Digitally Implemented Charge Control for LLC Resonant Converters

This paper proposes a digitally implemented charge control for LLC resonant converters. By analyzing the small-signal characteristics of the resonant capacitor, it is revealed that the resonant capacitor has exactly the same functionality as a resettable integrator in the charge control for a PWM converter. Based on the analyzed results, the proposed charge control is implemented simply by sensing the voltage across the resonant capacitor in a switching period. The proposed method features simple implementation with resistive voltage divider and suitability for digital control as well as good performance as other multiloop controls have. The design guideline for the charge controller is given based on the loop gain analysis. Finally, the operation and performance of the proposed method are verified with a 240-W LLC resonant converter.