Design for High-Voltage-Step-Down LLC Resonant Converter Based on Low-Winding-Turn-Ratio Planar Transformer

LLC resonant converter is widely used in big data centers and telecom rectifiers because of its ZVS, ZCS, and high efficiency. These applications require a high voltage step down LLC converter to convert high-voltage input such as 380Vdc to low-voltage output such as 12Vdc. However, with the traditional resonant transformer structure of LLC converter, because of the high voltage step down, even if the secondary windings' turn number is designed only 1, the primary winding's turn number will be about 32. In this article, a novel transformer configuration is proposed that significantly reduces the number of primary winding's turns in high-voltage-step-down LLC resonant converters. The transformer's core is designed with a central primary leg and four separated side secondary legs. The primary winding's turn number is reduced to 32/4 = 8 turns, making PCB-based planar transformer's design viable. Its four secondary windings' eight output terminals are all symmetrically placed on the same side of the transformer, to minimize the output path length and make the four secondary currents sharing. The design method, loss calculation method, and magnetic circuit model are presented. A LLC converter's prototype of 380V/12V/2kW is built to verify the low-winding-turn-ratio planar transformer structure and its magnetic circuit model.