Secondary-Side Voltage Control via Primary-Side Controller for Wireless EV Chargers

In inductive power transfer (IPT) system, the magnetic coupling has significant effects on power and system efficiency. The voltage and current on the secondary side are coupling dependent. Load measurements are not possible through sensors on the primary side. This article proposes the secondary-side voltage controller based on the primary-side control by means of bifurcation approach. The main advantage of operation under bifurcation is that the coupling and load on the secondary have no effect on the voltage gain. This enables the secondary voltage control through the primary voltage. With the proposed sensing coil on the primary side, the bifurcation detection is possible. The proposed control can utilize the frequency split phenomenon and track the bifurcation frequency to ensure maximum efficiency in various coupling conditions. In addition, the design guidelines of the parallel-parallel compensation configuration are introduced for a desired load profile and transfer distance. The constant voltage gain with respect to conditions of magnetic coupling variation is verified by using finite element analysis and experimental study. A 1-kW IPT prototype is constructed to validate the proposed bifurcation detection and voltage control. Experimental results show that the proposed IPT can achieve up to 90% of maximum efficiency while maintaining constant secondary voltage.