Damping impact on dynamic analysis of LLC resonant converter

In this paper, the small-signal model of LLC resonant converter is derived using extended describing function (EDF) methodology and the dynamics of the LLC resonant converter are studied based on the small signal model taking into account the circuit parasitic resistance. When designing the controller, this damping effect induced by the parasitic resistance is usually neglected. While it will be shown in this paper that for large transformer turns ratio, the secondary side parasitic resistance introduces significant damping in the resonant tank. A controller is designed for an example LLC circuit. It is shown that the control loop phase margin and the cross over frequency are highly dependent on the damping factor, which should be considered in the control loop design to have the desired transient performance. The experimental verification is provided as well.