Time To Switch?


Can a cheaper SiC switch deliver next-generation power grids, asks Compound Semiconductor.

The FREEDM Super-Cascode SiC switch targets power conversion applications in the next generation electrical grids.

Late last year, researchers at US-based North Carolina State University unveiled a SiC high voltage and high frequency power switch that they hope will gain commercial acceptance where other SiC devices have not.

The three terminal power device - called the FREEDM Super-Cascode - comprises a 1.2 kV SiC MOSFET and twelve 1.2 kV SiC JFETs, connected in series to give the 15 kV, 40 A switch.

Device pioneer, Professor Alex Huang, says the switch provides the high efficiency and high switching speed of an equivalent monolithic 15 kV SiC MOSFET from Wolfspeed.

Crucially, Huang also claims his switch is relatively cheap.

"The FREEDM Super-Cascode costs only one third of the estimated high voltage SiC MOSFETs," he says. "The 15 kV SiC MOSFET has attracted a lot of interest because of its potential in high voltage, high frequency power converter applications for the next generation Smart Grid."

"But devices are still under development, and high cost due to expensive materials growth and fabrication could limit future adoption," he adds.

Professor Alex Huang is developing cost effective SiC switches and more. [Roger Winstead, NC State University]

From a system integration viewpoint, Huang also points out how the switch only requires one gate signal to turn it on and off, easing implementation and making it less complicated than more widely established IGBT-series devices.

And thanks to a high thermal dissipation capability, the device can operate over a wide range of temperatures and frequencies, making it suitable for power conversion in the medium voltage drives, solid state transformers, high voltage transmission and circuit breakers of tomorrow's power grids.

The cheaper alternative

Huang established the FREEDM (Future Renewable Electric Energy Delivery and Management) Systems Center in 2008 following funding from the National Science Foundation and additional industry support to create the energy network of the future.

As he puts it: "We're trying to create a new electric grid infrastructure that we call the energy Internet. We're looking at the whole distribution system; that's a huge, very complex engineering system."

Huang is intent on introducing new power semiconductors and advanced power electronics to the future power grid, and has already delivered myriad devices including a 6.5 kV Si/SiC hybrid 'FREEDM-Pair' switch for high power applications and 6 kV SiC hybrid FREEDM Super-Cascode power switch based on a 1.2 kV SiC MOSFET and four 1.2 kV SiC JFETs.

The researcher believes such devices are more cost effective than the monolithic SiC device equivalent. As he highlights: "Lower voltage devices, for example, are more mature, in terms of manufacturing, and are already commercially available. These devices are the building blocks of our Super-Cascode devices making a clear path to commercialisation."

"We are also developing [single] high voltage devices in our laboratory, without using many devices in combination," he adds. "But the Super-Cascode design is lower risk and we can scale voltage and current very quickly."

Indeed, Huang's tests on both his latest 15 kV FREEDM Super-Cascode switch and the 15 kV SiC MOSFET from Wolfspeed reveal the former has a faster switching performance.

The high power switch has the potential to work more efficiently and cost less than conventional devices.

Meanwhile, thermal analyses on each indicate the Super-Cascode device has a higher power dissipation capability than a single monolithic SiC MOSFET.

"We are introducing this Super-Cascode switch as a single-packaged device, but the beautiful thing this, is how we have made many components work together as one," says Huang. "Synchronisation is key and this is our major innovation."

"We have achieved an extremely fast switching speed with our FREEDM Super-Cascode device, and this paves the way for power switches to be developed in large quantities with breakdown voltages from 2.4 kV to 16 kV," he adds.

Huang now intends to demonstrate his switch in a power electronics operating system, a feat he expects to achieve by Spring this year.

"We've demonstrated the technology as a single device but we need system-level verification as well," he says. "With this, we will be discussing the technology with potential licensees and are also looking at university start-up opportunities."

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