Today, with the increasing development of hybrid electric cars and the increasing use of computers in home and industrial applications, the issue of electric power supply of these applications has become one of the most challenging issues in the world. These applications require high-current DC / DC converters, which in cases that the input and output isolation is not required, synchronous buck structure is used due to its simplicity, low cost, and low component count. This structure uses a synchronous rectifier instead of the diode of the buck, to reduce the conduction losses. However, synchronous buck converter in high current high step-down applications suffer from disadvantages such as high conductive losses of MOSFETs, narrow duty cycle, switching losses due to hard switching condition of the main switch, and reverse recovery loss of the synchronous rectifier body diode. The narrow duty cycle increases the current stress of the main switch and control complexity of the converter. The interleaved structure of the buck converter is used to improve the transient response of the converter, reduce the conduction losses, and also improve the efficiency over a wide range of output currents. This method also reduces the output current ripple; however, interleaved structure has little effect on narrow duty cycles. Another challenge is the proper timing for triggering the synchronous rectifier, which might cause the short circuit of input source, and damages switches of the converter. Various methods have been proposed to improve the performance of the high step-down buck converters, one of which is altering the structure of buck, in order to increase the duty cycle, and, if possible, provide soft switching conditions for the main switch. The objective is to apply as little modification as possible to the synchronous buck converter to eliminating its disadvantages while maintaining the simplicity of the buck converter. The general idea behind these methods is to create a voltage source in reverse to the input to reduce the effective voltage. For this purpose, coupled inductors or capacitors are used in the power path. In this thesis, three non-isolated converters based on the coupled inductor buck structure along with two isolated converters based on forward topology are introduced. In the first proposed non-isolated converter a partially zero-voltage switching condition is provided at turn-off time of the main switch by using a lossless passive snubber circuit, which results in the improved efficiency of the converter. In addition, a self-drive circuit with gate energy recovery is used to automatically generate the synchronous rectifier control pulses, and reduce the complexity of the converter. In order to further reduce switching losses and reduce the number of converters, a second proposed non-insulated structure was introduced. In this converter, ZCS condition of the main switch is achieved without requiring any extra snubber or clamp circuits. Thus, the switching losses are limited to output capacitance loss of the main switch. Finally, the third proposed non-insulated structure with the same number of components as the second converter is introduced, which provides switching at zero voltage conditions of the MOSET. As such, switching losses are minimized and the efficiency of the converter is dramatically increased. Introduced isolated structures, in addition to providing the soft switching conditions, automatically generates the control pulses of synchronous rectifier. This is especially important in isolated applications that require the floating control signals for SRs. The advantage of the second isolated converter over previous circuits is that the proper self-drive pulses are generated for a wide range of input voltage. Key words:High step down, Synchronous rectifier, Soft switching