Micromachined inductors and transformers for miniaturized power converters.

摘要

Switched-mode dc-dc power converters are a ubiquitous part of modern, feature-rich portable electronic devices and are essential for efficiently transferring electrical energy out of battery sources and into various power-hungry loads, such as microprocessors, displays, sensors, and communications systems. These converters often comprise a significant portion of total system size/weight, and the largest offenders are often the associated power inductors and transformers. A significant reduction in the size the inductors and transformers would have a transformative effect in enabling new applications, such as mobile autonomous microsystems.;Increasing the switching frequency of the power converters offers to reduce the values of the required passives. However, the expected switching frequencies of next generation power converters fall into a gap between magnetic film inductors and transformers operable at 1 GHz. In answer, a new class of air-core microinductors and microtransformers is presented in this document that leveraged microfabrication-enabled advancements to attain high performance in the desirable very high frequency (VHF) switching range and to enable fully integrated power management systems in the smallest possible packages.;In order to design these devices, models were analyzed to uncover the ideal characteristics for operating in the VHF range. Compared to traditional air-core components, these new ones featured thicker windings and had more intricate windings for lower loss and higher density. A multilevel microfabrication process was developed for molding three-dimensional (3D) copper parts with the necessary characteristics of thickness, minimum feature size, and out-of-plane stacking.;The 3D copper process enabled the microfabrication of inductors with measured inductance densities up to 170 nH/mm2 and quality factors as great as 33. Transformers were measured with even greater inductance densities: up to 325 nH/mm2 was obtained in a configuration for voltage gain of 3.5 with up to 78% efficiency. Performance figures for both inductors and transformers were shown to outstrip a number of other microfabricated examples found in the literature, particularly in the frequency range of 10 MHz--1 GHz.;Microfabricated inductors were tested within the circuits of both a prototype 100 MHz switched-mode hybrid boost converter and a commercially-available surface-mount converter with up to 4 MHz switching frequency. With up to 37% efficiency at a conversion ratio of 6, the performance of the prototype 100 MHz converter when using a 14 nH microfabricated inductor largely matched that obtained when a larger 43 nH surface-mount inductor was used in the same converter at up to 1 mA load current. A packaging solution was devised for testing with the surface-mount converter. An embedded multilevel copper module consisting of both an inductor and interconnects was detached from its silicon fabrication substrate and served as a platform to which a surface-mount converter and capacitors were soldered.