Two Modulation Techniques for WPT Systems

The wireless power transfer technology is widely studied for applications related to recharging of the batteries of electric vehicles, but finds applications also in air, marine and medical sectors. A common feature of the different implementations of the wireless power transfer is a transmitting coil supplied by a two-legs high frequency inverter. Different modulation techniques have been proposed for the generation of the switching commands of the inverter, aimed at enhancing the output voltage control or the overall system efficiency. In any case, only one control variable is in charge of generating the commands for both the legs of the inverter. This paper proposes two modulation techniques that use two independent control variables to control the two inverter legs. No any relation is imposed between the control variables and, for any possible combination of them, the switching behavior of the inverter and the harmonic content of the generated voltage are analyzed. It is then recognized that some combinations of the control variables, which define the inverter control strategies, achieve better efficiency performance than the conventional phase shift technique, usually applied in the wireless power transfer systems. The presented modulation techniques are implemented in a prototypal system and experimented under different operating conditions. Consistent with predictions from theoretical analysis, experiments showed that by applying the proposed modulation techniques it is possible to evenly distribute the switching voltage solicitations between the two legs of the inverter and to achieve higher efficiency. With respect to the conventional phase shift modulation technique, depending on the used modulation technique and the considered power range, the improvement of the overall system efficiency was as high as 10% whilst the improvement of the inverter efficiency reached 20%.