An automatic stage transition converter for an inductive power transfer system is presented in this paper. An effective control strategy with two working stages of independent energy injection stage and free resonance stage is employed in the proposed converter. With the automatic stage transition strategy, when the frequency of the resonance network changes, the ending time of the free resonance stage is automatically determined. At the same time, the phase angle of the free resonance stage is automatically set as half a resonant cycle. As the stage transition is not triggered by the switches, the switch motion can be executed in advance of the transition moments. Time margins are offered for every switch in the converter, which make the switching moments of the switches flexible and the control simple. Another feature of this converter is that during the energy injection stage, the energy is injected into the inductor independently. Therefore, the input power can be easily regulated by adjusting the energy injection time. A prototype for the converter and the inductive power transfer system was implemented experimentally. From the experimental results, the automatic stage transition and power regulation capability of the proposed converter are verified. The switches all operated at the soft switch condition. When the energy injection time was adjusted from 10 μs to 25 μs, the output power changed from 143 W to 740 W.

The Independent energy injection and free resonance (IEIFR) system is a type of incomplete-resonance wireless power transfer system, which has a great tolerance with control strategy. This article uses a discrete-time modelling method to determine its all possible and rational operation points. This method samples the state variables with the switching cycle and once the system achieves steady-state, the mapping between those samples is a fixed point mapping. Combined with the known mode analysis, the rationality of those possible operation points can be verified. After the experiment verification, this method is helpful to analysis the incomplete-resonance wireless power transfer system.

For the fully resonant wireless power transfer (WPT) system, the high coupling of the converter and the resonant network introduced many problems, such as frequency splitting, the power curve peak limit, and the strict switch strategy. To solve these problems, this paper proposed a new six-switch topology based on the full–-bridge converter. With the unique structures containing two capacitor-isolated switches and a source-isolated diode, the system decouples the converter and the resonant network, and its modes have been decoupled, called the independent power injection and free resonance WPT (IPIFR–WPT) system. The capacitor-isolated switches and the source-isolated diode make the converter operate only when the voltage on the primary capacitor is equal to the source voltage, and the source will be isolated by the diode when the capacitor voltage is great than the source, which provides a wide time margin for the switches of the converter to turn on in advance. In this margin, the operation point is self-determined the same whenever the switches turn on so that the system’s performance is consistent. Based on this characteristic, the system can self-adapt a dynamic change in system parameters, with at least 15% tolerance for the coupling coefficient and 14% for the load resistance.