A 6.78-MHz Multiple-Transmitter Wireless Power Transfer System With Efficiency Maximization by Adaptive Magnetic Field Adder IC

A 6.78-MHz multiple-transmitter (TX) wireless power transfer (WPT) system was presented. An adaptive magnetic field adder (AMFA) IC was proposed, for the first time, to enable the maximization of the system efficiency (
$\eta _{\mathrm {SYS}}$
) by adaptively optimizing the amplitude and phase of the current in each TX coil on the basis of the coupling coefficient (
$k$
) between each TX coil and the receiver (RX) coil. Under the optimal condition, the current in each TX coil is proportional to
$k$
between the TX coil and the RX coil. For the independent control of the current in each TX coil, a selectively activated shared-half-bridge (HB) power amplifier (PA) together with an alternate TX coil array was proposed. To sense a small
$k$
, duty-ratio control was proposed in the integrated
$k$
sensor. The AMFA IC was fabricated by a 0.18-
$\mu \text{m}$
CMOS process with 1.8-V devices. The peak power conversion efficiency of the proposed PA reached 74%. The
$k$
sensor could accurately measure
$k$
with a percentage error within ±2.5%. A WPT system consisting of a
$4\times 4$
TX coil array driven by four AMFA ICs and a single RX coil was implemented. Experimental results showed that, compared with the conventional system,
$\eta _{\mathrm {SYS}}$
was increased from 0.11% to 51% with a load power of 576 mW when the RX coil was perpendicular to the TX coils. When the RX coil was parallel to the TX coils, a
$\eta _{\mathrm {SYS}}$
of 63%, which is higher than those in previous works, was also achieved.