Design and Optimization Techniques of Over-Chip Bond-Wire Microtransformers With LTCC Core

This paper describes the realization of bond-wire micromagnetics by using standard bonding wires and a toroidal ferromagnetic low-temperature co-fired ceramic core with high resistivity. The proposed fabrication procedure is suitable for the development of magnetic components on the top of an integrated circuit with a small profile and a small size (<15 mm²). A transformer is designed and applied over chip, working in the MHz range with high inductance (<inline-formula> <tex-math notation="LaTeX">$\sim 33~\mu \text{H}$ </tex-math></inline-formula>) and high effective turns ratio (~20). Applications include bootstrap circuits and micropower 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 <inline-formula> <tex-math notation="LaTeX">$2~\mu \text{H}$ </tex-math></inline-formula>/mm², along with an inductance per unit core volume of 15.6 <inline-formula> <tex-math notation="LaTeX">$\mu \text{H}$ </tex-math></inline-formula>/mm³, and a dc inductance-to-resistance ratio of 2.23 <inline-formula> <tex-math notation="LaTeX">$\mu \text{H}/\Omega $ </tex-math></inline-formula>. The presented technique allows to obtain over-chip magnetics trough a postprocessing of the core, and it is also suitable for high-density power supply in package and power supply on-chip. Finally, a series of optimization techniques for planar core magnetic devices in order to maximize the inductance per unit area is discussed and applied to the considered case.