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A controller design methodology to regulate the output voltage of a phase-shifted resonant converter is presented. The design objective is to overcome the highly nonlinear characteristics of the phase-shifted resonant converter observed when the load is unknown. The control goal is to maintain the output voltage at desired values in the presence of large load perturbations by varying the phase angle of the full-bridge converter. The controller is derived using passivity theory which ensures that the closed-loop system is exponentially convergent and that only a single output feedback is required. An adaptive version is deduced to estimate the output load resistance for the passivity control. This feature allows the use of an extra output current sensor to be avoided. The structure of the controller derived using passivity theory is very simple. Laboratory experiments performed using a digital signal processor system confirm the robustness of the converter with the passivity-based controller.