Zero phase difference capacitance control (ZPDCC) for magnetically resonant wireless power transmission

In a magnetically resonant wireless power transmission system, a DC-DC power transmission efficiency (η_TOTAL) at an inherent resonant frequency (f_RES) is degraded when the distance between a transmitter (TX) coil and a receiver (RX) coil is short, because the frequency dependence of η_TOTAL has two peaks. In order to solve the efficiency degradation, a zero phase difference capacitance control (ZPDCC) is proposed, which is suitable for the integration to LSI's. In ZPDCC, either of the two peaks is shifted to fRES and η_TOTAL is increased by tuning the capacitance (C) of the resonator in TX and RX to keep the zero phase difference (θ = 0) between the voltage and the current in TX at Δθ / ΔC > 0. Both TX and RX circuits are fabricated in a 3.3V, 180nm CMOS. By introducing ZPDCC, the measured η_TOTAL at fRES of 13.56MHz increases 1.7 times from 16% to 27% at the distance of 2.5mm between the TX and RX coils with a diameter of 40mm.