A wide variety of power semiconductor devices are available, however power handling capacity is still a significant limitation for many applications. An increase in the capacity of a module is usually accompanied by decreasing switching frequency which reduces achievable system bandwidth. Increased capacity can be attained without bandwidth reduction by using multiple lower power modules in parallel. Products based on parallel module topologies are already available in the market. However, there are many problems in parallel system. These problems are due to the control method and topology used and none of the systems combine high performance and high power with high reliability and easy maintainability. Pulse Width Modulation (PWM) inverters are widely used in Uninterrupted Power Supplies (UPS) for critical loads such as communication systems, computer systems, and hospital equipment in case of an interruption in the main power. One of the challenges in designing commercial PWM inverter systems is to design a PWM inverter which is capable of delivering high power, with high reliability, and at a low cost. High power inverters need to dissipate a large amount of heat and handle large currents; these two factors result in low reliability and high cost. One solution for this problem is to design lower power inverter modules and applying them in parallel to achieve high power and reliability. The main goals are to achieve balanced load sharing among all inverter modules and to eliminate any circulating currents in the system. For example in an inverter system which consists of N inverter modules connected in parallel, each inverter module should provide 1/N of the load current, however in parallel structure, circulating currents may cause the output current of each module to exceed limitations and damage the inverters. Several methods are proposed in the literatures for load sharing. Some methods provide current sharing, but the modules are dependent which reduced the reliability. Also, droop method is presented. In this method the modules are independent, however, load sharing is achieved by drooping the output voltage and frequency of the inverter. Furthermore, the control circuit is very complex. In this thesis current droop method is introduced to achieve current sharing. In the proposed technique, the modules are independent and also, control circuit is simple. The proposed idea is discussed and analyzed. Simulation and experimental results are presented to verify the theoretical analysis. Keywors: 1- PWM inverters 2- Parallel inverters 3- control methods 4- High power
