Control Architecture for Full Bridge LLC Series Resonant Converters Using Output Diode Current

The controller design for LLC resonant dc-dc converter is challenging due to large number of poles whose locations varies with operating conditions. A controller's ability to reject input disturbance is required to reduce the input filter size and increase power density. Also, high bandwidth of the controller allows for fast transient response. This article proposes a control architecture utilizing the output diode current measurement that essentially reduces the control-to-output transfer function for an LLC resonant converter to a first order and provides a very high degree of input voltage disturbance rejection with a minimal increase in control-loop complexity. A small-signal model for the proposed control variable is derived and loop analysis using Bode plot highlights the advantages of the proposed method. Simulation results are given to show the controller's transient performance. The control architecture is validated using a 150W hardware prototype. Experimental results confirm the proposed method's stability and fast transient response.