Boost Assist Control of LLC Resonant Converter for Wide Voltage Range by Use of Secondary-side MOSFETs with ZVS using Reverse Recovery Current of Body Diodes

LLC converters are generally highly efficient; however, the operating voltage range is not wide. There are generally two methods for maintaining output voltage by increasing the voltage boost ratio when input voltage is decreased. One is changing the turns ratio of the transformer; however, the operating point at rated conditions is far from the resonance frequency. The other approach is decreasing the magnetizing inductance of the transformer for charging a resonant capacitor; however, the magnetizing current increases. This decreases efficiency at rated conditions. We propose a control method for extending the voltage range without changing circuit parameters. To assist boost operation, the resonant-capacitor voltage is increased by applying secondary voltage to the transformer in the opposite direction to the primary-side output using secondary-side MOSFETs for synchronous rectification. The MOSFETs are turned on under zero-voltage-switching (ZVS) conditions using reverse recovery current of their body diodes. We evaluated the proposed control method through simulation using designed parameters for 3-kW LLC converter. The simulation results indicate that maximum output voltage increased by 22% and allowable minimum input voltage decreased from 333 to 285 V at rated output current with the proposed control method.