On Using CNTFs-Based Wires for High Frequency Wireless Power Transfer Charging Systems

In many practical wireless power transfers (WPT) applications, such as cell phone wireless charger, electrical vehicles, and implantable cardiac pacemakers, the weight and power losses in the coupled coils are primary design issues. Currently, metallic wires are normally used to fabricate the WPT coils. The metallic wires have several problems, mainly involving large weight, poor mechanical performance, corrosion, and resistive losses (especially at high frequency due to both of skin and proximity effects). Carbon Nanotube Fibers (CNTFs)-based wire is a promising candidate to replace the conventional metallic wires in these applications. CNTFs-based wires have superior electrical properties at high frequencies. Furthermore, it shows high mechanical performance, light weight, and high thermal conductivity. This paper uses CNTF wires to investigate a planar spiral transformer suitable for WPT charging systems. Efficiency of the transformer is investigated theoretically and experimentally over several MHz frequencies and ampere currents. The CNTFs-based wireless transformer operation was compared with copper (Cu) based wireless transformer fabricated with similar dimensions and DC resistance values. The AC resistance of both transformers is calculated using the conventional model of solid round conductor and the FEM simulation. It is found that the calculated and the FEM AC resistance values are matched in case of Cu transformer with 4194A Impedance/Gain-Phase Analyzer measurements while they are mismatched in case of CNTFs one. Therefore, a correction factor for the AC resistance of CNTFs wire is proposed to match the calculated AC resistance with the measured one. Furthermore, both transformers were tested experimentally within 50 W/6.78 MHz WPT charging systems at different ampere currents. The charger efficiency measurements verify the analyzer results showing better efficiency with CNTF based transformer in comparing to copper one.