Nowadays inverters have found many applications in industry such as induction motors drives with speed control, uninterruptible power supplies, AC power supplies with voltage and frequency control capability and active power filters. In order to reduce the size of output filter, increase the current or voltage quality, and to eliminate audio noise in circuit magnetic elements, high switching frequency is indispensable. However, switching losses increase at higher switching frequencies. In order to reduce switching losses and switches voltage and current stresses, usually turn on and turn off snubbers are employed which increase total inverter losses. Typical method employed to reduce switching losses and stresses, is applying soft switching techniques. In these techniques, extra elements are added to circuit that approximately eliminate overlap of voltage and current at turn off and turn on instants. These inverters are divided in to load resonant inverters and DC-link resonant inverters. Load resonant inverters have high number of auxiliary switches and are used in high power applications. In medium power applications, usually DC-link resonant inverters are used. In these inverters, only one resonant circuit exists in their DC-link. In recent years, the research is mainly concentrated on increasing the efficiency and decreasing the number of extra elements in inverters. There are usually two auxiliary switches or more in the inverter DC-link auxiliary circuit. In current thesis, reducing extra elements particularly auxiliary switches is achieved by employing new topologies. To achieve this goal, three novel quasi-resonant DC-link inverters are suggested and the converter operation and theoretical waveforms are presented. Also, design consideration of each converter is provided. To validate the theoretical analysis, two prototype converters are implemented and the results are compared with theoretical analysis. Keywords: 1- Three phase inverter, 2- Soft switching, 3- Quasi-parallel resonant DC-link, 4- Pulse width modulation, 5-Zero voltage switching, 6- Zero current switching.
