Zero Voltage Switching of a Low-Power IPT System With High-Step Down Voltage Ratio

A two-stage configuration is attractive for the wireless chargers due to its simple control scheme and high-efficiency operation. This configuration usually consists of a regulation stage and a unregulated inductive power transfer (IPT) stage. The unregulated IPT stage is widely used to simplify the control. This stage is able to offer isolation and voltage level conversion. It is promising to introduce such an IPT stage for off-line charger. However, it is easy to lose the zero voltage switching, especially for high-voltage low-current applications. This paper explores a systematic approach to analyze the ZVS lost mechanism. Numerical method is used to derive the ZVS boundary. The proposed analysis is also valid for any other high-order resonant converters. Finally, a 25-W wireless fast charger is designed with optimal design parameters for ZVS operation.