Stability Analysis and Efficiency Optimization of an Inductive Power Transfer System with a Constant Power Load

Series-series compensated inductive power transfer (SSIPT) systems have been widely studied and characterized for constant resistance loads (CRLs) and constant voltage loads (CVLs), but much less so for constant power loads (CPLs), although CPLs have numerous applications. In this work, we address some of the fundamental knowledge gaps for SSIPT/CPL systems that we believe have not been fully explored in the literature. First, we apply Middlebrook's stability criterion to derive a closed-form impedance-based stability condition for SSIPT/CPL systems. The derivation of the equilibrium solution is based on small-signal analysis and we show its consistency with intuitive results from perturbation-based arguments. Second, we show that the power transfer efficiency is minimum at the resonant frequency of the primary resonator. Third, the stability criterion is used to develop a straightforward approach for finding the operating frequency and input voltage that achieves near-maximum power transfer efficiency. This solution is useful as a starting point for a more meticulous parameter sweep to find the optimum input voltage and frequency values. Our analytical results are validated by performing frequency sweep measurements with two SSIPT experimental setups - one tuned to 165 kHz and the other to 6.78 MHz. We also provide an intuitive description and comparison of voltage-driven and current-driven CPLs. This topic is rarely treated in an intuitive manner and largely ignored, but we believe a solid conceptual understanding of voltage-driven and current-driven CPLs is beneficial for designers.