A Donut-Shaped Magnetic Field Lets This Wireless Charger Power Devices at Any Position, Orientation

A team of engineers at Finland's Aalto University have designed a new, cylindrical transmitter for wireless charging systems — producing a donut-shaped field that, they claim, will be more convenient and reliable for consumer use than the traditional flat-mat variety.

"We set out to create a simple, low-cost system using only a single power source," explains postdoc research Nam Ha-Van of the team's work, which focuses on a cylindrical power coil created so that the wire at the top of the coil is wound in the opposite direction to the wire at the bottom with a Z-shaped bridge between them.

When energized, the current flows through both halves of the coil in opposing directions and in doing so produces a complementary pair of magnetic fields: One field flowing from the middle of the cylindrical coil around the top winding and back in again; the other from the middle to the bottom and back in again. The result: An even magnetic field round the middle of the coil.

That field can then be used to charge or power portable devices wirelessly, and comes with some benefits over traditional wireless power transmission systems: Devices can be placed anywhere in the charge area, and regardless of position or orientation will charge with equal efficiency.

In testing a prototype implementation of the system achieved 72.4 percent DC-to-DC efficiency, supporting a load of 13W at a 6.78MHz operating frequency. "This was just a proof of concept," explains co-author Yining Liu. "Now we can work to improve the efficiency — maybe to around 90 percent — and also the power."

The team's work has been published under open-access terms in the journal IEEE Transactions on Industrial Electronics as an early access article.