A 6.78MHz 94.2% Peak Efficiency Class-E Transmitter with Adaptive Real-Part Impedance Matching and Imaginary-Part Phase Compensation Achieving a 33W Wireless-Power-Transfer System

Wireless charging of portable devices has received explosive growth due to its convenience. However, high efficiency is usually achieved at a specific load point in current wireless power transfer (WPT) systems rather than in a wide impedance range. The load varies greatly when charging state changes as shown in Fig. 9.7.1. And this variation will cause both real and imaginary parts of the transmitter's (TX) load impedance to vary over a wider range reflected from the coil coupling. To overcome this drawback, some TX designs [1]-[5] utilize a Class-D architecture due to its ease of control. Nevertheless, Class-E power amplifiers have garnered interest benefit by their reduced switching losses with zero-voltage-switching (ZVS), zero-voltage-derivative-switching (ZVDS) as well as smaller number of switches, which presents a higher potential for efficiency, especially at high frequency operation [6]-[7]. In comparison to Class-D architectures, Class-E systems is more vulnerable to the fluctuations in switching states and exhibits heightened sensitivity to variations in load conditions. To compensate the load imaginary part of the detuning problem, the dual-loop regulation bilateral bias capacitor is designed in [8]-[5]. Nevertheless, using with discrete power MOSFETs as resonant capacitors leads to a large size. Moreover, the inherent nonlinearity of the MOS capacitor constrains dynamic range and accuracy of the control system, thereby posing challenges for the TX in achieving higher efficiency over a wider load range. A compact single-stage regulated Class-E is employed in [10] with adaptive ZVS control to achieve enhanced end-to-end (E2E) efficiency across load and distance. However, the impedance characteristics of TX were not evaluated and only the ZVS switch state was considered. Under high input current conditions, the lack of ZVDS control can lead to a decrease in efficiency.