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This article has shown the design and characterization of silicon-integrated thin-film micro-transformers and their application in a quasi-resonant flyback converter. Two micro-transformers with different turn ratios of 4:4 and 3:3 were designed. The micro-transformers in this study have elongated spiral shape windings sandwiched between two layers of the thin-film magnetic core, and this structure can leverage the anisotropic core property. The micro-transformers were built on a silicon substrate using micro-electro-mechanical systems (MEMSs) fabrication process to ensure a high integration level and high power density. The inductance of 4:4 and 3:3 micro-transformers are 57.3 and 31.5 nH, respectively, their coupling coefficients are 90.5% and 82.7% at 30–40 MHz, and the measured quality factors are 7.82 at 21 MHz and 8.94 at 35 MHz, respectively. A resonant flyback converter topology deploying with a high electron mobility transistor [Gallium nitride (GaN)] was used to evaluate the electrical performance of two micro-transformers. Two resonant flyback converters operating at zero voltage switching (ZVS) reached the maximum conversion efficiency of 46.4% (3:3) and 45.7% (4:4), respectively, with 3.3 V input. The 3:3 micro-transformer developed in this work achieved the highest power density of 128 mW/mm2 with a similar efficiency compared to other cored micro-transformers. The loss analysis of the flyback conversion circuit shows that the main losses in the circuit are the copper winding loss of the micro-transformer and the eddy current loss of the magnetic core. The conversion efficiency of the circuit will be improved by applying thicker copper and core lamination in the future. [ABSTRACT FROM AUTHOR]

What is the future of integrated inductor design on silicon for power conversion applications at frequencies up to 100 MHz—is it magnetic-core or air-core inductors. This study presents measured results for two microfabricated inductors (magnetic core and air core), which have been designed to operate at 20 MHz and occupy a substrate area of less than 10 mm^2. The inductor technology and design are briefly discussed. An optimized inductor design study is, then, presented. Both magnetic-core and air-core inductor designs are compared and evaluated, in terms of inductance and efficiency per unit area for frequencies up to 100 MHz. The design of the microinductors is discussed and an analytical design optimization program is used to model the devices for the maximum efficiency and inductance. The introduction of laminations with high-frequency core inductors will also be examined within the study. A 100 MHz magnetic-core inductor design with three laminations gives a 36 nH inductance, with 96.4% efficiency and an area of 3 mm^2. A comparable air-core design also gives a 36 nH inductance, with 93.45% efficiency and an area of just 2.6 mm^2. [ABSTRACT FROM AUTHOR]