Design and Wafer-Level Fabrication of Stacked-Type Transformers for High-Density Power Converters

In response to the emerging demands in the miniaturization and integration of power conversion systems, the stacked-type transformer chips with significantly improved power efficiency for sub-MHz applications are developed in this article. The silicon-embedded high-density multilayer spiral winding structures in the transformer chips are fully metalized using our group developed Zn--Al alloy microelectromechanical system casting technique, which ensures the wafer-level batch fabrication with high consistency. After being assembled with dual E-shape magnetic cores, the fabricated stacked-type transformer chips are measured to demonstrate high inductance integration density of over 1.54 mu H/mm(2) and superior coupling coefficient of 0.985 at the targeted operating frequency of 100 kHz. Owing to the compact, low-resistance thick-metal structures, Q-factors of 20.5 at 100 kHz and transformer efficiency of above 85% in a wide frequency range are obtained from the two-port small signal measurements. Switching mode fly-back dc-dc converters are constructed to further validate the power conversion capability of the developed device. Benefiting from the implementation of the stacked-type transformers, the isolated dc-dc converters are measured to operate with an overall power efficiency of up to 82.46% at the 7.5 W power output condition, where an ultra-high power density of 17.64 W/cm(3) is also obtained.