Design and Optimization Techniques of Over-Chip Bondwire MicroTransformers with LTCC core

This paper describes the realization of bond-wire micro-magnetics by using standard bonding-wires and a toroidal ferromagnetic LTCC core with high resistivity. The proposed fabrication procedure is suitable for the development of magnetic components on top of an IC with small profile and small size (< 15 mm2). A transformer is designed and applied over-chip, working in the MHz range with high inductance (~33 μ H) and high effective turns-ratio (~20). Applications include bootstrap circuits and micro-power conversion for energy harvesting. Measurements demonstrate a maximum secondary Q-factor of 11.6 at 1.3 MHz, and a coupling coefficient of 0.65 with an effective turns ratio of 19, which are among the highest values reported for toroidal miniaturized magnetics. The achieved inductance density is 2 μH/mm2, along with an inductance per unit core volume of 15.6 μH/mm3, and a DC inductance-to-resistance ratio of 2.23 μH/Ω. The presented technique allows to obtain over-chip magnetics trough a post-processing of the core, and it is also suitable for highdensity power supply in package (PwrSiP) and power supply on chip (PwrSoC). Finally, a series of optimization techniques for planar core magnetic devices in order to maximize the inductance per unit area are discussed and applied to the considered case.