Continuously Variable Multi-Permeability Inductor for Improving the Efficiency of High-Frequency DC–DC Converter

This paper proposed a method for one step fabricating the continuously variable multi-permeability (CVMP) toroid core by applying a specific gradient sintering temperature from the inner wall to the outer wall, i.e., along the radius direction, of a toroid core in a tube furnace using a high-temperature resistant stainless steel pipe which is used for flowing the cooling medium. The sintering temperature gradient was designed based on the relationship between permeability and sintering temperature. A CVMP core with the permeability gradually increasing continuously along the radius could be made in this way. It is beneficial for improving the uniform distribution of magnetic flux density of the core by this way. The distribution of magnetic flux density and the inductance was analyzed in theoretical calculation and finite-element analysis. A CVMP core and a single-permeability core were fabricated to evaluate their performance using an 8-V input, 1-V output dc-dc converter. The results show that the CVMP core is possible to balance the inductance and improve the output efficiency compared with a single-permeability core employed in a dc-dc converter at heavy load. It can be concluded that the CVMP core has the capacity of taking higher current than a single-permeability core.