Abstract: Todays, the use of renewable resources has been expanded to reduce the consumption of fossil fuels. Photovoltaic cells and fuel cells are the most popular renewable resources, while their generated output DC voltage is low. For increasing the output voltage level, step-up and high step-up DC-DC converters are suggested for using an interface between the renewable resources and the AC inverter stage. The step-up and high step-up DC-DC converters have isolated and non-isolated structures, and due to the high volume and weight of isolated converters, non-isolated converters are more preferred. Adopting the interleaved structure for step-up and high step-up converters is a suitable approach to increase the power level and reduce the input current ripple. Also, to reduce the switching losses and increase the efficiency of the converter, many soft-switching techniques are presented. In this thesis, the challenges of step-up and high step-up converters are analyzed and then different structures and topologies along with the advantages and disadvantages are investigated. Further, various types of conventional soft-switching methods presented in state of the art journal papers are reviewed. Finally, non-isolated step-up and high step-up structures using the suitable soft switching methods are proposed. The procedure of presenting proposed converters is that, a non-isolated interleaved converter with the minimum number of passive elements is proposed as a basic topology, and the advantages and disadvantages of the basic topology are analyzed. Then, some modifications are applied to the basic converter for improving the performance in terms of voltage gain, voltage stress, and efficiency. In order to improve the voltage gain of this converter, a voltage multiplier cell is added to the circuit structure and a high step-up converter is proposed. Also, by reducing the number of elements in the power flow path, conduction losses are also decreased. In order to eva‎luate and validate the performance of the results, a laboratory prototype of the proposed high step up interleaved converter is implemented and the obtained results are given by simulation of the converter in PSIM soft-ware In the following, a new converter is proposed to further improve efficiency and reduce the switching losses, but the voltage gain is reduced compared to the previous interleaved converter. This converter has an inherent passive clamp for the main switches and by adding a ZVT auxiliary soft-switching cell, ZVS conditions for the switches is attained by using only one auxiliary switch. Due to the presence of leakage inductors, the auxiliary switch is also operate under ZCS conditions. Also, the leakage inductance of coupled inductors are employed instead of a resonance inductor for the ZVT auxiliary cell which increases the power density of the converter. In addition to the soft-switching operation of the switches, almost all diodes are also turned-off under ZCS or very close to ZCS condition which removes the reverse recovery losses of the diodes. Furthermore, the capacitive turn-on losses of the main switches, and the switching losses of all the switches are prevented. In order to eva‎luate and validate the performance of this converter, the simulation results are also given. Finally, the proposed topologies are compared and the advantages and disadvantages are investigated and some suggestion for the future work are presented. Keywords: Step up converters, High step up converters, Interleaved converters, Soft switching, passive clamp, ZVT.

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