Proterial has developed a magnetic sheet for electric vehicle wireless charging systems made of its nanocrystalline soft magnetic material Finemet, and a related series of magnetic sheet panels. The MS-HiQ sheets are designed to improve efficiency and reduce the size and weight of wireless charging systems for EVs.
Background
Wireless charging initially found application with cardiac pacemakers and other medical equipment, then entered widespread use after it was introduced to smartphones. Lately, wireless charging has been attracting attention as a technology for supplying power to EVs. In 2020, the SAE J2954 standard for wireless power transfer for electric vehicles was published. It envisioned that the system would be used in parking facilities. Full-scale studies towards practical application are now underway. However, wireless charging systems have a drawback in that they are not as efficient as wired charging systems.
Enhancing the efficiency of wireless charging systems requires a coil composition with its quality (Q) factor reciprocal to the loss factor. This means that the loss is smaller when the Q factor is larger. For this purpose, it is effective to place a magnetic sheet on the back of the coil (or on the back of the wireless charging unit), as illustrated in Figure 1.
In this configuration, the sheet acts as a shield as well as a back yoke. Cellphones need thin magnetic sheets with a high Q factor, characteristics that have generated a large market share for Proterial’s Finemet. While wireless charging systems for cellphones have a power capacity of 20 W or less, those for EVs have a very large power capacity of 3 kW to 11 kW. It is now common to develop a technology of increasing the size and thickness with the use of ferrite, a magnetic ceramic, for magnetic sheets. Proterial, however, believed that the characteristics of Finemet could also be useful to automotive applications and embarked on efforts to develop magnetic sheets for EV wireless charging systems.
Outline
Now, Proterial has recently developed two original technologies for heightening the quality factor for wireless charging systems.
- At the time of heat treatment of Finemet ribbons as constituents of a magnetic sheet, magnetic permeability control is performed while tensile strength is applied to the ribbons in their longitudinal directions to give them anisotropy. A wireless charging system equipped with magnetic sheets formed using the ribbons exhibits an improved Q factor.
- The MS-HiQ magnetic sheet created by the technology described in (1) above is cut. A magnetic sheet panel is then formed by placing the cut pieces in a radial pattern to ensure that the longitudinal directions of the ribbons are identical with the directions of magnetic force lines, as portrayed in Figure 2. A wireless charging system with this panel exhibits a higher Q factor.
The Proterial development team found that for producing a Q factor equivalent to that of a wireless charging system with ferrite on the back of the coil, a magnetic sheet panel consisting of four MS-HiQ elements, as seen in Figure 2(a), requires a thickness that is about half that of the ferrite, and a magnetic sheet panel comprising eight MS-HiQ elements, as depicted in Figure 2(b), requires a thickness of around one third that of the ferrite. Their experimentation confirmed that massive thickness and weight reduction of EV wireless charging systems could be achieved with this method. For more info, see www.proterial.com.
