Bifurcation Analysis in Weakly-Coupled Inductive Power Transfer Systems

This work reveals and analyzes an unfamiliar bifurcation in weakly-coupled inductive wireless power transfer systems (IWPT) employing diode rectifier. This paper firstly provides a local analysis calculating the required RF source power, based on the rectifier ac-dc conversion gain and the two-port analysis of the coupled coils. The local analysis is supported by measured results on silicon. Successively, we demonstrate that if the secondary coil and the diode rectifier have a high quality factor, the rectifier diode varactor can create a turning-point bifurcation in the solution curve of the output dc voltage versus the RF source power. With the bifurcation, the local analysis becomes invalid and a higher source power is required. Bifurcation analysis based on large-signal capacitor modeling and conversion matrix analysis are provided, substantiating the results. The introduced bifurcation analysis is verified by three IWPT examples on PCB. A proposed design with an auxiliary nonlinear varactor is proposed to mitigate the bifurcation with improved performance.