Practical Issues with Unloaded Resonant Inductive WPT Link Operating in Load-Independent Regime

The paper reveals practical issues present in series-series compensated unloaded inductive wireless power transfer system operating in load-independent regime. It was recently shown, that due to existence of parasitic resistances, corresponding output voltage is not completely load-independent. Based on time-domain analysis, DC voltage gain of an unloaded system was predicted for a wide range of operation frequencies. However, parasitic lumped capacitance of the receiving coil was not taken into account. It is therefore shown that parasitic oscillations appearing in an unloaded secondary impose significant overvoltage at DC output, which may cause safety issues. Two possible solutions are discussed, allowing damping the oscillations thus making the unloaded operation feasible. The presented analysis is well-verified by simulation results and experiments of a 400V, 1kW inductive wireless power transfer link.