High-frequency Inverter Design for a Wide Range of Resistive and Reactive Load Variation

This paper proposes a design methodology for a high-frequency resonant inverter module consisting of two inverters in parallel to deliver constant output power with high efficiency under load impedance variations. Thanks to zero-voltage-switching (ZVS) with a ground-referenced device, a single-ended resonant inverter such as a class Φ
2
inverter is suitable for high-power and high-frequency applications such as plasma generation and magnetic resonance imaging (MRI). However, when the resistive or reactive load varies, the class Φ
2
inverter loses ZVS and changes its output power, which reduces the efficiency of these applications. To address this issue, we propose an inverter module consisting of two inverters in parallel, one with an inductive and another with a capacitive load network. First, thanks to a designing process of the class Φ
2
inverter for ZVS operation (mainly determined by the input inductance), we achieve ZVS even with a capacitive load network. Therefore, when the reactive load varies, the output power from the inductive and capacitive load networks changes in the opposite direction, which allows us to maintain constant output power. Also, when the resistive load varies, a phase shift between two inverters is applied to maintain the output power at the desired level. To ensure load variation does not disrupt ZVS, we design the inverters when the output load network adds the maximum inductance or capacitance to the circuit. Consequently, when the load varies, the added inductance or capacitance only reduces to achieve ZVS across the entire range of load variations. A mathematical model is established to determine a proper phase shift between two inverters for the output power control and the design of the closed-loop controller. The prototype was fabricated to deliver a constant 350 W rated power at 13.56 MHz with an efficiency of 89 - 92% over a load impedance range of 35±j50 Ω − 100±j50 Ω.