In medium and high power applications full bridge topology is mostly used. This topology consists of four controllable switches. For high switching frequencies (more than 50 kHz) MOSFET is usually applied. Since switching losses increases at high switching frequencies, to reduce losses, zero voltage switching is preferred specially for MOSFET. Nowadays IGBT is the most popular semiconductor device that is used from medium to high power converters. This is due to IGBT advantages such as high current density, low cost and simple gate drive, in addition to reliability and lower conduction losses in high voltage applications. The main disadvantage of IGBT is tailing current problem. In other words, while switching from ON state to OFF state; the current will become zero after several hundreds of nanoseconds. So the overlap between this nonzero current and voltage will result in unwanted power losses. Therefore, in high frequency and high power converters soft switching techniques such as zero voltage switching and zero current switching should be applied to reduce switching losses and EMI noises. In the converters implemented with IGBT switches, due to tailing current problem at turn off instant, ZCS (zero current switching) is more effective and vastly applied. Phase shift pulse width modulation is mostly applied to drive full bridge converter in order to achieve ZVS (Zero voltage switching) or ZVZCS (Zero voltage Zero current switching). In ZVZCS converters, leading leg switches will operate under ZVS condition and lagging leg will operate under ZCS condition. Various types of ZVZCS converters are presented in the literatures. Some of these do not apply any additional auxiliary circuit in the secondary side. In these converters, ZVZCS condition is achieved by adding blocking capacitor and a saturating inductance in series with the primary side of the main transformer or using unidirectional switches in the lagging leg instead of a saturating inductance. These two types have minimum additional components and are easily controlled. However, the losses of the output rectifier remain high due to diode reverse recovery problem. In some other types, the auxiliary circuit is used in order to achieve ZVZCS condition and also reduce the output filter losses, but the numbers of additional elements arehigh which results in control circuit complexity and relatively expensive auxiliary circuit. A zero voltage and zero current switching full bridge converter with series resonance tank is presented in this thesis. This converter is based on standard full bridge topology and a series capacitor is added in the primary side to reset the leakage inductance current without any additional auxiliary circuit. Leakage inductance of the transformer is used as the resonance inductance. Using series resonance tank and applying control pulses with fix frequency, zero voltage switching for leading leg and zero current switching for lagging leg is achieved. The output power is controlled using phase shift technique. In the proposed converter soft switching condition is attained for wide range of load variation. Due to its high performance and minimum additional components with respect to regular converter, this converter can be applied for medium to high power applications. In this thesis, topology and Operating modes of the proposed converter are discussed and the validity of theoretical analysis is verified by prototype experimental results. Key Words Soft switching, ZVS, ZCS, ZVZCS
