Control system analysis and design of a resonant inverter with the variable frequency variable duty cycle scheme

This paper proposes a general modelling analysis method for a complete closed-loop control system for resonant inverters, such as LC series resonant inverter, LLC and LCC series-parallel resonant inverters, which use variable frequency variable duty cycle scheme. A half-bridge series resonant induction cooker with the variable frequency asymmetrical pulse width modulation (VFAPWM) control is used as a modelling example. Two control loops are implemented for this control system, by which the inverter is operated at zero-voltage-switching (ZVS) over wide load range. Phase-locked loop (PLL) is used as frequency tracking control, asymmetrical pulse width (APWM) as power regulation. The complete closed-loop dynamic model of a half bridge series resonant inverter system is solved. Then, the analysis and design of these control loops are discussed, and design considerations are provided. Finally, experiment results are given.