Jizheng Qiu

Integrating Magnetics for On-Chip Power: A Perspective

Integration of efficient power converters requires technology for efficient, high-power on-chip inductors and transformers. Increases in switching frequency, facilitated by advances in circuit designs and silicon or wide-bandgap semiconductors, can enable miniaturization, but only if the magnetics technology works well at the higher frequencies. Technologies, geometries, and scaling of air-core and magnetic-core inductors and transformers are examined, and their potential for integration is discussed. Air-core inductors can use simpler fabrication, and increasing frequency can always be used to decrease their size, but magnetic cores can decrease the required thickness without requiring as high a frequency.

A Technology Overview of the PowerChip Development Program

The PowerChip research program is developing technologies to radically improve the size, integration, and performance of power electronics operating at up to grid-scale voltages (e.g., up to 200V) and low-to-moderate power levels (e.g., up to 50W) and demonstrating the technologies in a high-efficiency light-emitting diode driver, as an example application. This paper presents an overview of the program and of the progress toward meeting the program goals. Key program aspects and progress in advanced nitride power devices and device reliability, integrated high-frequency magnetics and magnetic materials, and high-frequency converter architectures are summarized.

Design and Fabrication of VHF Tapped Power Inductors Using Nanogranular Magnetic Films

The design and fabrication of coupled inductors with low profile, low cost, and high power density for tapped-inductor high-step-down dc-dc power converters in the very-high-frequency (30-300 MHz) range are presented. The inductor consists of a multilayer thin-polyimide printed circuit board with nanogranular thin-film magnetic material deposited on both sides and on beveled cuts to form a fully linked closed core. The magnetic material Co-Zr-O has good soft magnetic properties, high resistivity, and high flux density capability. Multilayer Co-Zr-O/ZrO2 thin films are used to improve the performance of the magnetic core. A winding loss model of this coupled inductor is presented. Coupled inductors with turns ratios 2 and 10 for several different converter applications have been designed, fabricated, and measured. Loss in the magnetic material is investigated. Small-signal and large-signal measurements show good performance of the fabricated components with high-frequency, high-field excitation.

An improved AC loss model for the optimization of planar-coil inductors

This paper presents a computationally efficient method for calculating ac winding losses in planar-coil inductors. This loss model is employed in a multi-objective optimization algorithm to determine inductor designs which minimize power loss while maximizing power density. The ac winding losses in the optimized inductor designs are compared to finite-element simulations - as well as to estimates from other loss model approaches - and exhibit less than 7% error across the tested optimized designs. The loss model additionally provides the designer with an understanding of how to reduce ac conduction losses by balancing the current distribution in each turn of a winding.

Radial-anisotropy nanogranular thin-film magnetic material for toroidal inductors

Thin-film nanogranular magnetic materials are attractive for micro-fabricated very-high-frequency power magnetics, but their anisotropy makes them difficult to use in toroidal geometries. A method to deposit magnetic materials with the radial anisotropy required for high-performance toroidal inductors is introduced in this paper. The radial anisotropy is induced during sputter deposition of Co-Zr-O thin films by an applied radial magnetic field. A magnetic field orientation fixture which consists of two magnets and iron components is designed to create a radial field predominantly parallel to the substrate plane. Initial prototype toroidal cores with 8-mm inside diameter, 16-mm outside diameter, and thickness of 6 μm have been fabricated. Hysteresis measurements along the radial direction (easy axis) and the circumferential direction (hard axis) of deposited samples show the desired anisotropy orientation. Small-signal measurements show that the fabricated toroidal core has flat relative permeability near 60 to several hundred megahertz, and reaches a quality factor higher than 100 in the range of tens of megahertz.

Inductor design for VHF tapped-inductor dc-dc power converters

The design and fabrication of coupled inductors with low profile, low cost, and high power density for tapped-inductor high-step-down dc-dc power converters in the very high frequency (30–300 MHz) range are presented. The inductor consists of a multi-layer thin-polyimide printed circuit board (PCB) and nano-granular thin-film magnetic material deposited on both sides and on beveled cuts to form a fully linked closed core. The magnetic material Co-Zr-O has good soft magnetic properties, high resistivity and high flux density capability. Multi-layer Co-Zr-O/ZrO2 thin films are used to improve the performance of the magnetic core. A winding loss model of this coupled inductor is presented. Coupled inductors with turns ratios 2 and 10 for several different converter applications have been fabricated.

Modeling and measured verification of stored energy and loss in MEMS toroidal inductors

This paper presents the derivation and verification of a sinusoidal steady-state equivalent-circuit model for microfabricated inductors developed for use in integrated power electronics. These inductors have a low profile, a toroidal air core, and a single-layer winding fabricated via high-aspect-ratio molding and electroplating. Such inductors inevitably have a significant gap between winding turns. This makes the equivalent resistance more difficult to model. The low profile increases the significance of the energy that is stored in the winding, which together with the winding gap makes the equivalent inductance more difficult to model. The models presented here account for these effects. Finally, the models are verified against results from 2-D and 3-D finite-element analysis (2-D FEA and 3-D FEA) direct measurement, and from in-circuit experimentation. In all cases, the equivalent-circuit model is observed to be accurate to within several percentage.

Design of toroidal inductors with multiple parallel foil windings

This paper presents the design of toroidal inductors with multiple parallel foil windings. Compared to single-winding toroidal inductors, parallel-winding inductors have the same inductance and core loss, but can achieve higher voltage operation, reduced capacitance, reduced leakage flux, and reduced winding resistance if designed well. The parallel-winding design concept, optimization of an example, and detailed calculation of loss including a new analysis of circumferential-current associated loss are discussed in this paper. It is predicted that for a toroid with outer radius of 1 mm, inner radius of 0.6 mm, height of 0.3 mm, 4 turns, the ac winding loss can be decreased by 32% at 50 MHz with double windings, and can be decreased by 39% with five sections of windings in parallel.

A toroidal power inductor using radial-anisotropy thin-film magnetic material based on a hybrid fabrication process

This paper presents improvements for the design and fabrication of high-frequency toroidal power inductors with and without radial-anisotropy thin-film magnetic material. An improved winding resistance model for toroids is developed considering an angle factor for actual winding shape, the effect of spacing between turns and loss associated with exterior current in the circumferential direction. A hybrid process for fabricating low-profile magnetic-core toroids is presented. The process uses standard flex printed circuit board (PCB) technology for the top and bottom winding layers with vias electroplated after sandwiching the magnetic core between the two winding layers. Thin-film magnetic material (Co-Zr-O) is deposited in the presence of a radial magnetic field, which induces radial anisotropy in the toroidal core. Small-signal measurements show that the toroidal core has a relative permeability over 40 at frequencies up to several hundred megahertz, and very high quality factor in the frequency range below 100 MHz. Air-core and magnetic-core toroidal inductors using this hybrid fabrication process were built and tested at small-signal levels. Magnetic-core toroids showed a higher inductance and quality factor than air-core toroids at frequencies below 100 MHz. This winding loss model and fabrication process can be applied to both air-core and magnetic-core toroidal inductors.

Measured performance and micro-fabrication of racetrack power inductors

This paper presents micro-fabrication results and electrical performance measurements for chip-scale racetrack power inductors. Magnetic components with sputtered Co-Zr-O magnetic cores are demonstrated in a multi-megahertz dc-dc converter for solid-state lighting applications. The dc-dc converter achieves an 89% conversion efficiency at 5 MHz with an inductor power density of 1 W/mm2 of substrate area. Small-signal measurements of the inductors are compared with modeled predictions to validate the design optimization approach. Fabricated components achieved inductance values of 1.2 μH and peak quality factors of 15.1 at 8.3 MHz.