Design of High-frequency, Load-independent Resonant Inverter Using Phase-Shift Control Method

This paper presents a phase-control method in paralleled resonant inverters to provide constant output power independent of load variation at MHz frequencies. A single-ended resonant inverter such as a class Phi(2) inverter operates at 10's of MHz frequencies while providing high efficiency thanks to zero-voltage-switching (ZVS). However, when the load varies, their output power is not constant, which is not suitable for high-power applications such as plasma generation. To overcome this challenge, we propose a new approach, designing two inverters in parallel to maintain constant rated output power by controlling a phase shift between two inverters. First, we optimize the output resonant tank to minimize the phase shift to achieve high efficiency and reduce the value of the input inductance to ensure the ZVS operation for the entire range of load variation. Then we model the two-inverter design to find a proper phase shift at different load values. The desired PWM signals with the phase shift are applied to each switching device in the inverters through a PI controller. The proposed inverter was implemented to deliver a constant 300 W rated power at 13.56 MHz with an efficiency of the module above 92% over a load range of 25 Omega - 55 Omega.