New approach for cascaded-transformers-based multilevel inverter with an efficient switching function

A new multilevel PWM inverter is presented to improve the system characteristic of a prior 11-level shaped PWM inverter scheme. In appearance, it consists of three full-bridge modules and three cascaded transformers; therefore, the configuration of the proposed multilevel PWM inverter is equal to that of the prior one. Only the turn-ratio of a transformer and its corresponding switching function are different from each other. Based on these differences, the proposed multilevel PWM inverter has two promising advantages. First, output voltage levels increase almost two fold. Consequently, it can generate more sinusoidal output voltage waves. Second, due to a suitable switching pattern, it lightens power imposed on the transformer, which is used for compensating output voltages with chopped pulses between steps. Operational principle of the proposed 19-level shaped PWM inverter is analysed with comparisons of the prior 11-level shaped PWM inverter. The validity of the proposed inverter system is verified by computer-aided simulations and experimental results based on a 1kW prototype.