Enrico Macrelli

Modeling, Design, and Fabrication of High-Inductance Bond Wire Microtransformers With Toroidal Ferrite Core

This paper presents the design of miniaturized bond wire transformers assembled with standard IC bonding wires and NiZn and MnZn ferrite toroidal cores. Several prototypes are fabricated on a printed circuit board substrate with various layouts in a 4.95 mm × 4.95 mm area. The devices are modeled by analytical means and characterized with impedance measurements over a wide frequency range. Experimental results on 1:38 device show that the secondary self-inductance increases from 0.3 μH with aircore to 315 μH with ferrite core; the coupling coefficient improves from 0.1 with air-core to 0.9 with ferrite core; the effective turns ratio enhances from 0.5 with air-core to 34 with ferrite core. This approach is cost effective and enables a flexible design of efficient micromagnetics on top of ICs with dc inductance to resistance ratio of 70 μH/Ω and an inductance per unit area of 12.8 μH/mm2 up to 0.3 MHz. The design targets the development of bootstrap circuits for ultralow voltage energy harvesting. In this context, a low-voltage step-up oscillator suitable for thermoelectric generator sources is realized with a commercial IC and the proposed microtransformers. Experimental measurements on a discrete prototype report that the circuit bootstraps from voltages down to 260 mV and outputs a dc voltage of 2 V.

Design and fabrication of a 315 μΗ bondwire micro-transformer for ultra-low voltage energy harvesting

This paper presents a design study of a new topology for miniaturized bondwire transformers fabricated and assembled with standard IC bonding wires and toroidal ferrite (Fair-Rite 5975000801) as a magnetic core. The microtransformer realized on a PCB substrate, enables the build of magnetics on-top-of-chip, thus leading to the design of high power density components. Impedance measurements in a frequency range between 100 kHz to 5 MHz, show that the secondary self-inductance is enhanced from 0.3 μH with an epoxy core to 315 μH with the ferrite core. Moreover, the micro-machined ferrite improves the coupling coefficient from 0.1 to 0.9 and increases the effective turns ratio from 0.5 to 35. Finally, a low-voltage IC DC-DC converter solution, with the transformer mounted on-top, is proposed for energy harvesting applications.

Design of Low-Voltage Integrated Step-up Oscillators with Microtransformers for Energy Harvesting Applications

This paper describes the modeling of startup circuits in battery-less micropower energy harvesting systems and investigates the use of bond wire micromagnetics. The analysis focuses on step-up Meissner oscillators based on magnetic core transformers operating with input voltages down to ≈100 mV, e.g, from thermoelectric generators. As a key point, this paper examines the effect of core losses and leakage inductances on the startup requirements obtained with the classical Barkhausen criterion, and demonstrates the minimum transconductance for oscillations to occur. For validation purposes, a step-up oscillator IC is fabricated in a STMicroelectronics 0.32 μm technology, and connected to two bond wire microtransformers, respectively, with a 1:38 MnZn ferrite core and with a 1:52 ferromagnetic low-temperature co-fired ceramic (LTCC) core. Coherently with the proposed model, experimental measurements show a minimum startup voltage of 228 mV for the MnZn ferrite core and of 104 mV for the LTCC core.

Design Optimization of Integrated Magnetic Core Inductors

This paper presents a design study of novel geometrical configurations for miniaturized toroidal planar core thin-film inductors. A standard core with square-shaped toroidal geometry is compared with a new core with serpentine toroidal geometry. A design procedure is provided to maximize the inductance value of a square toroid in a certain footprint area. In addition, the mathematical model of the inductor demonstrates the possibility to further increase the maximum inductance value for closed or air-gap core inductors in a given footprint area (up to 35% in 16 mm2), by means of a more efficient use of the core surface area in the case of serpentine geometry, assuming the same coupling factor for both windings. Furthermore, the presented serpentine geometry increases performances in terms of magnetic energy density and quality factor, while retaining higher inductance values, at the cost of a limited increase of layout complexity. This paper also provides a more accurate estimation of the inductance of a square-shaped toroidal inductor based on the evaluation of the mean magnetic path length accounting for the non-uniformity of the magnetic flux density in the core cross section and for the corner effect.