Systematic Design of Coils in Series-Series Inductive Power Transfer for Power Transferability and Efficiency

The quality factor of coils should be high enough to deliver power efficiently during inductive power transfer. To achieve a higher quality factor, coils are designed either with a larger size and thicker wire or with a special structure, making the coils bulky and complicating the fabrication. This paper presents a systematic method to design the coils in series-series inductive power transfer with the smallest possible value of quality factor to realize the requirements of power transferability and efficiency. The electrical parameters of the coils are normalized, decoupled in equations, and designed sequentially. It is found that the smallest value of the quality factor for required efficiency is achieved using an optimal normalized impedance. The physical parameters of the coils are selected sequentially to realize the electrical parameters. This paper presents two operating conditions of series-series inductive power transfer, and example sets of coils with a 100-mm air gap are designed for these operating conditions using the same procedure. The coils were fabricated and tested in a 3.3-kW converter. The measured output voltage (200-400 V for 400 V input) and the coils' efficiency (98.9% at peak power) met the specifications. All switches turned on with zero voltage.