Abstract

This paper presents a wireless power transfer (WPT) system that maintains high efficiency and low output voltage ripple under large variations in coupling and load conditions. To address these variations, the proposed system employs an adaptive mode switching scheme with coupling-insensitive sensing. This is implemented using a fixed-reference sensor topology with filtered integrated sensor outputs and synchronization of RX 0X-to-1X transitions with TX 0X-to-1X mode transitions. For stable output regulation, a voltage-racing hysteretic controller is introduced to achieve low output voltage ripple, which is difficult to obtain with conventional analog feedback- or comparator-based hysteretic controllers. In addition, an on-chip load detector enables automatic detection of heavy- and light-load conditions. Fabricated in a 0.18- μ m BCD process, the proposed system was measured with coil distances from 7 to 20 mm, corresponding to coupling coefficient (k) variations from 0.42 to 0.08. The system supports an output power range from 2.6 mW to 147 mW while achieving a low ripple voltage of 18 mV and achieves a peak end-to-end efficiency of 69.1%.

Implantable medical devices must continue operating reliably despite the body's constant movement—from walking and breathing to simply changing position during sleep. But those everyday movements can disrupt wireless charging by altering the alignment between the external charger and the implanted device.

Researchers at UNIST have developed a wireless power transfer system that adapts to changes in body movement and device power demand. By maintaining stable power delivery while reducing unnecessary energy loss and heat generation, the system could make long-term wireless powering of implantable medical devices more practical.

Many implantable medical devices alternate between active treatment and standby modes, causing their power requirements to change over time. At the same time, body movement can shift the distance or alignment between the transmitter and receiver coils, reducing wireless charging efficiency. Together, these challenges make it difficult to deliver stable power when and where it is needed.

Led by Professor Franklin Bien of the Department of Electrical Engineering, the team designed the system to distinguish changes in device power demand from changes in coil alignment caused by body movement. Rather than relying solely on signal strength, it detects characteristic changes in communication between the transmitter and receiver, allowing it to switch reliably between high- and low-power modes as charging conditions change.

The researchers also developed a new Voltage-Racing Hysteretic Controller (VRHC) to stabilize the receiver's output voltage. Instead of comparing voltages directly, the new controller detects voltage changes by measuring tiny differences in signal propagation time within the circuit. This allows it to respond more quickly and keep the output voltage remarkably stable.

In laboratory tests, the system remained stable across transmission distances ranging from 7 mm to 20 mm. Even when the implanted device's operating current increased sharply from 6 mA to 16 mA, it maintained a constant 3.3 V output with a voltage ripple of only 18 mV. The system achieved a peak end-to-end power transfer efficiency of 69.1%, representing an improvement of up to 51.3 percentage points over a comparable design without the adaptive mode-switching technology.

“Wireless charging systems need to adapt as conditions change, whether because a patient moves or a device's power demand shifts,” said Professor Bien. "By continuously adjusting power delivery in real time, our system improves both efficiency and stability. We hope it will help make implantable medical devices more practical for long-term use."

The study was co-first authored by Sungmin Shin and Seongbin Kwon of UNIST. The research was supported by the Ministry of Science and ICT (MSIT) and the Institute for Information & Communications Technology Planning & Evaluation (IITP), and published in   IEEE Transactions on Circuits and Systems I: Regular Papers (IEEE TCAS-I) on June 19, 2026.

Journal Reference

Sungmin Shin, Seongbin Kwon, Kiju Lee,  et al ., "A Wireless Power Transfer System With Adaptive Mode-Switching Insensitive to Coupling Variations Achieving Low Ripple and High Efficiency,"  IEEE TCAS-I,  (2026).