Load Observing System Based on Wave Reflection for Soft-Switching Control of MHz-Frequency Resonant Inverter Against Load Fluctuations

Applying MHz-frequency inverters to radio frequency (RF) power sources for plasma applications has been challenging because capacitive loads caused by load fluctuations cause destructive hard-switching. This paper proposes a MHz-frequency load impedance estimation method for soft-switching control. The proposed system estimates the load impedance by utilizing a directional coupler that is typically built into an RF power source to observe traveling and reflected waves. Therefore, unlike typical low-frequency power electronics systems, the proposed system does not require output current sensors which are not suitable for MHz-frequencies. In the experiment, the proposed method was tested using the auxiliary switch-inductor circuit of a 3.75MHz gallium nitride (GaN) full-bridge inverter as the control target. Capacitive load detection by the proposed method was experimentally verified, and soft-switching control was realized under capacitive load. The transient response of the control under a fluctuating load that changes from inductive to capacitive was also measured. The inverter transitioned to soft-switching within 2.31<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>s after a load change, and hard-switching took only 8 cycles.