Forward and flyback isolated DC-DC converters are widely used in low and medium power supply applications, due to simple structure and low number of components. Switching power supplies are the main source of electromagnetic interferences (EMI) in electrical devices. Also, these converters have a significant effect on the weight, volume, and the total efficiency of devices. The main solution of power density augmentation is increasing the frequency of switching converters. However, it causes to increment of both switching losses and EMI, especially in high frequency applications. In order to reduce switching losses and electromagnetic interferences of high frequency converters, soft switching methods are proposed. Lossless snubbers, resonant and quasi-resonant converters, and also soft transition methods are the main techniques to provide soft switching condition in switching power supplies. Soft transition converters usually have low number of components in comparison to lossless snubbers. Also, unlike resonant converters, PWM control is prepared for soft transition circuits, due to short period of resonance. In order to implement soft transition methods, ordinarily one auxiliary switch and several passive components are used, which can provide soft switching condition independent of line and load variations for semiconductor components of converter. In soft transition circuits, soft switching of all switches, current and voltage stress, and also capacitive turn-on loss of switches are the important criteria of evaluation. In addition, employing minimum number of components can be effective to achieve high power density and lower parasitic elements which results in further reduction of EMI. In order to reduce EMI and meet very stringent requirements, other methods should be investigated, in addition to soft switching methods. Some of these EMI reduction methods are EMI filter, noise compensation methods, balance and symmetry methods, and spread spectrum algorithms, which can be used along with soft switching techniques to reduce EMI. Increase in the weight and volume of converters, complexity to implement, and also adverse effects on the efficiency are the main drawbacks of these methods. In this thesis, existing soft transition forward converters along with various soft switching two-switch forward converters are discussed and then, the principles of conducted electromagnetic interferences and the solutions of EMI reduction in switching power supplies are investigated. After that, a new ZVT forward converter is proposed which has low number of auxiliary components comprising a switch and a capacitor. Also, in the proposed converter, due to the usage of leakage inductance as a resonant inductor, no extra electromagnetic element is used. Also, in the proposed converter, the auxiliary switch has low stress and thus, low capacitive turn-on losses. Moreover, in order to employ this converter in high input voltage applications, the proposed auxiliary circuit is applied to two-switch forward converter which retains all benefits of the proposed ZVT forward circuit. In order to evaluate the EMI of the proposed ZVT forward converter, the EMI model of this converter is extracted. Finally, in order to reduce EMI of the proposed converter and meet requirements, a symmetric structure is proposed for this converter. The proposed converters in this thesis are analyzed in details and design considerations are presented. Also to verify the theoretical analysis, practical results of the converter’s prototype in addition to simulation results are presented. Moreover, in order to verify noise analysis and meet EMI requirements, EMI of the prototypes are measured practically and compared each other. At the end, conclusions and suggestions are presented. Key words: Switching power supplies, Soft switching, Soft transition, Electromagnetic interferences (EMI)