Ting Ge

Gap design for nonlinear ferrite cores to maximize inductance

Conventional design of ferrite-cored inductor employs air gaps to store magnetic energy. In this work, the gap length is allowed to be smaller than the conventional value so that the nonlinear ferrite material is biased in the region with low permeability and, hence, significant energy density. A peak in the inductance-gap relationship has thus been uncovered where the total energy stored in the gaps and the core is maximized. A reluctance model is formulated to explain the peaking behavior, and is verified experimentally. Curves of inductance versus gap length are generated to aid the design of swinging inductance and reduce the core size.

UV-assisted 3D-printing of soft ferrite magnetic components for power electronics integration

For ease of fabricating and integrating ferrite magnetic components in power electronics circuits to improve the power density and efficiency, we explored a paste-extrusion additive manufacturing method to fabricate ferrite core and conductive winding in a single platform. A UV-sensitive low-temperature (< 950 °C) sinterable NiCuZn ferrite paste formulated by ourselves as well as a commercial nanosilver paste were used as feed materials for 3D printing ferrite cores and silver windings, respectively. A planar inductor with one turn of silver winding embedded was 3D-printed to demonstrate the feasibility of fabrication. After the 3D structure of the planar inductor was printed, it was sintered to 920 °C for 14hrs. The sintered ferrite paste had relative permeability of 70, and the sintered nanosilver paste had a DC resistivity 1.5 times of pure bulk silver. The measured inductance and DC winding resistance of the 3D-printed planar inductor after sintering were 792 nH and 15 mΩ, respectively. The measured value of both inductance and DC resistance were in less than 10% error with those predicted by 3D finite-element-analysis simulation. Microstructure of the sintered planar inductor was characterized by scanning electron microscopy (SEM).

Design and additive manufacturing of multi-permeability magnetic cores

Uneven distribution of magnetic flux in a conventional magnetic core limits improvement of power density of the magnetic component. By making a magnetic core comprising materials with different permeabilities, henceforth called multi-permeability core, a more uniform flux distribution can be achieved without complex configuration and geometry of the core and winding. In this work, we designed a three-ring toroid core to produce a flux uniformity factor, α, defined as Bmin/Bmax, in each of the three rings by increasing permeability from inner to outer ring. With the same inductance, volume of the three-permeability toroid core was significantly smaller than that of a single-permeability toroid core. For ease of fabricating multi-permeability magnetic cores, a commercial multi-extruder paste-extrusion 3D printer was explored to process different magnetic pastes into 3D structures. The magnetic pastes were formulated in our laboratory. Two types of magnetic paste systems, a high-temperature (> 900°C) pressure-less sinterable ferrite and a low-temperature (< 250°C) pressure-less curable powdered-iron, were developed and tested in the 3D printer. By varying the magnetic/organic composition in each of the two material systems, we produced pastes that are compatible with the 3D printer and can be processed into core materials with relative permeability ranging from 20 to 70. With the same dimensions, a 3D-printed three-permeability toroid core had a higher inductance than a single-permeability core.

Two-Dimensional Gapping to Reduce Light-Load Loss of Point-of-Load Inductor

Point-of-load converter at light load has low efficiency owing to the “fixed losses” such as core loss and ac winding loss. This paper focuses on two-dimensional (2-D) gapping of a ferrite core to shape inductance versus load current to reduce inductor loss at light load. Since the maximum inductance of conventional stepped gap is limited by the cross-sectional area of the thin gap, a 2-D gap is formed by joining two orthogonal gaps to gain flexibility. Higher inductance is achieved at light load compared with uniform-gap and stepped-gap geometries having the same volume and dc resistance. AC resistance is reduced at light load thanks to a magnetic path that steers ac flux away from the winding. Two C-cores with 2-D gap were fabricated and tested on a buck converter with 50% reduced total inductor loss at 10% load current.

Point-of-load inductor with high swinging and low loss at light load

Point-of-load converter at light load has low efficiency owing to the “fixed losses” such as core loss and ac winding loss. This paper focuses on two-dimensional (2D) gapping of a ferrite core to shape inductance versus load current to reduce inductor loss at light load. Since the maximum inductance of conventional stepped gap is limited by the cross-sectional area of the thin gap, a 2D gap is formed by joining two orthogonal gaps to gain flexibility. Higher inductance is achieved at light load compared with uniform-gap and stepped-gap geometries having the same volume and dc resistance. Ac resistance is reduced at light load thanks to a magnetic path that steers ac flux away from the winding. Two C-cores with 2D gap were fabricated and tested on a buck converter with 50% reduced total inductor loss at 10% load current.