A 6.78-MHz Coupling Coefficient Sensorless Wireless Power Transfer System Charging Multiple Receivers With Efficiency Maximization by Adaptive Magnetic Field Distributor IC

Targeting the simultaneous wireless charging of multiple receiver (RX) coils, we developed a coupling coefficient (
${k}$
) sensorless wireless power transfer (WPT) system enabled by the proposed adaptive magnetic field distributor (AMFD) IC. By simply measuring the voltages and currents on the transmitter (TX) side free of knowing the
${k}$
between each pair of coils, the system can optimize the current in each TX coil to optimize the generated magnetic fields at each RX coil to maximize the system efficiency (
$\eta _{\mathbf {SYS}}$
). A current sensor calibration (CSC) technique was also proposed to guarantee the accurate on-chip current sensing in the AMFD IC. The AMFD IC was fabricated by a
$0.18~\mu $
m CMOS process with 1.8 V devices. Owing to the CSC technique, an accurate on-chip current sensing could be performed with a percentage error within ±4.5 %. A WPT system consisting of 2 TX coils driven by 2 AMFD ICs and 2 RX coils was implemented. Experimental results showed that, compared with the conventional WPT method,
$\eta _{\mathbf {SYS}}$
in the developed system was increased from 16 % to 65 % with a load power of 276 mW when the RX coils were perpendicular to the TX coils. When the RX coils were parallel to the TX coils, a high
$\eta _{\mathbf {SYS}}$
of 78 % was also obtained.