AC/PV dc-dc Power converters

The voltage step-up converters have always been the subject of intense research in the domain of power electronics, because of the different topologies as well as the variety of applications, such as UPS, electric vehicle systems, battery chargers and renewable energy sources, i.e. wind power or photovoltaic systems, either autonomous, or connected to the utility grid. Given the depletion of energy sources from fossil fuels, the price increase of these resources, but also the environmental impacts associated with their use, there is always a "hunt" for new energy sources, as well as the best exploitation of the existing ones. In the photovoltaic systems, the electronic power converter plays a key role in the system, since it is responsible to adjust the output voltage of the photovoltaic module in order to provide the maximum possible energy to the storage medium (a battery, or the grid via an inverter). Additionally, since solar energy is an unlimited resource, the set objective is the greatest possible penetration of these systems in the electrical network of each country.

The newest technology in the field of home solar power is the AC/PV modules, systems that integrate the solar panel, together with a dc-ac converter which connects to the wall outlet. Key criteria for the development of these systems are the degree of performance, cost and lifetime, and therefore the research to improve the operation of the converters is constant. Methods to achieve this are to invent new topologies, or to modify the existing ones, improving their most important characteristics, which vary depending on the application. Possible approaches to properly select the components (active and passive) of the power converter, or, on the other hand by designing the proper control strategy. Typical optimization criteria set for those converters are their energy efficiency, their size (volume and mass), their reliability, response to changes or cost, especially when it comes to mass-produced devices.

The integrated converter can be either single-stage or double-stage. In the double-stage approach, a dc-dc converter is used to step-up the voltage of the photovoltaic panel and perform MPPT. After that a power inverter supplies ac power to the grid. During the initial stages of my PhD thesis, I studied various topologies for use in AC/PV modules and different MPPT algorithms. Two laboratory experimental prototypes were built to investigate the converters' performance.

The first one is a single switch boost dc/dc converter, which consists of a boost converter connected in series with a flyback converter but with a single active switch. Because of the two converters, four different operating modes exist for the combined converter, based on the current flowing through each magnetic component. The different operating modes were analytically presented and the transfer function for each operating mode, as well as the boundary condition between the various modes were derived.

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The second family of dc/dc voltage step-up converters are voltage multipliers. Voltage multipliers are ac/dc voltage step-up converters. However, if a dc/ac inverter is used, their combination leads to a high step-up ratio dc/dc converter. A high-frequency resonant inverter was used as an input stage of the voltage multipliers, which were designed, in order to validate the theoretical analysis of using different capacitance ratios for each stage. The designed converters were a Half-Wave Cockroft-Walton Voltage Multiplier and a Dickson Voltage Multiplier.

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