A Charge Normalization Based Burst Mode Control Strategy for the LLC Converter Without Current Sensing

Burst mode has been widely used in
LLC
converters at light load. Traditionally, the load current sensing is a practical solution to determine the light load operation condition for
LLC
converters to enter burst mode. With introducing the charge control for
LLC
converters, it opens up a potential solution to implement burst mode control by only using the charge because the charge contains load power information. However, due to the nonlinear relationship between the charge and the load power, different load powers may correspond to the same charge, leading to the difficulty of determining the burst mode entry points without sensing the load current. By normalizing the charge through switching frequency to obtain the approximately linear relationship between the charge and load current, this letter proposed a current-sensing-free burst mode control for the charge-controlled
LLC
converter. Therefore, a new control variable representing the normalized charge can directly reflect the load current and trigger the burst mode at light load. The simulation and experimental results implemented on a 400/12.2 V 1-kW
LLC
prototype are in good agreement with the theoretical analysis.