Abstract
This paper presents a study on long-distance wireless power transfer (WPT), which formulates the voltage gain in terms of the coupling coefficient between the power transmitting unit (PTU) and the power receiving unit (PRU) coils. It is proposed that maximum power transfer efficiency (PTE) can be reached when maximum voltage gain is achieved under a matching condition between the coil quality factor and the coupling coefficient. In order to achieve maximum power delivered to load (PDL), we need to elevate the input voltage as high as the high breakdown-voltage of gallium nitride (GaN) high-electron mobility transistors (HEMT) along with class E amplifier circuit topology. In order to promote voltage gain, knowledge of the coupling coefficient between two coils including the factors of the coil diameter, wire diameter, coil turns, and the coil resistance are derived. It was observed that a lower coil resistance leads to a reduced parallel quality, which facilitates long-distance wireless power transfer. Experimental results support the findings that the maximum PTE occurred at the maximum voltage gain existing at a specific distance matches the coupling coefficient between coils. A maximum power point tracking (MPPT) method is also developed to achieve maximum PDL. At a distance of 35 cm, experiments with more than 100 W successfully receive a PTE of 57% at the PRU when the received voltage reached 1.4 kV. This is used to verify the concepts and analysis that are proposed in this paper.