On the Minimal Loading of Sensorless Series-Series Compensated Inductive WPT Link Operating at Load Independent Voltage Output Frequency Without Feedback

It is well-known that continuous conduction mode (CCM) operation at certain frequency yields nearly load-independent voltage gain in practical series-series compensated inductive wireless power transfer links (SS-IWPTL). Such an operation is usually referred to as “load independent voltage output” (LIVO). Residual load dependence is due to the presence of non-zero equivalent series resistances in both transmitter and receiver. On the other hand, it has been shown that voltage gain remains load-dependent at the same operating frequency in discontinuous conduction mode (DCM) irrespectively of equivalent series resistances values. Consequently, the output voltage of SS-IWPTL operated at LIVO frequency resides within a range with lower bound corresponding to rated load and upper bound corresponding to zero loading for a given input voltage. The upper bound expression (typically attained in CCM) is derived using the first harmonic approximation (FHA) based equivalent circuit and is well-established by previous works. On the contrary, derivation of the lower bound (achieved in DCM) requires differential equation (DE) based analysis and was barely treated in the literature. The paper demonstrates that lumped capacitance of receiving side coil and input capacitance of receiving side rectifier must be taken into account during DE-based analysis since they impose significant receiving side voltage oscillations, which may give rise to safety issues. In order to damp these oscillations and keep the output DC voltage below a predetermined limit, minimal loading should be applied at the SS-IWPTL output, creating an RCD-snubber equivalent. Guidelines for the minimal load sizing are derived in the paper analytically. Simulations and experiments based on a 400V, 1kW SS-IWPTL operating at LIVO frequency demonstrate excellent matching, validating the proposed analysis.