Surface-Oxidized Amorphous Alloy Powder/Epoxy-Resin Composite Bulk Magnetic Core and Its Application to Megahertz Switching LLC Resonant Converter

To realize a compact, lightweight and high-efficiency megahertz (MHz) switching DC-DC converter using SiC/GaN power device, inductor/transformer core must have small core loss at such high frequency to maintain high efficiency of the converter. This paper focuses on Fe-based amorphous (Fe-AMO) alloy powder used in metal composite bulk magnetic core for MHz switching DC-DC converter, where fine Fe-AMO powder with a mean diameter of 2.56 μm was used to suppress MHz band eddy current inside the Fe-AMO powder body. When applying Fe-AMO powder to the closely packed composite core together with epoxy resin, high electrical-resistivity layer must be formed on the Fe-AMO powder surface in order to suppress the overlapped eddy current between adjacent Fe-AMO powders. In this paper, about 10 nm-thick oxidized layer of the Fe-AMO powder surface was successfully formed by using annealing in dry air. This paper describes on the surface-oxidized Fe-AMO powder/epoxy-resin composite bulk core transformer and its application to GaN power device MHz switching LLC resonant DC-DC converter. By using the Fe-AMO composite core transformer, the fabricated 48 V-input/24 V-output LLC resonant converter exhibited 90% over efficiency in the output power range of 24 to 120 W, which was higher efficiency when using the Ni-Zn ferrite core transformer.