the output current of forward converter is not pulsating and it is suitable for low output current ripple applications. The core reset of the forward transformer is a significant problem, which is generally solved by using a tertiary winding. However, this increases the volume and cost and also limits the duty cycle. The second basic problem in the conventional forward converter is hard switching. ZVT auxiliary circuits provide zero-voltage at turn-on for the main switch. The criterions of evaluation in ZVT cells include soft switching of both main and auxiliary switches, voltage and current stress of the auxiliary and main switches, efficiency and applicability to other converters. In this thesis, at first, existing soft transition forward converters with their advantage and disadvantage are investigated. Then,a new ZVT forward converter is proposed in which ZVS conditions for the main switch and ZCS condition for the auxiliarywitchis provided. In addition, by eliminatingcapacitive turn-on loss of the main switch and eliminating the reverse recovery loss of all diodes due to ZCS turn off, theefficiency of the presented converter has increased. In addition, the voltage stress across the auxiliary switch is clamped to the input voltage. In the proposed converter, no isolated gate drivers are required for the main and the auxiliary switches while the converter is PWM. The primary windings of the forward converter is utilized to transfer power from the input to the output, providing ZVS condition and the core reset of the power transformer. As result,the tertiary winding is omitted. In the introduced converter, the power transformer is discharged during a resonance process and provides the capability of operating at duty cycles more than 0.5. Soft switchingcondition is independent from the input voltage and output load, just like other ZVT converters. This ZVT cell can be used for flyback converter, too. The proposed flyback converter provides soft switching for both the auxiliary and main switches. Also, all the above mentioned benefits attained for the forward converter, are provided for the flyback converter.The complete analysis of the proposed forward converter, its operating principals and design considerations are discussed in this thesis. Also to verify the theoretical analysis, the converter simulation results and practical results are presented.Then, operating interval of the proposed flyback converter is illustrated. At the end, the conclusions of this thesis and suggestions for future research are explained. Keywords 1-Forward converter2-oft switching3-zero voltage transition (ZVT)
