Abstract
The LLC resonant converters are used as server power supplies in data centers, and further efficiency improvements are required as the information society develops. The LLC transformer used in LLC resonant converters requires a coupling coefficient of approximately 0.8 to utilize the resonance phenomenon between the leakage flux and the resonant capacitor. Typically, the coupling coefficient is adjusted by introducing a gap in the inner leg of the core. However, this causes fringe flux to act on the windings, leading to AC copper losses due to proximity effects. This paper confirms that using AC copper loss reduction windings in LLC transformers reduces the coupling coefficient from 0.93 to 0.88 while maintaining excitation inductance, simultaneously decreasing AC resistance by 37.9%. This improves the efficiency of LLC resonant converters by 0.54 percentage points and reduces losses by up to 4.1W
