps; it is also interesting to modulate the output voltage of the converter with a specific bandwidth. In order to accomplish this, the question of what is it that limits the dynamic response of power converters should be answered. Analyzing this question leads to the conclusion that the dynamic response is limited by the power topology and specifically, by the filter inductance of the converter which is found in series between the input and the output of the converter. The series inductance is the one that determines the gain of the converter and provides the regulation capability. Although the energy stored in the filter inductance enables the regulation and the capability of filtering the output voltage, it imposes a limitation which is the concern of this Thesis. The series inductance stores energy and prevents the current from changing in a fast way, limiting the slew rate of the current through this inductor. Different solutions are proposed in the literature in order to reduce the limit imposed by the filter inductor. Many publications proposing new topologies and improvements to known topologies can be found in the literature. Also, complex control strategies are proposed with the objective of improving the dynamic response in power converters. In the proposed topologies, the energy stored in the series inductor is reduced; examples of these topologies are Multiphase converters, Buck converter operating at very high frequency or adding a low impedance path in parallel with the series inductance. Control techniques proposed in the literature, focus on adjusting the output voltage as fast as allowed by the power stage; examples of these control techniques are: hysteresis control, V 2 control, and minimum time control. In some of the proposed topologies, a reduction in the value of the series inductance is achieved and with this, the energy stored in this magnetic element is reduced; less stored energy means a faster dynamic response. However, in some cases (as in the high frequency Buck converter), the dynamic response is improved at the cost of worsening the efficiency. In this Thesis, a drastic solution is proposed: to completely eliminate the series inductance of the converter. This is a more radical solution when compared to those proposed in the literature. If the series inductance is eliminated, the regulation capability of the converter is limited which can make it difficult to use the topology in one-converter solutions; however, this topology is suitable for power architectures where the energy conversion is done by more than one converter. When the series inductor is eliminated from the converter, the current slew rate is no longer limited and it can be said that the dynamic response of the converter is independent from the switching frequency. This is the main advantage of eliminating the series inductor. The main objective, is to propose an energy conversion strategy that is done without series inductance. Without series inductance, no energy is stored between the input and the output of the converter and the dynamic response would be instantaneous if all the devices were ideal. If the energy transfer from the input to the output of the converter is done instantaneously when a load step occurs, conceptually it would not be necessary to store energy at the output of the converter (no output capacitor COUT would be needed) and if the input source is ideal, the input capacitor CIN would not be necessary. This last feature (no CIN with ideal VIN) is common to all power converters. However, when the concept is actually implemented, parasitic inductances such as leakage inductance of the transformer and the parasitic inductance of the PCB, cannot be avoided because they are inherent to the implementation of the converter. These parasitic elements do not affect significantly to the proposed concept. In this Thesis, it is proposed to operate the converter without series inductance in order to improve the dynamic response of the converter; however, on the other side, the continuous regulation capability of the converter is lost. It is said continuous because, as it will be explained throughout the Thesis, it is indeed possible to achieve discrete regulation; a converter without filter inductance and without energy stored in the magnetic element, is capable to achieve a limited number of output voltages. The changes between these output voltage levels are achieved in a fast way. The proposed energy conversion strategy is implemented by means of a multiphase converter where the coupling of the phases is done by discrete two-winding transformers instead of coupledinductors since transformers are, ideally, no energy storing elements. This idea is the main contribution of this Thesis. The feasibility of this energy conversion strategy is first analyzed and then verified by simulation and by the implementation of experimental prototypes. Once the strategy is proved valid, different options to implement the magnetic structure are analyzed. Three different discrete transformer arrangements are studied and implemented. A converter based on this energy conversion strategy would be designed with a different approach than the one used to design classic converters since an additional design degree of freedom is available. The switching frequency can be chosen according to the design specifications without penalizing the dynamic response or the efficiency. Low operating frequencies can be chosen in order to favor the efficiency; on the other hand, high operating frequencies (MHz) can be chosen in order to favor the size of the converter. For this reason, a particular design procedure is proposed for the ‘inductorless’ conversion strategy. Finally, applications where the features of the proposed conversion strategy (high efficiency with fast dynamic response) are advantageus, are proposed. For example, in two-stage power architectures where a high efficiency converter is needed as the first stage and there is a second stage that provides the fine regulation. Another example are RF power amplifiers where the voltage is modulated following an envelope reference in order to save power; in this application, a high efficiency converter, capable of achieving fast voltage steps is required. The main contributions of this Thesis are the following: The proposal of a conversion strategy that is done, ideally, without storing energy in the magnetic element. The validation and the implementation of the proposed energy conversion strategy. The study of different magnetic structures based on discrete transformers for the implementation of the proposed energy conversion strategy. To elaborate and validate a design procedure. To identify and validate applications for the proposed energy conversion strategy. It is important to remark that this work is done in collaboration with Intel. The particular features of the proposed conversion strategy enable the possibility of solving the problems related to microprocessor powering in a different way. For example, the high efficiency achieved with the proposed conversion strategy enables it as a good candidate to be used for power conditioning, as a first stage in a two-stage power architecture for powering microprocessors.

Ion beam analysis of as-received, H-implanted and post implanted annealed fusion steels

The elemental distribution for as-received (AR), H implanted (AI) and post-implanted annealed (A) Eurofer and ODS-Eurofer steels has been characterized by means of micro Particle Induced X-ray Emission (μ-PIXE), micro Elastic Recoil Detection (μ-ERD) and Secondary Ion Mass Spectrometry (SIMS). The temperature and time-induced H diffusion has been analyzed by Resonance Nuclear Reaction Analysis (RNRA), Thermal Desorption Spectroscopy (TDS), ERDA and SIMS techniques. μ-PIXE measurements point out the presence of inhomogeneities in the Y distribution for ODS-Eurofer samples. RNRA and SIMS experiments evidence that hydrogen easily outdiffuses in these steels even at room temperature. ERD data show that annealing at temperatures as low as 300 °C strongly accelerates the hydrogen diffusion process, driving out up to the 90% of the initial hydrogen.

Damage tolerance of cold drawn ferritic-austenitic stainless steels wires for prestressed concrete

Damage tolerance of high strength cold-drawn ferritic–austenitic stainless steel wires is assessed by means of tensile fracture tests of cracked wires. The fatigue crack is transversally propagated from the wire surface. The damage tolerance curve of the wires results from the empirical failure load when given as a function of crack depth. As a consequence of cold drawing, the wire microstructure is orientated along its longitudinal axis and anisotropic fracture behaviour is found at macrostructural level at the tensile failure of the cracked specimens. An in situ optical technique known as video image correlation VIC-2D is used to get an insight into this failure mechanism by tensile testing transversally fatigue cracked plane specimens extracted from the cold-drawn wires. Finally, the experimentally obtained damage tolerance curve of the cold-drawn ferritic–austenitic stainless steel wires is compared with that of an elementary plastic collapse model and existing data of two types of high strength eutectoid steel currently used as prestressing steel for concrete.

Minimum time control for multiphase buck converter: analysis and application

The combination of minimum time control and multiphase converter is a favorable option for dc-dc converters in applications where output voltage variation is required, such as RF amplifiers and dynamic voltage scaling in microprocessors, due to their advantage of fast dynamic response. In this paper, an improved minimum time control approach for multiphase buck converter that is based on charge balance technique, aiming at fast output voltage transition is presented. Compared with the traditional method, the proposed control takes into account the phase delay and current ripple in each phase. Therefore, by investigating the behavior of multiphase converter during voltage transition, it resolves the problem of current unbalance after the transient, which can lead to long settling time of the output voltage. The restriction of this control is that the output voltage that the converter can provide is related to the number of the phases, because only the duty cycles at which the multiphase converter has total ripple cancellation are used in this approach. The model of the proposed control is introduced, and the design constraints of the buck converters filter for this control are discussed. In order to prove the concept, a four-phase buck converter is implemented and the experimental results that validate the proposed control method are presented. The application of this control to RF envelope tracking is also presented in this paper.

Effect of fire on the microstructure and mechanical response of structural steels for building construction

The research is an exhaustive study of the microstructure and of the stress-strain curves of structural steel S460N at temperatures typical of a fire. It includes a fractographic study of the fracture suifaces of cylindrical specimens, tensile tested at different fire scenarios, explaining the relationship between the failure micromechanisms and temperature. The paper ends with the comparison between the experimentally found strain-stress curves with that one's proposed by the EUROCODE EC3, resulting that in the case of steel S460N these are on the side ofsafety.

Improvement of the Dynamic Performance of a Multiphase Current Controlled Buck Converter using an Auxiliary Synchronous Buck Converter as an Additional Energy Path

This work is related to the output impedance improvement of a Multiphase Buck converter with Peak Current Mode Control (PCMC) by means of introducing an additional power path that virtually increases the output capacitance during transients. Various solutions that can be employed to improve the dynamic behavior of the converter system exist, but nearly all solutions are developed for a Single Phase Buck converter with Voltage Mode Control (VMC), while in the VRM applications, due to the high currents, the system is usually implemented as a Multiphase Buck Converter with Current Mode Control. The additional energy path, as presented here, is introduced with the Output Impedance Correction Circuit (OICC) based on the Controlled Current Source (CCS). The OICC is used to inject or extract a current n-1 times larger than the output capacitor current, thus virtually increasing n times the value of the output capacitance during the transients. Furthermore, this work extends the OICC concept to a Multiphase Buck Converter system while comparing proposed solution with the system that has n times bigger output capacitor. In addition, the OICC is implemented as a Synchronous Buck Converter with PCMC, thus reducing its influence on the system efficiency.

Effect of fire on the microstructure and mechanical response of structural steels for building construction

The research is an exhaustive study of the microstructure and of the stress-strain curves of structural steel S460N at temperatures typical of a fire. It includes a fractographic study of the fracture suifaces of cylindrical specimens, tensile tested at different fire scenarios, explaining the relationship between the failure micromechanisms and temperature. The paper ends with the comparison between the experimentally found strain-stress curves with that one's proposed by the EUROCODE EC3, resulting that in the case of steel S460N these are on the side ofsafety.

Multiphase Current Controlled Buck Converter with energy recycling Output Impedance Correction Circuit (OICC)

This work is related to the output impedance improvement of a Multiphase Buck converter with Peak Current Mode Control (PCMC) by means of introducing an additional power path that virtually increases the output capacitance during transients. Various solutions that can be employed to improve the dynamic behavior of the converter system exist, but nearly all solutions are developed for a Single Phase Buck converter with Voltage Mode Control (VMC), while in the VRM applications, due to the high currents, the system is usually implemented as a Multiphase Buck Converter with Current Mode Control. The Output Impedance Correction Circuit (OICC) is used to inject or extract a current n-1 times larger than the output capacitor current, thus virtually increasing n times the value of the output capacitance during the transients. Furthermore, this work extends the OICC concept to a Multiphase Buck Converter system while comparing proposed solution with the system that has n times bigger output capacitor. In addition, the OICC is implemented as a Synchronous Buck Converter with PCMC, thus reducing its influence on the system efficiency

Multiphase Parallel Interleaved And Primary-Parallel Secondary-Series Forward Micro-Inverter Comparison.

Using multiphase technique is interesting in PV AC-module application due to light-load efficiency improvement by applying phase shedding, and the possibility of low-profile implementation. This paper presents a comparison, in terms of size and efficiency, of the parallel interleaved and the parallel-series connected multiphase configurations, as a function of the number of phases, for a forward micro-inverter operated in DCM. 8-phase prototypes of both multiphase configurations are built and compared between them and with the single phase forward micro-inverter, validating the presented analysis.

Biofouling on PM stainless steels

Powder metallurgy (PM) consists in obtaining pieces of powder metal that are processed at high temperatures and pressure. Due to its characteristic manufacturing process, the materials can have a specific and controlled porosity, which makes it possible to obtain porous parts such as ball bearings, gears, and roller bearings, etc. This porosity is what made us think about how easy biofouling would be on these materials and its possible environmental applications.

Multiphase Design and Control Techniques Applied to a Forward Micro-Inverter

En la última década la potencia instalada de energía solar fotovoltaica ha crecido una media de un 49% anual y se espera que alcance el 16%del consumo energético mundial en el año 2050. La mayor parte de estas instalaciones se corresponden con sistemas conectados a la red eléctrica y un amplio porcentaje de ellas son instalaciones domésticas o en edificios. En el mercado ya existen diferentes arquitecturas para este tipo de instalaciones, entre las que se encuentras los módulos AC. Un módulo AC consiste en un inversor, también conocido como micro-inversor, que se monta en la parte trasera de un panel o módulo fotovoltaico. Esta tecnología ofrece modularidad, redundancia y la extracción de la máxima potencia de cada panel solar de la instalación. Además, la expansión de esta tecnología posibilitará una reducción de costes asociados a las economías de escala y a la posibilidad de que el propio usuario pueda componer su propio sistema. Sin embargo, el micro-inversor debe ser capaz de proporcionar una ganancia de tensión adecuada para conectar el panel solar directamente a la red, mientras mantiene un rendimiento aceptable en un amplio rango de potencias. Asimismo, los estándares de conexión a red deber ser satisfechos y el tamaño y el tiempo de vida del micro-inversor son factores que han de tenerse siempre en cuenta. En esta tesis se propone un micro-inversor derivado de la topología “forward” controlado en el límite entre los modos de conducción continuo y discontinuo (BCM por sus siglas en inglés). El transformador de la topología propuesta mantiene la misma estructura que en el convertidor “forward” clásico y la utilización de interruptores bidireccionales en el secundario permite la conexión directa del inversor a la red. Asimismo el método de control elegido permite obtener factor de potencia cercano a la unidad con una implementación sencilla. En la tesis se presenta el principio de funcionamiento y los principales aspectos del diseño del micro-inversor propuesto. Con la idea de mantener una solución sencilla y de bajo coste, se ha seleccionado un controlador analógico que está originalmente pensado para controlar un corrector del factor de potencia en el mismo modo de conducción que el micro-inversor “forward”. La tesis presenta las principales modificaciones necesarias, con especial atención a la detección del cruce por cero de la corriente (ZCD por sus siglas en inglés) y la compatibilidad del controlador con la inclusión de un algoritmo de búsqueda del punto de máxima potencia (MPPT por sus siglas en inglés). Los resultados experimentales muestran las limitaciones de la implementación elegida e identifican al transformador como el principal contribuyente a las pérdidas del micro-inversor. El principal objetivo de esta tesis es contribuir a la aplicación de técnicas de control y diseño de sistemas multifase en micro-inversores fotovoltaicos. En esta tesis se van a considerar dos configuraciones multifase diferentes aplicadas al micro-inversor “forward” propuesto. La primera consiste en una variación con conexión paralelo-serie que permite la utilización de transformadores con una relación de vueltas baja, y por tanto bien acoplados, para conseguir una ganancia de tensión adecuada con un mejor rendimiento. Esta configuración emplea el mismo control BCM cuando la potencia extraída del panel solar es máxima. Este método de control implica que la frecuencia de conmutación se incrementa considerablemente cuando la potencia decrece, lo que compromete el rendimiento. Por lo tanto y con la intención de mantener unos bueno niveles de rendimiento ponderado, el micro-inversor funciona en modo de conducción discontinuo (DCM, por sus siglas en inglés) cuando la potencia extraía del panel solar es menor que la máxima. La segunda configuración multifase considerada en esta tesis es la aplicación de la técnica de paralelo con entrelazado. Además se han considerado dos técnicas diferentes para decidir el número de fases activas: dependiendo de la potencia continua extraída del panel solar y dependiendo de la potencia instantánea demandada por el micro-inversor. La aplicación de estas técnicas es interesante en los sistemas fotovoltaicos conectados a la red eléctrica por la posibilidad que brindan de obtener un rendimiento prácticamente plano en un amplio rango de potencia. Las configuraciones con entrelazado se controlan en DCM para evitar la necesidad de un control de corriente, lo que es importante cuando el número de fases es alto. Los núcleos adecuados para todas las configuraciones multifase consideradas se seleccionan usando el producto de áreas. Una vez seleccionados los núcleos se ha realizado un diseño detallado de cada uno de los transformadores. Con la información obtenida de los diseños y los resultados de simulación, se puede analizar el impacto que el número de transformadores utilizados tiene en el tamaño y el rendimiento de las distintas configuraciones. Los resultados de este análisis, presentado en esta tesis, se utilizan posteriormente para comparar las distintas configuraciones. Muchas otras topologías se han presentado en la literatura para abordar los diferentes aspectos a considerar en los micro-inversores, que han sido presentados anteriormente. La mayoría de estas topologías utilizan un transformador de alta frecuencia para solventar el salto de tensión y evitar problemas de seguridad y de puesta a tierra. En cualquier caso, es interesante evaluar si topologías sin aislamiento galvánico son aptas para su utilización como micro-inversores. En esta tesis se presenta una revisión de inversores con capacidad de elevar tensión, que se comparan bajo las mismas especificaciones. El objetivo es proporcionar la información necesaria para valorar si estas topologías son aplicables en los módulos AC. Las principales contribuciones de esta tesis son: • La aplicación del control BCM a un convertidor “forward” para obtener un micro-inversor de una etapa sencillo y de bajo coste. • La modificación de dicho micro-inversor con conexión paralelo-series de transformadores que permite reducir la corriente de los semiconductores y una ganancia de tensión adecuada con transformadores altamente acoplados. • La aplicación de técnicas de entrelazado y decisión de apagado de fases en la puesta en paralelo del micro-inversor “forward”. • El análisis y la comparación del efecto en el tamaño y el rendimiento del incremento del número de transformadores en las diferentes configuraciones multifase. • La eliminación de las medidas y los lazos de control de corriente en las topologías multifase con la utilización del modo de conducción discontinuo y un algoritmo MPPT sin necesidad de medida de corriente. • La recopilación y comparación bajo las mismas especificaciones de topologías inversoras con capacidad de elevar tensión, que pueden ser adecuadas para la utilización como micro-inversores. Esta tesis está estructurada en seis capítulos. El capítulo 1 presenta el marco en que se desarrolla la tesis así como el alcance de la misma. En el capítulo 2 se recopilan las topologías existentes de micro-invesores con aislamiento y aquellas sin aislamiento cuya implementación en un módulo AC es factible. Asimismo se presenta la comparación entre estas topologías bajo las mismas especificaciones. El capítulo 3 se centra en el micro-inversor “forward” que se propone originalmente en esta tesis. La aplicación de las técnicas multifase se aborda en los capítulos 4 y 5, en los que se presentan los análisis en función del número de transformadores. El capítulo está orientado a la propuesta paralelo-serie mientras que la configuración con entrelazado se analiza en el capítulo 5. Por último, en el capítulo 6 se presentan las contribuciones de esta tesis y los trabajos futuros. ABSTRACT In the last decade the photovoltaic (PV) installed power increased with an average growth of 49% per year and it is expected to cover the 16% of the global electricity consumption by 2050. Most of the installed PV power corresponds to grid-connected systems, with a significant percentage of residential installations. In these PV systems, the inverter is essential since it is the responsible of transferring into the grid the extracted power from the PV modules. Several architectures have been proposed for grid-connected residential PV systems, including the AC-module technology. An AC-module consists of an inverter, also known as micro-inverter, which is attached to a PV module. The AC-module technology offers modularity, redundancy and individual MPPT of each module. In addition, the expansion of this technology will enable the possibility of economies of scale of mass market and “plug and play” for the user, thus reducing the overall cost of the installation. However, the micro-inverter must be able to provide the required voltage boost to interface a low voltage PV module to the grid while keeping an acceptable efficiency in a wide power range. Furthermore, the quality standards must be satisfied and size and lifetime of the solutions must be always considered. In this thesis a single-stage forward micro-inverter with boundary mode operation is proposed to address the micro-inverter requirements. The transformer in the proposed topology remains as in the classic forward converter and bidirectional switches in the secondary side allows direct connection to the grid. In addition the selected control strategy allows high power factor current with a simple implementation. The operation of the topology is presented and the main design issues are introduced. With the intention to propose a simple and low-cost solution, an analog controller for a PFC operated in boundary mode is utilized. The main necessary modifications are discussed, with the focus on the zero current detection (ZCD) and the compatibility of the controller with a MPPT algorithm. The experimental results show the limitations of the selected analog controller implementation and the transformer is identified as a main losses contributor. The main objective of this thesis is to contribute in the application of control and design multiphase techniques to the PV micro-inverters. Two different multiphase configurations have been applied to the forward micro-inverter proposed in this thesis. The first one consists of a parallel-series connected variation which enables the use of low turns ratio, i.e. well coupled, transformers to achieve a proper voltage boost with an improved performance. This multiphase configuration implements BCM control at maximum load however. With this control method the switching frequency increases significantly for light load operation, thus jeopardizing the efficiency. Therefore, in order to keep acceptable weighted efficiency levels, DCM operation is selected for low power conditions. The second multiphase variation considered in this thesis is the interleaved configuration with two different phase shedding techniques: depending on the DC power extracted from the PV panel, and depending on the demanded instantaneous power. The application of interleaving techniques is interesting in PV grid-connected inverters for the possibility of flat efficiency behavior in a wide power range. The interleaved variations of the proposed forward micro-inverter are operated in DCM to avoid the current loop, which is important when the number of phases is large. The adequate transformer cores for all the multiphase configurations are selected according to the area product parameter and a detailed design of each required transformer is developed. With this information and simulation results, the impact in size and efficiency of the number of transformer used can be assessed. The considered multiphase topologies are compared in this thesis according to the results of the introduced analysis. Several other topological solutions have been proposed to solve the mentioned concerns in AC-module application. The most of these solutions use a high frequency transformer to boost the voltage and avoid grounding and safety issues. However, it is of interest to assess if the non-isolated topologies are suitable for AC-module application. In this thesis a review of transformerless step-up inverters is presented. The compiled topologies are compared using a set benchmark to provide the necessary information to assess whether non-isolated topologies are suitable for AC-module application. The main contributions of this thesis are: • The application of the boundary mode control with constant off-time to a forward converter, to obtain a simple and low-cost single-stage forward micro-inverter. • A modification of the forward micro-inverter with primary-parallel secondary-series connected transformers to reduce the current stress and improve the voltage gain with highly coupled transformers. •The application of the interleaved configuration with different phase shedding strategies to the proposed forward micro-inverter. • An analysis and comparison of the influence in size and efficiency of increasing the number of transformers in the parallel-series and interleaved multiphase configurations. • Elimination of the current loop and current measurements in the multiphase topologies by adopting DCM operation and a current sensorless MPPT. • A compilation and comparison with the same specifications of suitable non-isolated step-up inverters. This thesis is organized in six chapters. In Chapter 1 the background of single-phase PV-connected systems is discussed and the scope of the thesis is defined. Chapter 2 compiles the existing solutions for isolated micro-inverters and transformerless step-up inverters suitable for AC-module application. In addition, the most convenient non-isolated inverters are compared using a defined benchmark. Chapter 3 focuses on the originally proposed single-stage forward micro-inverter. The application of multiphase techniques is addressed in Chapter 4 and Chapter 5, and the impact in different parameters of increasing the number of phases is analyzed. In Chapter 4 an original primary-parallel secondary-series variation of the forward micro-inverter is presented, while Chapter 5 focuses on the application of the interleaved configuration. Finally, Chapter 6 discusses the contributions of the thesis and the future work.

Processing and characterization of plasticized PLA/PHB blends for biodegradable multiphase systems

Blends of poly(lactic acid) (PLA) and poly(3-hydroxybutyrate) (PHB) plasticized with a lactic acid oligomer (OLA) added at three different concentrations (15, 20 and 30 wt% by weight), were prepared by an optimized extrusion process to improve the processability and mechanical properties of these biopolymers for flexible film manufacturing. Morphological, chemical, thermal, mechanical, barrier and migration properties were investigated and formulations with desired performance in eco-friendly films were selected. The efficiency of OLA as plasticizer for PLA_PHB blends was demonstrated by the significant decrease of their glass transition temperatures and a considerable improvement of their ductile properties. The measured improvements in the barrier properties are related to the higher crystallinity of the plasticized PLA_PHB blends, while the overall migration test underlined that all the proposed formulations maintained migration levels below admitted levels. The PLA_PHB blend with 30 wt% OLA was selected as the optimum formulation for food packaging, since it offered the best compromise between ductility and oxygen and water vapor barrier properties with practically no migration.