A New LLC Converter with Wide Output Voltage Range and Improved Efficiency at Medium or Low Output Voltage

This paper proposes an LLC resonant converter with two added resonant cavities in the primary side of the converter and an added rectifier bridge in the secondary side (TR LLC). This converter can work in the three operating modes so that it can get a wide output voltage range. In addition, the proposed circuit can obtain any output voltage in the wide range only through the pulse frequency modulation (PFM). And then, the proposed circuit can avoid the disadvantages of phase shift modulation (PSM) method which has a narrow range of ZVS and a relatively large loop current. So, the TR LLC can avoid the loop current and achieve zero voltage switching (ZVS) in the all output voltage range and improve the efficiency significantly. This paper introduces the structure and working modes of the proposed circuit and analyzes its characteristics, which includes gain characteristic and ZVS conditions. Two 1000W prototypes of the proposed circuit and the conventional full-bridge LLC converter are established to verify the analysis and compare the efficiency.