Cree fabricated the SiC power devices and Kansai Electric (KEPCO) constructed the SiC modules and inverter using these devices, as a part of the collaborative eight-year effort funded by KEPCO. These inverters could eventually reduce conversion losses by more than 50% compared to existing silicon (Si) inverters, according to Cree.

An inverter is used in a variety of applications to convert power from DC to AC or for changing the operating frequency in variable-speed motor drives. Applications include heat pumps, industrial motors, and electric vehicles. The inverter in an automobile, for example, converts direct current (DC) power from a fuel cell or a battery to alternating current (AC) power for the motor. Inverters are also used to transfer power onto the grid from battery, wind and solar sources.

In the future, a tremendous amount of energy savings could be realized in industry by switching from Si [silicon] inverters to SiC inverters because the power loss of the inverters is estimated to be reduced by more than 50 percent. Kansai Electric plans on further increasing the power capability of these inverters in order to apply them to a variety of power systems.

—Dr. Yoshitaka Sugawara, executive researcher and manager of the SiC program at KEPCO

We are advancing SiC power device technology through a variety of improvements ranging from lower defect density crystal growth to improved epitaxial processes, device design and processing. While more development is required, these improvements are moving the technology closer to the point where a significant reduction in the need for new power plants and reduction in greenhouse gas emissions are possible.

—Dr. John Palmour, Cree executive vice president of advanced devices

The capability of the new Cree-KEPCO inverter is about 60% greater than that currently being targeted for the power electronics for hybrids and fuel-cell vehicles: DOE’s FreedomCAR program specifies continuous power of 30 kW with peak power of 55 kW for 18 seconds by 2010.

(Watts and Volt-amperes are dimensionally equivalent—they are both measures of power. Conversion from one to the other requires the application of a power factor. Cree estimates that in the worst case, the power factor for this inverter would be 0.8—yielding a minimum power output of about 88 kW.)

Significant reduction in weight and size of SiC power converters with an increase in the efficiency is projected. SiC unipolar devices such as Schottky diodes, VJFETs, MOSFETs, etc. have much higher breakdown voltages compared to their Si counterparts which makes them suitable for use in traction drives replacing Si pn diodes and IGBTs.

—from “A 55 kW Three-Phase Automotive Traction Inverter with SiC Schottky Diodes”

For the FreedomCAR program, Oak Ridge National Laboratory worked with Cree and Semikron to build a 450V, 55kW SiC inverter by replacing the Si pn diodes in Semikron’s inverter with Cree’s SiC Schottky diodes. That unit is under evaluation by FreedomCAR, along with next-generation inverters from other suppliers.

KEPCO-Cree. The SiC switching device jointly developed by KEPCO and Cree for the new 110 kVA inverter is a 4.5kV, 100A design called the SiC Commutated Gate Turn-off Thyristor (SiCGT). The chip size is 8mm x 8mm, enabled by a significant reduction in crystalline defect densities. This switch, which can turn off or on in less than 2 microseconds, has a switching speed 10x faster than that of an equivalently rated silicon Gate Turn-off Thyristor (GTO). This device does not require a snubber circuit, a commonly used protective circuit for GTOs, thus reducing the part count and heat dissipation.

A SiCGT module was then developed that utilizes one SiCGT and one 6mm x 6mm SiC PiN diode in a metal can package. The module can operate at higher temperatures (300°C) than conventional silicon modules (125°C), by utilizing a new high-temperature resin for dielectric insulation. Using six of these modules, the three-phase Pulse Width Modulation (PWM) inverter demonstrated an output power of about 110 kVA. The PWM frequency was 2 kHz.

Resources:

• ORNL: Impact of SiC Power Electronic Devices for Hybrid Electric Vehicles

• ORNL: A 55 kW Three-Phase Automotive Traction Inverter with SiC Schottky Diodes