Dual-loop controller for LLC resonant converters using an average equivalent circuit

LLC resonant converters are widely employed in industrial applications due to their high efficiency and power density. The design of closed-loop controllers for these converters is particularly challenging due to its non-linear behaviour and many different operating modes. The traditional linear techniques that are usually employed for this application require complex analysis or the use of empirical methods to obtain the small signal transfer functions of the system. This paper proposes a method to design a dual-loop controller using a simplified average model of the LLC resonant converter at resonant frequency. It is shown that the resonant condition represents the worst-case scenario from the control point of view, and that can be used as a reference point for designing the compensators. The theoretical analysis is validated by simulation and experimental results using a 150 W LLC resonant converter prototype.