Wireless power transfer—delivering energy through magnetic fields rather than physical cables—has moved well beyond smartphone charging pads. Researchers are now targeting more demanding applications: surgical implants, autonomous robots, and low-altitude aircraft, where compact size, high power, and reliable efficiency are all required simultaneously. Operating at megahertz (MHz) frequencies is one promising path toward these goals, as higher frequencies allow smaller components and greater power density. But MHz-range systems introduce stubborn engineering obstacles, particularly in maintaining efficiency when power levels are high or when the load on the system fluctuates.
A research team led by Associate Professor Fu Minfan from the School of Information Science and Technology (SIST) at ShanghaiTech University has now reported two advances that directly address these obstacles. Both findings were published in IEEE Transactions on Industrial Electronics.
The more ambitious of the two results concerns kilowatt-level power transfer at 6.78 MHz—a frequency band designated by the international AirFuel wireless charging standard. A central challenge at this scale is that conventional circuit designs optimized for a single operating condition collapse in efficiency when the load varies, as it inevitably does in real-world deployment.
The team developed a new impedance-based design methodology that keeps the system in an energy-efficient switching mode—known as zero-voltage switching, which minimizes heat and power loss during the rapid on-off transitions of the circuit—across a tenfold variation in load. A 1-kW prototype validated the approach, achieving a peak system efficiency of 94.9% and stable output voltage throughout, a combination that has been difficult to demonstrate at this frequency and power level.
The second study addresses a complementary problem at the receiver end of these systems. In any wireless power system, the received AC signal must be converted back to DC for use—a process called rectification. At MHz frequencies, conventional rectifier components lose efficiency rapidly, and keeping the receiver synchronized with the transmitter without a dedicated communication link is non-trivial. The team introduced a synchronous rectification scheme that extracts its timing signal directly from the receiver’s own resonant circuit, eliminating the need for transmitter-side coordination entirely. Operating at 5 MHz, the rectifier achieved a peak efficiency of 97.5% across a wide load range—meaningfully higher than the roughly 92–93% typical of commercial implementations at comparable frequencies.
Together, the two studies form a coherent set of solutions: one targeting the power delivery architecture, the other the receiving end. The results point toward wireless power systems that are simultaneously more compact, more efficient, and more robust under the variable conditions that practical deployment demands—including EV charging infrastructure, industrial robotics, and implantable medical devices.
Doctoral student Gao Shiqi at SIST is the first author of the rectifier paper, “A Megahertz Synchronous Rectifier Based on Resonant Gate Drive.” The kilowatt-level transfer paper, “Load Insensitivity Design for a 6.78 MHz kW-Level Inductive Power Transfer Using Bridge Topology,” was co-authored by doctoral students Wang Xinlin and Yang Jun. Prof. Fu is the corresponding author.