Nowadays, due to adverse environmental impacts caused by the use of fossil fuels and the reduction of fossil fuels sources, the use of renewable energy sources such as solar energy has received increasing attention. However, most of these energy sources are inherently low voltage. So in order to track the maximum power point as well as adjust the voltage level for the next stage inverter, a DC-DC step-up converter is required. On the other hand, in the field of renewable energy, utilization an energy storage source is inevitable since the instantaneous values of the input and output power are not equal. Instead of using multiple DC-DC converters to transfer power from each input source to the output load, a multi input converter can be used. By integrating several converters in a multi input converter the cost, size and complexity of the system can be reduced. Multi input converters are widely used in photovoltaic systems, hybrid electric vehicles and uninterruptable power supplies. Due to the widespread use of multi input converter with high voltage gain, extensive research is carried out to achieve appropriate structures and to improve them. One of the factors that should be considered in design of the multi input converters is reducing the number of the converter components. Also, by using soft switching methodes, the efficiency of these converters is increased. Multi input converters are categorized into isolated and non-isolated. The goal of this thesis is to analyze and improve the efficiency of non-isolated high step-up multi input converters. At first, the methods of creating multi input converters are described. Then, the most prominent high step-up multi input converters in literature are investigated and their performance is reported. To overcome the drawbacks of the existing converters, the first proposed converter is presented. In this converter, a high voltage gain is obtained by using the coupled inductors technique while only one magnetic element is used in this converter. Also, the number of switches used in this converter is minimal. In order to prevent voltage spikes of switches and recycle the leakage inductance energy, a clamp circuit is used. In this structure, it is possible to expand the number of input sources. The second proposed converter has a high voltage gain. In this converter, all input sources can transfer power simultaneously or individually to the load. In order to reduce switching losses and improve the efficiency of the converter, soft switching conditions are provided. The performance and design process of the proposed converters are investigated. The first introduced converter is implemented and the theoretical analyses is validated by the experimental results. 1-Multi Input Converters, 2-High Step-up Converters, 3-Soft Switching
