Design and Control Method of T-Type Three-Level LLC Converter for Wide Input Voltage Application

The LLC converters feature high efficiency, soft switching, and low cost. However, its limited gain range makes it challenging to apply the conventional method to wide voltage input applications, such as fuel cell and photovoltaic power generation, which are compatible with wide input. This paper proposes a new modulation method for T-type three-level half-bridge LLC converters. This paper analyses the existing operating modes of T-type LLC converters and proposes the upper half bridge (UHB) mode and lower half bridge (LHB) mode of T-type LLC converters, which have only half the DC voltage gain of normal pulse frequency modulation (PFM). The problem of voltage unbalance between the voltage divider capacitors is avoided by using the method of switching between UHB mode and LHB mode. Meanwhile, the transition method between UHB mode and LHB mode is designed by using the plane trajectory analysis method, which achieves fast and smooth transition between UHB mode and LHB mode. Finally, a simulation model and an experimental prototype are built, and the effectiveness of the method is demonstrated by circuit simulation and prototype.