Duty and Phase Control of a Class E Rectifier with Nonlinear Capacitance of FETs

This paper presents a control strategy of a class E rectifier with the nonlinear capacitance of active devices at MHz frequencies for wireless power transfer (WPT) systems. The class E rectifier benefits high-frequency (HF) WPT systems thanks to a zero voltage switching (ZVS) operation with a single and ground-connected switching device. However, to control the output power of the rectifier, the nonlinear capacitance C oss of field-effect transistors (FETs) may distort the shape of the drain-voltage waveform, which causes non-ZVS or partial diode operation. To address this issue, we propose an α (=duty ratio) and β (=phase delay) control method while considering the nonlinearity of C oss of enhancement Gallium Nitride (eGaN) FETs to generate proper gate signals for power control. The selected pair of α and β values at a given output power offers the optimum control path to achieve high efficiency. In experiments, the rectifier showed an efficiency of 94% at the nominal power (240W) with the proposed αβ-control method. Also, the method presented higher efficiency than α- or β-control over a wide range of output power, 100 − 240 W. Lastly, the effectiveness of the proposed control method was also demonstrated in closed-loop control during dynamic operation from 200 to 150 W, with a very small ripple (1.7%) in the output power.