Via-Winding Magnetics: Optimal Design for Ultra-Thin Vertical Multiphase Coupled Inductors

This paper presents the principles of via-winding magnetics and introduces ultra-thin multiphase coupled inductor structures optimized for vertical power delivery. By integrating vertical straight-through winding vias and vertically coupled pinwheel magnetic cores, these inductors achieve balanced vertical current flow with very low interconnect impedance and high efficiency. A systematic optimization method was introduced to guide the design of such inductors under performance and volume constraints. Two representative designs are demonstrated and compared: a 1.8 mm high, 8×8 mm2 inductor with 0.09 mΩ perphase winding resistance supporting 100 A total current, and a 2.5 mm high, 9×9 mm2 variant with 0.12 mΩ per-phase resistance handling 160 A total current. Both designs are integrated into a four-phase buck VRM module and tested, achieving a current density of 1 A/mm2 , a power density of 3,960 W/in3 , and a peak efficiency of 91.5% and 93.5%, respectively, at a 2 MHz switching frequency for point-of-load 4 V to 1 V conversion.