Cian O. Mathuna

One of the primary limiting factors presently inhibiting device miniaturisation is the current roadblock posed by the large size of inductors employed in power processing units. The miniaturisation and integration of the inductor is a challenge requiring innovations in novel inductor topologies. This paper reports the design, modeling, and analysis of a novel vertical magnetic-core inductor structure: coaxial cross-section of copper conductor and a magnetic core surrounding it. To evaluate the performance of the proposed inductor structure, we considered a range of performance parameters: inductance, saturation current, DC ratio of inductance to resistance (Q_dc), AC quality factor (Q_ac), etc., which are essential figures-of-merit (FOM) of micro-inductors that are employed for energy storage in PwrSiP/PwrSoC applications. A novel two-dimensional graphical approach that correlates AC performance and DC performance is introduced.We derived an inductance model for the proposed structure. The measured relative permeability (μ_r) and the magnetic loss tangent (tan δ) of Co-Zr-Ta-B magnetic thin-films, developed in-house, are used to estimate the AC quality factor. Finally, the primitive structure of the proposed inductor is fabricated, and experimental results are presented to demonstrate that it is possible to realize the proposed structure in practice.

Soft magnetic properties of thin films for use as a core material are critical for the realization of future miniaturized electromagnetic devices operating at frequencies of tens or hundreds MHz. Laminated stacks consisting of alternate thin layers of magnetic material and dielectric material are widely used to suppress eddy current losses that dominate especially at a higher frequency of operation. Thus, identifying a suitable dielectric layer, its optimum thickness, and the understanding of its effect on the performance of the laminated core is important. In this paper, six different CoZrTaB (CZTB) laminated cores are reported featuring a variety of dielectric materials (AlN, SiN, Al ₂ O ₃ , and oxide CZTB) and/or dielectric thickness (5, 15, and 50 nm). This study shows that stacks with different dielectric material have a varied residual stress that plays an important role in inducing magnetic anisotropy, thus affecting the permeability. CZTB stacks with oxide CZTB dielectric show the best combination of high permeability, low coercivity, and low losses at high frequency.

Sputtered FeCoN _x thin films with various nitrogen fraction are investigated in the quest of low lossy, soft magnetic thin films (SMFs) for high frequency inductor core. 100-nm thick FeCoN _x films were deposited in N ₂ +Ar environment (5 mTorr) with increasing N ₂ volume percent, x % = N ₂ /(Ar + N ₂ ), up to 30%. While FeCo thin film is isotropic, incorporation of nitrogen in the film exhibits a well-defined uniaxial magnetic anisotropy (UMA) as a result of reduced stress in the FeCoN _x films. FeCoN _17.5 thin film with UMA shows superior all-round soft magnetic properties (saturation flux density ~ 1.7 T, coercivity ~ 7.8 Oe, real permeability ~ 381.7 and resistivity ~ 201 μΩ.cm). Study of the dynamic magnetic properties of FeCoN _x ( x = 15, 17.5 and 19%) films reveal two ferromagnetic resonance (FMR) peaks. Double resonance peaks in films with high concentration of nitrogen might be related to FeCo and Fe-N multiphase in the films. Furthermore, evolution of the real (μʹ) and imaginary (μʺ) part of the permeability spectra in the presence of dc magnetic bias field is also examined.

This article presents an analysis of low-power, multiphase interleaved buck converters to illustrate the extent to which adding more phases is beneficial for reducing the passive components’ sizes. The analysis focuses on a typical converter application with a wide operating duty cycle range and requires good load transient. It is shown that by restricting the phase current ripple, the theoretical reduction in total inductor peak energy predicted for increased phase numbers is limited. The article assesses the benefit of inductor coupling and presents guidelines for coupling factor selection to avoid steady-state inductance roll-off over the wide input voltage range. Standard printed circuit board (PCB) manufacturing design rules are shown to place a practical phase count limit considering the reduction in total inductor size. PCB integrated, air-core solenoid and spiral inductors are implemented in both single- and two-phase 5.4 W, 20 MHz buck converter prototypes. When compared with single-phase designs, two-phase spiral inductors are 44% smaller, and the two-phase solenoid has a 54% lower loss. The two-phase prototype converter achieves better overall efficiency and peaks at almost 90% at V_IN = 4.5 V, V_OUT = 1.8 V, and F_SW = 20 MHz.

This article presents a novel design procedure for 4 ^th order and 4 ^th order resonance (4thRes) output filters, for given buck converter specifications, making components selection a straightforward process. An accurate filter analysis is provided to predict the filter component currents and voltages in both frequency and time domains. Application of the analysis in a design study of a 20 MHz, 5.4 W buck converter shows that the 4thRes filter has the potential to reduce the output passive components for a wide duty cycle range. As compared with a 2 ^nd order filter at V _IN = 6.6 V to V _OUT = 1.8 V, total inductance, inductor energy, capacitance, and capacitor energy are 58%, 35%, 45%, and 31% lower, respectively. Air-core PCB integrated solenoid inductors are considered for implementation and testing within a prototype converter to show the impact of these filters on the converter performance. The 4thRes filter achieved 3.7% and 3.6% higher full-load efficiency than the 2 ^nd and 4 ^th order filters, respectively, and a better load transient performance.

This Special Issue captures a significant portion of the current sensors research excellence in Ireland

Present needs in efficiency and integration drive research toward the miniaturization of power converters. Among the latest components to achieve the desired degree of integration are cored micro-inductors that are still one of the hardest devices to optimize, due to the high number of freedom degrees in their fabrication. In this article, a comprehensive design procedure for these micro-inductors is presented. The proposed method makes it possible to design the optimal device in a single iteration. It also allows the designer to easily ascertain the limits of the inductor in terms of handled current and losses and provides valuable physical insights into the output of the process.

This work presents analytical expressions for the dc inductance of 3-D air-core inductors with circular cross-sectional pillars (CCSPs) and rectangular cross-sectional pillars (RCSPs). We consider the following four types of inductor structures: 1) a toroid with CCSP; 2) a toroid with RCSP; 3) a solenoid with RCSP; and 4) a solenoid with CCSP. For each type, a unique analytical model is developed for obtaining dc inductance. High-frequency (1–100 MHz) effects on inductance are also discussed. The inductance values predicted by the proposed analytical models of the first three types of inductor structures are in an acceptable agreement with numerical finite-element analysis (FEA) solutions, where the maximum difference is 7.3%. Also, our analytical model for the fourth-type inductor reduces the error, when correlated with FEA inductance value, up to $6\times $ compared with previously published models. A comparison of results using the proposed analytical expressions with published measured values as well as our measurement data demonstrates the error ranging from 0.5% to 16.2%, while conventional formulas show errors of up to 143%. The results of the proposed models could serve as a good initial estimate for power supply-on-chip (PwrSoC) and power supply in package (PSiP) applications.

This paper investigates a novel approach to the integration of multi-layer thin film magnetics embedded in PCB for use in Power Supply in Package (PwrSip) applications. A solenoid inductor structure is formed by sandwiching a multi-layer CZTB magnetic sheet between two distinct PCBs with copper tracks on each which form the upper and lower windings of the device. A solder reflow process, utilizing solder balls mounted on the exposed NiAu pads of one of the PCBs, is used to create the electrical connection between the two boards, hence forming closed current loops for the windings of the inductor. This novel inductor structure enables the use of net 4oz (140 μm) copper to enclose the multi-layer CZTB magnetic sheet, and hence demonstrates very low Rdc. The multi-layer CZTB magnetic sheet was characterized using a Ryowa Permeameter. The magnetic sheet exhibited a permeability of 367. The manufactured Flip-Chip inductor was electrically characterized using a Vector Network Analyser. The device has a footprint of 7.25 mm2 and has an inductance of 52nH. The fabricated PCB embedded inductor achieved a peak quality factor of 23 at 40 MHz.

The integration of power inductors is a roadblock in realizing highly miniaturized power supply in package (PSiP) and power supply on chip. Inductors in such power systems are used for energy storage and filtering, but they dominate in size and loss. This paper presents a novel three-dimensional in-silicon through-silicon via (TSV) magnetic-core toroidal inductor for PSiP. The magnetic powder based core is embedded into a TSV air-core inductor using a casting method. The unique air-core inductor design with a hollow core and suspended windings enables a complete core filling with microscale magnetic powders. The proposed casting method is simple, scalable, and generic for a wide range of magnetic powders. TSV magnetic-core inductors are fabricated in a compact size of 2.4 mm × 2.4 mm × 0.28 mm with the core content varying from 63 to 88 wt% of soft ferrite NiZn powders. The TSV magnetic-core toroidal inductors are fabricated and electrically characterized. Small-signal measurements show a threefold higher inductance of 112 nH and a 30% higher quality factor of 14.3 at 12.5 MHz for magnetic-core inductors compared with similar TSV air-core inductors. The small-signal measurement results are verified by the modeled results. The total core loss is characterized by large-signal measurements. A suitable inductor is implemented in a 12-MHz buck converter that operates in a zero-voltage-switching mode. The converter achieves a peak efficiency of 71.6% and an output power of 2.4 W while converting 12 to 5 V _dc .