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Engineers on a roll toward smaller, more efficient radio frequency transformers

Illinois engineers roll toward smaller, more efficient radio frequency transformers that take 10 to 100 times less space on a microchip. Credit: Wen Huang

University of Illinois engineers say they have been able to minimise the size of the transformer – a notoriously large element of ICs.

"The size of transformers may become a key obstacle to overcome in the future for wireless communication and IoT," principal investigator, Professor Xiuling Liof Illinois, said.

Transformers use coiled wires to convert input signals to specific output signals for use in devices like microchips. Previous researchers have developed some radio frequency transformers using a stacked conducting material to solve the space problem, but these have limited performance potential. This is due to inefficient magnetic coupling between coils when they have a high turns ratio, meaning that the primary coil is much longer than the secondary coil, or vice versa, Prof. Li said.

These stacked transformers need to be made using special materials and are difficult to fabricate, and are bulky and unbendable. The new transformer design uses techniques Prof. Li's group previously developed for making rolled inductors.

3D rolled-up radio frequency transformers take 10 to 100 times less space, perform better when the power transfer ratio increases, and have a simpler fabrication process than their 2D progenitors, according to the team.

"We are making 3D structures using 2D processing," Prof. Li said. The team explained that they patterned metal wires onto stretched 2D thin films, and once the tension was released, the 2D films self-rolled into small tubes. This enabled the primary and secondary wires to coil and nest inside each other into a much smaller area for optimum magnetic induction and coupling. The nested 3D architecture leads to high turns ratio coils, Prof. Li added.

"A high turns ratio transformer can be used as an impedance transformer to improve the sensitivity of extremely low power receivers, which are expected to be a key enabler for IoT wireless front ends," said co-author, Professor Songbin Gong. Rolled transformers are also said to receive and process higher frequency signals than the larger devices.

"Wireless communication will be faster and use higher-frequency signals in the future. The current generation of radio frequency transformers simply cannot keep up with the miniaturisation requirements and high-frequency operation of the future," said lead author and postdoctoral researcher, Wen Huang. "Smaller transformers with more turns allow for better reception of faster, high-frequency wireless signals, as well as high-level integration in IoT applications."

The new transformers have a robust fabrication process - stable beyond standard foundry temperatures and compatible with industry-standard materials – according to the team. This study used gold wire, but the researchers claim to have successfully demonstrated the fabrication of their rolled devices using industry-standard copper.

"The next step will be to use thinner and more-conductive metal such as graphene, allowing these devices to be made even smaller and more flexible. This advancement may make it possible for the devices to be woven into the fabrics of high-tech wearables," Prof. Li concluded.

Bethan Grylls

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