Increasing tolerable coupling coefficients range of series-series compensated inductive wireless power transfer systems operating in restricted sub-resonant frequency region with constant current output

Frequency control of series-series compensated inductive power transfer links (SS-IWPTL) operated in sub-resonant region while feeding variable voltage-type loads (e.g. batteries) with constant current under uncertain coupling conditions allows to minimize volt-ampere (VA) rating of transmitting-side converter. In such systems, variations of coupling coefficient and load voltage are counteracted by operational frequency adjustment so that primary current magnitude remains nearly unchanged throughout the complete operational range, as opposed to input voltage and phase shift control approaches. In practice, allowable operational frequency band is typically restricted by certain application and/or standard (e.g. SAE J2954). As a result, tolerable load voltage span and coupling coefficients range are also limited. Classical design guidelines impose the resonant frequency of SS-IWPTL to coincide with the upper bound of allowable operational frequency band for operation with the minimum expected coupling coefficient and the lower bound corresponding to operation with the maximum expected coupling coefficient. The paper demonstrates that generalizing this approach by releasing the former constraint adds a degree of freedom, allowing to expand tolerable coupling coefficients range for a given operational frequency band at the expense of slight efficiency deterioration. Analytical relations describing the above-mentioned behavior are established, clearly indicating the arising trade-offs. Simulation and experiments validate the proposed methodology.