Modeling the Load Equivalent Resistance of a Series-Series Inductive-Coupled Resonant Wireless Power Transfer System

Resonant inductive coupled wireless power transfer systems are mainly implemented as opportunity charging stations in many applications, such as drones. The estimation of the load equivalent resistance seen before the bridge rectifier in the secondary circuit is crucial for the optimal design of this kind of systems. Generally, the equivalent load resistance model assumes the LC-filter capacitor large enough to neglect the ripple component of the current that flows into the battery. This paper investigates if the above assumption is still correct when the designer chooses a smaller value of the output capacitor, in order to maximize the power transfer at the expense of a nonoptimal efficiency and a larger ripple in the battery voltage and current. The load equivalent resistance was calculated by means of LTSpice simulations in this specific case. The results are compared to those obtained by the typical model generally used in the literature. They show that the development of a new analytical model to extract the load equivalent resistance is required to justify the large difference found.