Magnetic devices are indispensable parts of modern power systems. For decades, size of the active components in power systems kept shrinking rapidly but the size of the magnetic devices had not been reduced accordingly. In portable and wearable applications, granular power-supply-on-chip applications, and other size or cost sensitive applications, the large size of the conventional magnetic devices resulted in bulky systems, larger parasitics, higher costs, and less design flexibility. Thus, magnetic devices have become the major obstacle for power system miniaturization nowadays. In this thesis, novel silicon integrated magnetic devices are proposed, modeled, optimized, and demonstrated for power system miniaturization in various applications.

First, novel silicon integrated coreles...[ Read more ]

Magnetic devices are indispensable parts of modern power systems. For decades, size of the active components in power systems kept shrinking rapidly but the size of the magnetic devices had not been reduced accordingly. In portable and wearable applications, granular power-supply-on-chip applications, and other size or cost sensitive applications, the large size of the conventional magnetic devices resulted in bulky systems, larger parasitics, higher costs, and less design flexibility. Thus, magnetic devices have become the major obstacle for power system miniaturization nowadays. In this thesis, novel silicon integrated magnetic devices are proposed, modeled, optimized, and demonstrated for power system miniaturization in various applications.

First, novel silicon integrated coreless power inductors are proposed and demonstrated for high efficiency, compact power conversions. A tapered spiral inductor is designed and fabricated for reducing the power loss caused by proximity effect. It achieves a 97% improvement in Q factor and 56% reduction in AC power loss, making it suitable for zero-voltage-switching power conversions. Then by accommodating the coreless inductor in low permittivity and non-conducting material, SU-8, the substrate capacitive loss is suppressed. As a result, the AC resistance is reduced by more than 2 orders of magnitude for a 5 μH inductor in a MHz frequency range. In a discrete implementation of a compact off-line LED driver, this low substrate loss inductor achieves a power efficiency of 89%.

Second, an analytical model based on Maxwell’s equations is proposed to calculate the AC resistance due to proximity effect in spiral inductors with thick coils. The proposed model takes into account the unique 2-D magnetic field and current redistribution effects which achieves a discrepancy of less than 9.4% compared to experiment results for frequency below the peak Q factor. With the analytical model, procedures for design optimization of the inductor for integrated off-line LED drivers are discussed. The constraints and guidelines for a design of high efficiency power inductors are summarized.

Third, novel silicon integrated inductors with magnetic cores are designed and fabricated. Different core materials and novel core structures are used to improve the inductance density and L/R ratio of the inductors. A thick (~300 μm) MnZn composite magnetic core with relative permeability of 5 is used in a backside embedded toroidal inductor. The distributed air gaps in the composite core material resulted in a large saturation current (> 10 A), making it suitable for high current applications. Then a high relative permeability (~350) NiFe magnetic core is tried out in the inductor. By using a novel vertical laminated structure for the magnetic core, the eddy current suppression and hard axis alignment can be achieved simultaneously without the need of a complicated multilayer process. The inductor achieves more than seven times increase in L/R ratio compared with previously reported inductors with a similar area (~ 1 mm²), making it suitable for small area power conversion applications.