Wireless Power Transfer With Zero-Phase-Difference Capacitance Control

Wireless power transfer enables the frequent and ubiquitous charging of electronic devices. However, the variation of the efficiency and the received power with the transmission distance is an outstanding issue. To solve the problem of efficiency degradation of the magnetic resonance at short distances, zero-phase-difference capacitance control (ZPDCC), which is suitable for integration in large scale integrations (LSIs) is proposed in this paper. The proposed ZPDCC achieves adaptive capacitance control by a newly proposed control algorithm with a current-sensing circuit to control variable capacitors at a fixed frequency. Additionally, a theoretical analysis of the total DC-DC power transmission efficiency (ηTOTAL) including a power amplifier, coupled resonators, and a rectifier is demonstrated in this paper. The analysis indicates that the frequency (and capacitance) splitting of ηTOTAL is mainly due to the power amplifier; additionally, the efficiency of the power amplifier is maximized at the split peaks when the transmission distance (d) is short. A wireless power transfer system in magnetic resonance with ZPDCC is fabricated in a 3.3 V, 180 nm CMOS. By introducing ZPDCC, the measured ηTOTAL at 13.56 MHz increases 1.7 times from 16% to 27% at d=2.5 mm.