UC Davis

Wireless power transfer has significantly improved the user experience in device charging and is currently an area of active research. Some major limitations of wireless charging systems include size constraints due to low frequency operation, the need for close proximity and tight alignment for efficient power transfer, and limited use of the device while charging. This work proposes several techniques to passively boost the voltage harvested and decrease the size of the power module. The rectifier and DC-DC converter explored in this dissertation extend technology proposed in existing literature that uses a resonator as the high quality factor component in the antenna matching network and energy storage element, respectively. This project proposes further development of these technologies by multiplexing the resonator between the rectifier and converter to reduce power module size and decrease generated electromagnetic interference by eliminating the need for magnetics. Preliminary testing has shown a resonator is more effective than an inductor (by a maximum of 30 percent) in boosting the rectifier output voltage and can boost the rectifier output voltage by a maximum of 8 times compared to an unmatched rectifier at 18 MHz on a commercial-grade printed circuit board. Initial DC-DC converter simulations with NMOS switches and realistic diodes show a 100 kHz resonator can store energy in a boost DC-DC converter on the order of nanojoules for a 10 V to 14 V step up conversion with 68 percent efficiency. This is similar in performance to a simulated inductor-based converter, which has a 78 percent efficiency when operating with the same step up ratio. The resonator-based DC-DC converter was built on a printed circuit board (PCB) using discrete transistors and a 185 kHz ceramic resonator. Simulations and measurements both showed the proposed discrete transistors did not effectively boost a 5 V input voltage by 1.4 times as was shown previously. Different methods to address this issue such as a floating driver and PMOS switches were proposed. An integrated design was also implemented using the open-source SkyWater 130 nm CMOS process design kit (PDK). A magnetoelectric (ME) antenna is proposed to decrease the size of the antenna by operating at acoustic resonance. Another benefit is more lenient alignment rules, which provide more device usage flexibility while charging. The maximum voltage output from the fabricated ME antenna is 0.3 V with a Qi transmitter producing 0.8 W as the source. The size of the antenna is 2 cm by 1.5 cm, which is smaller than the commercial Qi receiving antennas of 4 cm by 4 cm. When used to harvest vibrational energy, a slightly larger antenna (3 cm by 1.5 cm) could provide a maximum output power of 0.3 uW for an input vibration acceleration of approximately +/- 8 g.