Power/Performance Bits: May 18

Efficient high-voltage power conversion
Researchers from École Polytechnique Fédérale de Lausanne (EPFL) and Enkris Semiconductor are working to design new power transistors with the aim of improving power converter efficiency.

“We see examples of electric power losses every day, such as when the charger of your laptop heats up,” said Elison Matioli, head of EPFL’s POWERlab, noting that it’s even more of a problem in high-power applications. “The higher the nominal voltage of semiconductor components, the greater the resistance.”

The researchers said their transistor has less than half as much resistance as conventional transistors, while holding voltages of over 1,000 V.

Two aspects are key to the device. First, it uses several conductive channels to distribute the flow of current. “Our multi-channel design splits up the flow of current, reducing the resistance and overheating,” said Luca Nela, a PhD student at EPFL.

Second, it uses nanowires made of gallium nitride (GaN). The nanowires have a diameter of 15 nm and a funnel-like structure enabling them to support high electric fields and voltages of over 1,000 V without breaking down.

The researchers said the combination leads to greater conversion efficiencies in high-power systems. “The prototype we built using slanted nanowires performs twice as well as the best GaN power devices in the literature,” said Matioli.

They also don’t see any major obstacles to large-scale production. “Adding more channels is a fairly trivial matter, and the diameter of our nanowires is twice as big as the small transistors made by Intel,” added Matioli. The team has filed several patents on the technology, and said there is interest in further development from several major manufacturers.

Structural batteries
Researchers at Chalmers University of Technology and KTH Royal Institute of Technology are advancing structural batteries by improving strength, stiffness, and energy storage capacity.

Structural batteries, also called ‘massless batteries,’ act as both energy storage mechanisms and also structural elements of the object itself. In the case of electric vehicles, this could be a load-bearing part of the car body (hence massless, as no extra weight is being used for a the battery alone).

The team said their carbon fiber-based structural battery provides improved multifunctional performance over previous prototypes. The battery has an energy density of 24 Wh/kg, or approximately 20% capacity compared to comparable lithium-ion batteries currently available. However, the researchers said that the weight of the vehicle would be reduced and require less energy to drive. The battery has a stiffness of 25 GPa, comparable with many other commonly used construction materials.

“Previous attempts to make structural batteries have resulted in cells with either good mechanical properties, or good electrical properties. But here, using carbon fiber, we have succeeded in designing a structural battery with both competitive energy storage capacity and rigidity,” said Leif Asp, a Professor at Chalmers.

The new battery has a negative electrode made of carbon fiber, and a positive electrode made of a lithium iron phosphate-coated aluminum foil. They are separated by a fiberglass fabric, in an electrolyte matrix.

The team is beginning a new project to improve performance by replacing the aluminum foil with carbon fiber as a load-bearing material in the positive electrode, providing both increased stiffness and energy density. The fiberglass separator will be replaced with an ultra-thin variant, which should provide faster charging cycles. The new project is expected to be completed within two years. Asp estimates that this battery could reach an energy density of 75 Wh/kg and a stiffness of 75 GPa, making it about as strong as aluminum with a lower weight.

“The next generation structural battery has fantastic potential. If you look at consumer technology, it could be quite possible within a few years to manufacture smartphones, laptops or electric bicycles that weigh half as much as today and are much more compact,” said Asp.

Calculating EM noise
Researchers at Osaka University are investigating ways to better understand electromagnetic noise. The researchers formulated a numerical method to visualize how signals propagate and radiate depending on the conductor’s shape, allowing them to anticipate the causes of electromagnetic noise in the circuit designing stage.

“It is necessary to simultaneously calculate electrical signals and external radiation phenomena to treat electromagnetic noise. To this end, we aimed to precisely solve the delay integral equations of the scalar potential (potential) U and the vector potential A derived from Maxwell’s equations,” said Souma Jinno of Osaka University.

The team said they achieved numerical stability through a calculation method that more strictly considers the delay time, and also developed an algorithm that simultaneously calculates Ohm’s law and the equation of continuity and the circuit element of the boundary.

They showed the effect of radiation on the conducted signal by applying their method to a circuit and comparing it with a method that ignores radiation, explained Jinno. “By comparing the two results, we can see that the calculation with radiation is immediately damped and converges to zero, while the calculation without radiation continues to oscillate without damping. Since the conductor’s resistance component is assumed to be zero, there is no loss due to conduction. From the above results, the conduction signal current attenuates due to external radiation. The attenuation indicates that a single conductor is not suitable for conducting a signal because of the extensive external radiation. Usually, electrical circuits use two conductors, which can conduct with less radiation.”

“Rapid electrification has potential problems such as tight supply and demand for electricity and invisible environmental pollution caused by electromagnetic noise,” added Masayuki Abe, of Osaka university. “Using the electromagnetic circuit simulator developed in this research, we aim to realize a ‘sustainable electrified society’ by further reducing noise and power consumption of electric circuits by correctly handling and consuming electric power.”