A High-Frequency Inverter for Variable-Load Operation

This paper presents a new inverter architecture suitable for driving widely varying load impedances at high frequency (HF, 3-30 MHz) and above. We present the underlying theory and design considerations for the proposed architecture along with a physical prototype and efficiency optimizing controller. The HF variable-load inverter (HFVLI) architecture comprises two HF inverters with independently controllable amplitude and phase connected together and to the load via a lossless power combining network, implemented here as an immittance converter. By controlling the amplitudes and relative phase of the two constituent inverters, the loading seen by each constituent inverter can be kept in a desirable range even for wide variations in load impedance. This allows for the use of highly efficient zero-voltage switching inverters that would otherwise be precluded or limited in applications presenting wide impedance ranges, such as wireless power transfer and RF plasma generation. The prototype HFVLI system demonstrates the benefits of the proposed approach. It operates at 13.56 MHz and can supply a maximum output power of 1 kW into a 21.8 + 0.3j load at an efficiency of 95.4% and is able to drive a wide range of capacitive and inductive loads at high power with high efficiency.