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SemiSouth Laboratories Inc of Austin, TX, USA says that its enhancement-mode silicon carbide (SiC) junction field-effect transistor (JFET) has significantly improved the efficiency of an off-the-shelf inverter commonly used in residential and commercial solar power energy systems. Guidance and precision test equipment required to measure and validate the efficiency improvement was provided by the Center for Advanced Vehicle Systems (CAVS) at Mississippi State University (MSU). SemiSouth was spun off from MSU in 2000, and since August 2006 has based its manufacturing in a 20,000ft2 MSU-owned facility in Starkville, MS.

CAVS validated that, by replacing the existing silicon transistors with SemiSouth’s enhancement-mode SiC JFETs, the inverter reduced losses in power semiconductors by as much as 50%. Most significantly, CAVS used the JFETs as ‘drop-in’ replacements for the silicon devices, without any design changes to the inverter. The grid-connected, low-frequency isolated inverter, purchased from commercial inventory of a leading solar inverter provider, was designed with conventional silicon insulated-gate bipolar transistors (IGBTs). “CAVS is putting a major emphasis on positioning itself for a lead in automotive power electronics R&D,” says Rand German, CAVS’ director and professor of Mechanical Engineering at MSU. “This result validates our belief that developing and testing applications for silicon carbide power electronics through our partnership with SemiSouth is leading to breakthroughs in energy efficiency.”

Beyond the dramatic reduction in both conduction and switching losses, the use of SiC JFETs as ‘drop-in’ replacements to achieve the efficiency gains is unprecedented, claims SemiSouth. “All that was necessary to drop-in the enhancement-mode JFETs was the addition of a few passive components in the control circuit to take into consideration the change in threshold voltage between the original IGBTs and these JFETs. We started the inverter and it ran without any problems,” says applications engineer Robin Kelley. “With this simple change, immediate improvement with respect to performance and efficiency were realized.” This opens the door to additional modifications to fully take advantage of the device, yielding additional improvements not only in performance and efficiency but also in overall system reliability, he adds.

The enhancement-mode SiC JFET are “cheaper to produce, smaller than any comparable silicon or silicon carbide transistor, and free of reliability concerns with gate-oxides that have plagued more conventional approaches to developing an enhancement-mode field effect transistor using silicon carbide,” claims chief technical officer Dr Jeff Casady.

The new enhancement-mode JFET can be used as a direct replacement for silicon MOSFETs and IGBTs in virtually any off-the-shelf converter or inverter design, SemiSouth claims. According to president and CEO Vess Johnson, this means the barrier to entry has been greatly reduced and that designers working with the enhancement-mode SiC JFETs can see immediate performance and efficiency improvements and will be able to drive new and better products to market much faster.

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