Watt-level wireless power transfer based on stacked flex circuit technology

This paper presents the design, simulation, fabrication, and experimental characterization of a multi-layer spiral inductor that acts as the receiver coil for watt-level wireless power transfer. The inductor was designed with multiple vertical laminations where 88-μm-thick copper coils were separated by 25-μm-thick Kapton films using a flexible PCB fabrication technique. This Cu-Kapton approach has the potential for lower-cost coil fabrication than relatively expensive Litz-wire winding techniques. Varying turn widths were implemented to account for proximity effects and maximize the coil current distribution uniformity inside the coil windings at a given frequency, as validated by two-dimensional electromagnetic simulations. The multi-layer design incorporating lamination of four layers together with width variation exhibited a Q-factor improvement of 150% in comparison to the single-layer inductor. It was measured to have an inductance of 17 μH and a Q-factor of 50 at 300 kHz with an outer diameter of 5 cm. With a Litz-wire inductor as the transmitter coil for wireless power transfer, a load power of 7 Watts was transferred at 300 kHz over a distance of 5 cm and 5 Watts over 10 cm, two times the coil diameter, achieving an overall efficiency (defined as the ratio of the received load power to the total input power to the driving circuitry) of 46% and 23% respectively. In comparison, a manually-wound Litz-wire receiver coil with same characteristics under similar conditions demonstrated an overall efficiency of 58% and 39% at one and two-diameter distances, respectively.