A Si-FET-Based High Switching Frequency Three-Level LLC Resonant Converter

This paper highlights the proposed silicon field-effect transistor (Si-FET)-based high switching frequency three-level (TL) LLC resonant converter. It provides a detailed operational analysis of the converter; the multilevel (ML) organization of cells; voltage-balancing principles; current-balancing principles; loss comparison between Si-FETs and gallium-nitride (GaN)-FETs; and an optimal design consideration based on loss analysis. This analysis reveals that the switching losses of all power switches can be considerably reduced as the voltage across each switch can be set to half of the input voltage without an additional circuit or control strategy. Moreover, the current of each resonant inductor is automatically balanced by a proposed integrated magnetic (IM)-coupled inductor. Therefore, the operating frequency can be easily increased to near 1 MHz without applying high-performance switches. In addition, the resonant tanks of the converter can be a group of cells for multilevel operation, which indicates that the voltage across each switch is further reduced as more cells are added. Based on the results of the analysis, an optimal design consideration according to the resonant tank and switching frequency is discussed. The proposed converter was validated via a prototype converter with an input of 390 V, an output of 19.5 V/18 A, and a frequency of 1 MHz.