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/mm
²
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.