A Gain Boosting Array Technique for Weakly-Coupled Wireless Power Transfer

Wireless power transfer using near-field inductive coupling is the predominating technology to power up wireless devices such as radio frequency identification tags, sensors, and medical implants. Miniaturization is a new key requirement for the application in the internet of things and implantable medical devices. Realization of efficient RF power transfer using conventional techniques based on 900 MHz and 2.4GHz is difficult, mainly due to the relatively large wavelength in these bands. On the other hand, conventional inductive coupling is very inefficient as the device becomes very small, limited by the small coupling factor and operating frequency. In this paper, we utilize a multicoil transmitting (Tx) array to power up very small receiving (Rx) devices in the near field by boosting the power gain to a much higher level. The power gain boosting is achieved by operating the array close to its self-resonance frequency and optimizing the phase relationship of the array elements. To validate the proposed technique, we implemented a prototype printed circuit board (PCB) to power a 1 cm × 1 cm small Rx coil at a distance of 20 cm. To demonstrate the improvement on power gain, we designed a 3 × 3 Tx coil array composed of nine 3 cm × 3 cm elements, as well as an individual large Tx coil with the same dimension of the array for fair comparison. Measured results show that the transducer power gain can be increased by 16.4 dB with the proposed technique.