Researchers at the FREEDM Systems Center have moved a step closer to perfecting a technology designed to block voltages in both directions on a circuit, a necessary feature in the solid-state devices now being developed to modernize the electrical power grid.
Working at the FREEDM headquarters at North Carolina State University, Research Assistant Professor Woongje Sung fabricated a symmetric blocking P-type gate turn-off thyristor out of silicon carbide, a semiconductor material that can handle the large currents generated by power electronic devices.
A gate turn-off thyristor, or GTO, acts as a switch to control the flow of current on a circuit, somewhat like a mechanical circuit breaker does in a house.
Previously fabricated silicon carbide GTOs block current flow in only one direction. Engineers added a diode in series with previous GTOs to block current in both directions. But that results in energy loss every time the diode conducts current. Because the new symmetric GTO supports both forward and reverse voltage, the diode can be eliminated, saving energy and cost. There are other benefits as well.
“Because we’re using silicon carbide, chip size can be smaller and the switching speed will be much faster,” Sung says. He notes that the material can handle far higher temperatures than silicon.
Sung says he achieved symmetric blocking during the dicing process on a silicon carbide wafer by applying a novel orthogonal bevel to the edge of the chip. That innovation, developed at FREEDM last year, allows the depletion region to extend wider along the bevel surface. In reverse blocking mode, that reduces the electric field by more than 50 percent compared to the active area.
Distinguished University Professor Jay Baliga, a FREEDM researcher responsible for many of the advances in power semiconductor technology over the past four decades, says the new silicon carbide GTO is a key component of a fault isolation device, or FID, now under development at the center. That device, coupled with a new solid-state transformer pioneered at FREEDM, form the core of the smart system that will revolutionize the nation’s power grid, allowing it to intelligently and efficiently manage alternative energy sources.
“Solid-state devices can respond very fast compared to mechanical devices,” he says. “So the goal of our work is to create an FID that interrupts current in a very short time, in the range of tens of microseconds.”
The FID also has to be robust, able to handle up to 15 kilovolts, and it has to be bidirectional, with the ability to block both forward and reverse voltage. That’s why Baliga and his fellow researchers are counting on development of the silicon carbide symmetric GTO.
“No symmetric blocking device has ever been developed at this voltage rating,” he notes.
In its first demonstration, the GTO’s performance topped out at 3.8 kilovolts. But researchers say design improvements will likely boost that over 10 kilovolts in coming months.
“There are still engineering challenges,” says Progress Energy Distinguished Professor Alex Huang, FREEDM’s founder and former director. “This is a very significant proof-of-concept first step. The next step is to achieve what you design.”
Huang is a global leader in power devices who pioneered the concept of the “energy Internet,” hailed as one of the 10 most important emerging technologies by MIT Technology Review in 2011. He says the GTO could be scaled down to work with lower voltages in numerous devices, from motor drives to low-voltage circuit breakers.