Nowadays non-isolated high step down DC-DC converters are vastly applied in microprocessors supply, LED lightning equipment, power factor correction and battery chargers. Because of development in microprocessors technology, their current demand and power consumption has increased, which can be reduced through decreasing supply voltage. Also precise control of light intensity, power factor correction and high efficiency are features highly needed by modern lightning systems. Therefore non-isolated high step down DC-DC converters can be an ideal solution to these problems. In this thesis the most basic non-isolated structure i.e. buck converter is investigated and the problems concerning its application in high step down application has been discussed. Then the previously mentioned solutions for reducing the impact of problems are discussed. There are six approaches to resolve these problems including, extending the duty cycle, limiting voltage and current spike of semiconductor elements, reduction of voltage and current stress of semiconductor elements, provide soft switching conditions for semiconductor elements, reducing conduction and reverse recovery losses and increasing the switching frequency. Thus four new topologies are proposed to overcome the drawback of high step down converters. The proposed topologies have low gain and their operating duty cycle is high. All switches of the proposed converters operate under soft switching conditions and only one auxiliary switch is used for each of them to provide soft switching condition. Considering the intrinsic active clamp structure of the proposed converters , Leakage inductor energy has been completely recovered and the resulting voltage spike has been limited. The performance of the proposed converters is analyzed and their corresponding theoretical analysis are provided. In addition to simulation results a laboratory prototype of each converter is implemented and the theoretical analysis is confirmed by experimental results. Finally an overall conclusion is provided. Key Words: High step down DC-DC converters , voltage and current spike , voltage and current stress, soft switching conditions, reverse recovery
