One of the most used power electronic converters in low power applications is flyback converter. The reason of the popularity of the converter is its simple structure. The single switch flyback converter is constituted only by a switch, a transformer, a diode and a capacitor. Because flyback converter doesn't use output filter inductor, the power conversion density of the converter is smaller than other ones. However flyback converter has the advantages above, it has some problems which make it hard to use at medium and high power applications. The biggest problem is the voltage stress due to the transformer leakage inductor. It makes the designer to use a clamp circuit. Another problem is a switch voltage stress larger than input DC voltage. It causes choosing an appropriate switch to become hard. In this thesis however it is tried to solve the above mentioned problems, there is tried to reduce the switching losses. Dual switch and interleaved flyback converters can reduce the single switch flyback converter problems and are able to be used in higher powers compared with the single switch one. In the thesis two flyback type converters are suggested and introduced. The first type isdouble switch flyback converters. For these converters three lossless passive snubbers are introduced which not only provide soft switching conditions for the converter switches but also provide soft switching condition for other semiconductor elements. The second type is half bridge interleaved flyback converters. The advantage of these structures is that the current which is seen by the output capacitor is continues. It effects on the quantity, size and weight of the output capacitor. In the thesis two half bridge interleaved flyback converters are suggested which use two lossless passive snubber circuits. Same as first type, these snubber circuits can provide soft switching condition for all semiconductor elements of the converter. Usually in order to provide soft switching condition in the passive techniques, the stored energy in parasitic elements is used. The principle idea of the suggested lossless passive snubber circuits is that the snubber circuit absorbs its required energy from the input source. Thus changin output power doesn't effect on the operation of the snubber circuit. Keywords: Flyback converters,Soft switching, Passive snubber, Interleaved, Double switc h