DC-DC converters are widely used in electrical and electronic systems. Applications of step up and high step up converters include solar cells, fuel cells, electric vehicles, motor drive systems, and battery chargers. Increasing the operating frequency in switching converters reduces the volume and weight of converter passive elements. But, increasing the switching frequency leads to higher switching losses. Soft switching methods are employed to reduce switching losses and increase efficiency. In this thesis, first, a review of common methods of increasing voltage gain in step up and high step up converters is provided. Then soft switching methods are analyzed. Switch resonator Converters are a dir=ltr The purpose of this thesis is to present high step up DC-DC converters with complete soft switching and the minimum number of semiconductor elements based on switch resonator converters. In this regard, first a high step up converter is presented which has a very low number of elements. In this converter, complete soft switching is established at zero voltage for the switch, and both converter diodes are turned off at zero current. This converter is fully analyzed and its design relationships are expressed. The converter is simulated by ice software and a laboratory converter prototype is implemented to evaluate the performance and accuracy of theoretical analysis. Then another high step up converter is introduced which has two phases. This converter is based on switch resonator converters with two switches, and to increase the gain in this converter, a traditional back-boost converter is integrated in the topology in such a way that no exta switch is added to the converter, and full soft switching is established. The performance of the converter is completely analyzed, its design relationships are expressed and the results from the converter simulation are presented. Keywords 1-Soft switching 2- High step up converter 3- Switch resonator converter 4- Zero voltage switching