Design of a High Efficiency, Wide Input Range 500 kHz 25 W LLC Resonant Converter

LLC resonant converter with secondary synchronous rectification is capable of achieving soft-switching both at the primary and secondary transistors, which reduces greatly the switching noise and offers great performance improvements over the other converter topologies. In this paper, the LLC converter's equivalent electrical circuit is established, and its frequency-gain characteristics is analyzed. Based on the theoretical analysis, a prototype of 500 kHz, high efficiency half bridge LLC resonant converter operating at zero voltage switching-discontinuous current mode is designed and full detailed design considerations are presented. The designed resonant converter is capable of generating a constant output voltage of 5 V based on a wide input voltage range of 20 V-40 V, with a nominal power of 25 W. The converter's frequency response is analyzed and a voltage regulation control scheme is proposed. Experimental results verify that the converter achieves soft switching both at the primary and secondary transistors with a high efficiency and a magnetic core volume miniaturization compared to the PWM converters. Furthermore, soft-start, over-current protection and a fast transient response are attained.