Highly-laminated, high-saturation-flux-density, magnetic cores for on-chip inductors in power converter applications
The objective of this project is development of a manufacturing process to realize thick highly-laminated metallic magnetic cores with suppressed eddy current losses for power applications and demonstration of integrated magnetic components using the process. The continual reduction in size and increase in functionality of portable electronic devices drives the development of ultracompact electrical power converters and therefore their associated inductors, which typically occupy the largest volume within the converter. This inductor size reduction can be achieved by increasing both the converter switching frequency as well as inductor operating flux density by utilizing appropriate magnetic core materials. Magnetically soft metallic alloys (e.g., NiFe and CoNiFe) are good core material candidates due to their superior magnetic properties (e.g., high saturation flux density and low coercivity) compared to conventional ferrite. However, to minimize eddy current losses, the core thickness is limited to the skin depth (typically less than 3 µm for 10 MHz operation); cores of this thickness are likely insufficient for handling high power (> 10W). Therefore, there is great interest in the development of highly-laminated metallic alloy cores with single lamination thickness in the sub-micron range to achieve suppressed eddy-current losses and high power handling capacity simultaneously. During the project, highly-laminated metallic alloy cores comprising up to 2000 layers of permalloy (Ni 80 Fe 20 ) or CoNiFe materials with single lamination thickness less than 1 µm have been fabricated based on automated multilayer electrodeposition technology compatible with CMOS manufacturing. The developed highly-laminated metallic cores preserve the high saturation flux density and low coercivity characteristics of metallic alloys, while suppressing eddy-current losses less than 1 W/cm3 at operation flux density of 0.5 T and frequency over 1 MHz. The developed highly-laminated metallic alloy cores have been further integrated into microfabricated windings to realize compact, on-chip inductors. Utilizing CMOS-compatible integration techniques, circular (toroid) and bar-shape (solenoid) microinductors have been fabricated exhibiting a constant inductance higher than 1 µH up to 50 MHz with peak quality factor as high as 35 at 5 MHz. The microfabricated inductor with highly-laminated core was tested in a power converter and demonstrated a converter efficiency of 93% at 5.5 MHz and 42 W under 100V input and 35V output operation.