Analysis and design of a multi-tapped high-frequency auto-transformer based inverter system

The Google Little Box Challenge, an open competition to build the world smallest 2 kW Photo-Voltaic (PV) inverter, triggered a trend in the power electronic community to design PV inverter systems with a very high power density. In particular, in this paper, a novel single-phase non-isolated PV inverter topology is presented, achieving both high efficiency and high power density. The proposed solution takes advantage of a highly efficient resonant DC-DC stage to generate multiple intermediate DC voltage levels; these levels are then modulated by means of a high-frequency operating tap-selector and the resulting waveform is filtered and unfolded to obtain a low-frequency sinusoidal voltage at the output. Only a fraction of the full DC-link voltage is switched in every commutation of the tap-selector, consequently the occurring switching losses are reduced with respect to single-stage PWM inverters. At the core of the DC-DC stage, a high-frequency auto-transformer with multiple output taps is employed. Since it is realized only with a single winding and a fraction of the input current directly flows to the output, its utilization increases the efficiency and reduces the volume with respect to an equivalent transformer based solution. A detailed analysis of the working principle of the investigated inverter is presented in this paper together with a Pareto optimal design, focusing in particular on the resonant operation and on the modeling of the autotransformer. The performance in terms of losses and volume is finally provided for the optimal solution.