Maximum Efficiency Point Tracking in Inductive Links: Series Versus Parallel Receiver's Compensation

The postregulation method of an inductive power link consists of adjusting its output voltage directly in the receiver by using a feedback dc-dc converter, thus without having to rely on back telemetry. In postregulated systems, the maximum efficiency point (MEP) of the link can be tracked by adjusting the transmitter voltage amplitude in closed-loop. In this paper, we analyze how the series and parallel receiver compensation affect differently the closed-loop control of the MEP tracking. The theoretical analysis proves that the MEP cannot be achieved with a series compensation but it can be attained with a parallel compensation. Additionally, the theoretical analysis predicts how any more complex resonant structure affects the attainability of the MEP. Finally, the analysis is validated by simulations and measurements. The proof-of-concept system postregulates the output voltage at 5 V and achieves the MEP of 40% in measurements while delivering 50 mW to the load, working at 13.56 MHz with 1-cm-air-gap and a 25mmx25mm receiver.