Zero-Voltage Switching Operation of Transformer Class-E Inverter at Any Coupling Coefficient

This paper presents a complete design methodology of a Class-E inverter with a loosely coupled transformer under zero-voltage switching conditions. At a selected maximum coupling coefficient, the inverter satisfies both zero-voltage switching (ZVS) and zero-derivative switching (ZDS) conditions to achieve a high efficiency. As the coupling coefficient is decreased from its maximum value to zero, the inverter satisfies the ZVS condition over a wide range of load resistance. The proposed method absorbs the magnetizing inductance and the leakage inductance into the inverter topology. The closed-form expressions for all the Class-E inverter parameters are derived in terms of the coupling coefficient, including: 1) input impedance of the transformer with a complex load impedance; 2) equivalent phase of the resonant circuit for both nominal and suboptimum operations; and 3) slope of the switch voltage waveform. The relationship between the voltage slope and the coupling coefficient is introduced, which indicates the proximity of the operating point from ZVS and ZDS conditions. A design example of the Class-E inverter is presented to achieve both ZVS and ZDS, at an optimum coupling coefficient of 0.77, and ZVS condition for any coupling coefficient lower than 0.77 and over a wide range of load resistance. Saber simulations and experimental results are given to validate the soft-switching capabilities over a wide range of the coupling coefficient and load resistance. The theoretical results were in good agreement with Saber simulations and experiments.