Whenever a electronic switch absorbs excess voltage, external clamping components must be connected to it to avoid damage when the current flow is interrupted. But the higher voltage, the more components required, significantly increasing both the cost and size of the system.
High voltage silicon MOSFET switches have been in use for 20 years
“We explored another avenue in the 90s: silicon MOSFET transistors that clamp voltage via avalanche breakdown. We designed and produced a 100 kV MOSFET breaker for the tokamak at the Cadarache nuclear research center to protect a microwave tube that could withstand up to 1MW of electrical power, and it's been in use for 20 years," said CEA researcher Daniel Chatroux.
Avalanche breakdown could easily be perceived as a kind of electric valve function, since MOSFET transistors maintain the voltage around the desired level (such as 600 V) when an excess occurs, by allowing the current to flow through. “The only drawback to silicon MOSFETs is that they failled in avalanche mode below their short circuit current. Otherwise, they perfectly perform their role," explained Daniel Chatroux.
SiC MOSFETs designed for high voltages
CEA-Liten have refocused their efforts on this area for two reasons: Firstly, SiC MOSFETs capable of withstanding several thousand volts are now available on the market. Secondly the increase in voltage of renewable energies, especially photovoltaics, “is aiming to handle higher [electrical] voltages, enhance efficiency, [and] reduce costs and volume."
By choosing the leading transistors on the market, carrying out mockup testing, and putting their MOSFETs to the test, researchers found that such MOFSETs perfectly withstand voltage shocks up until their short circuit current. This meant that a 1,700 V transistor was able to clip off 2,000 V voltages by allowing currents higher than 200 A to pass through. “Somehow, we managed not to break them," said Daniel Chatroux with a smile. “Their performance exceeded that of silicon MOSFETs."
Caption: 333 A current clamped via avalanche breakdown at 2,180 V
by a nominal 1,700 V 68 A SiC MOSFET (200 A, pulsed). .
A patent filed and a switchboard under development
These findings led to a patent being filed several months ago, as well as the development of a switchboard with six MOSFETs in series for demonstration purposes. Researchers also tested an innovative control law.
It is worth reminding ourselves that these particular transistors are not specially designed to clamp voltage via avalanche breakdown, and are not specified too, meaning that industries with strict security requirements, such as the aeronautical sector, will not have access to them. Meanwhile, their use is highly beneficial for rapidly developing sectors that handle high voltages, especially if they transmit electricity over long distances. Equally important, the clamping properties of SiC MOSFETs are down to their design rather than contingencies arising from a manufacturing process. In other words, there are still great possibilities to explore.