Design of High-Efficiency Inductive Charging System with Load-independent Output Voltage and Current Tolerant of Varying Coupling Condition

Maintaining constant-current (CC) and constant-voltage (CV) outputs for meeting the charging profile of lithium-ion batteries while realizing zero-voltage-switching on the primary bridge over wide ranges of load and coupling variations are rather challenging due to the presence of leakage and magnetizing inductances of a loosely coupled transformer. Although the challenges can be greatly relaxed by properly designing a compensation network with load-independent output characteristics, the desirable characteristics rendered by fixed compensation networks will be lost and cannot be fully restored by existing control methods once the designed compensation is deviated from the nominal coupling condition. In this article, a dynamic series/series-parallel compensation network, which originally aims to deliver constant output voltage only, is further investigated as a feasible topology for CC-CV battery charger adaptive to different sizes of air gap/misalignment of the coils. As benefited from the extra controllability offered by the parallel compensation capacitance as compared to the series/series compensation counterpart, this article provides alternative design criterion for the effective parallel compensation capacitance for designing and maintaining desirable charging currents in the CC stage while maximizing the efficiency in the CV stage under varying coupling condition. An experimental prototype with the air gap ranging from 10 to 16 cm is built to verify the idea.