A new variable-mode control strategy for LLC resonant converters operating in a wide input voltage range

This paper proposes a new variable-mode control strategy that is applicable for LLC resonant converters operating in a wide input voltage range. This control strategy incorporates advantages from full-bridge LLC resonant converters, half-bridge LLC resonant converters, variable-frequency control mode, and phase-shift control mode. Under this control strategy, different input voltages determine the different operating modes of the circuit. When the input voltage is very low, it works in a full-bridge circuit and variable frequency mode（FB_VF mode）. When the input voltage rises to a certain level, it shifts to a full-bridge circuit and phase-shifting control mode（FB_PS mode）. When the input voltage further increases, it shifts into a half-bridge circuit and variable frequency mode（HB_VF mode）. Such shifts are enabled by the digital signal processor（DSP）, which means that no auxiliary circuit is needed, just a modification of the software. From light load to heavy load, the primary MOSFET for the LLC resonant converter can realize zero-voltage switching（ZVS）, and the secondary rectifier diode can realize zero-current switching（ZCS）. With an LLC resonant converter prototype with a 300 W rated power and a 450 V output voltage, as well as a resonant converter with 20–120 V input voltage, the experiments verified the proposed control strategy. Experimental results showed that under this control strategy, the maximum converter efficiency reaches 95.7% and the range of the input voltage expands threefold.