Design of Planar Integrated Transformer for LLC Resonant Converters Based on Hybrid Magnetic Materials

LLC resonant converters are widely utilized in applications such as server power supplies, data center power systems, LED drivers, and electric vehicle onboard chargers. Traditional LLC converters employ wire-wound transformers and additional magnetic components as resonant inductors, significantly increasing the overall size of the converter. Planar magnetic integration technology enables the integration of resonant inductors and transformers, thereby enhancing power density. In various magnetic integration schemes using ferrite cores, air gaps are typically unavoidable, and the resulting fringing flux exacerbates conductor eddy current losses. This paper proposes a magnetic integration scheme for LLC resonant converters based on an E-core structure, where magnetic powder cores replace traditional air gaps to form a gapless integrated magnetic component with hybrid magnetic materials. The proposed design significantly reduces winding losses caused by fringing flux from air gaps, improves overall converter efficiency, and demonstrates more pronounced benefits at higher power levels. In an LLC resonant converter operating at 3.5 kW output power, the integrated transformer with the hybrid magnetic material solution achieves a 49.6% reduction in copper loss and 35.76% decrease in total magnetic component loss compared to the conventional air-gap design. The proposed hybrid magnetic material-based LLC converter attains a peak efficiency of 97.13%, demonstrating a 0.86% efficiency enhancement over the air-gap counterpart.