Wireless power transfer using a field-enhancing coil and a small-sized receiver with low coupling coefficient

This study investigates the near-field wireless power transfer system with small-sized receiver. The receiver is placed at a relatively long distance from the power emitter. The coupling between the receiver and the emitter is very weak, which results in low-power delivery and efficiency. A passive resonance coil, which enhances the field, is added to the emitter to address these problems. The necessary coupled-mode theory is developed and combined with optimal operation of the class-E amplifier. The impact of the enhancing coil (EX) on the energy distribution is discussed from the perspective of energy amplification between the source coil and the EX. The point of the maximum energy of the EX is found and necessary conditions for improving transfer efficiency of such structure are proposed. An experimental setup operating at a frequency of 300 kHz was built using the EX concept. The maximum secondary magnetic field generated by the EX was established and used to wirelessly power a 4 cm receiver with more than 40 mW at 0.5 m. The structure is also compared with a similar system without an EX. Test results show that the inclusion of the EX in the emitter strengthens the spatial magnetic field and improves the transmission efficiency.