Flux-Free Region Based Magnetically Integrated Fractional-Turn Matrix Transformer for LLC Converters

With the rapid growth of artificial intelligence and data center computing power demands, power supply systems face dual challenges of high power density and high efficiency. These trends drive the bus voltage toward high-voltage direct current (HVDC). However, existing transformer solutions struggle to simultaneously meet the requirements of extreme high step-down ratios and compact layouts. This paper proposes a magnetically integrated fractional-turn matrix transformer with a high step-down ratio for high-performance DC-DC conversion. Based on the flux cancellation principle, the proposed scheme systematically integrates discrete fractional-turn units into a single high-density magnetic component. To address circuit-magnetic compatibility challenges during integration, a vertical excitation loop is innovatively designed. A “flux-free region” via-hole is constructed in the center of the magnetic plate. This structure accurately reconstructs the magnetic coupling characteristics of fractional turns. Meanwhile, it forms a low-impedance current path and effectively eliminates parasitic interference in the drive loop. Based on this scheme, a 1.2 kW, 1 MHz LLC-DCX resonant converter prototype is designed and fabricated. This prototype achieves direct conversion from 400 V to 6.25 V (64:1). Experimental results demonstrate that the prototype achieves an ultra-high power density of 2.5 kW/in³ and a peak efficiency of 97.40%. These results verify the superiority of the proposed scheme for next-generation high-performance computing (HPC) applications.