Nissan Develops SiC Diode-based Automotive Inverter

Sep 9, 2008
Tadashi Nezu, Nikkei Electronics

Nissan Motor Co Ltd announced that it developed an automotive inverter with SiC diodes for the first time in the world.

The company mounted this inverter on its fuel-cell vehicle "X-Trail FCV" and started test runs. By switching from Si diodes to SiC diodes, in the future, inverters can be reduced in size and weight and their reliability can be enhanced, the company said. So far, the layout of electric vehicles has been restricted by the large size of inverters.

SiC elements are drawing attention as the next-generation power semiconductor devices that have superior characteristics. The dielectric breakdown field of SiC is approximately one figure larger than that of Si; therefore, in theory, the on resistance of SiC can be two figures less than that of Si. It is because on resistance is in inverse proportion to the cube of dielectric breakdown field.

Because of its small on resistance, SiC can reduce power loss when used in power supply circuits. In addition, the thermal conductivity of SiC is higher than that of Si, and it has a larger thermal diffusion. Therefore, the adoption of SiC is expected to reduce the size of cooling systems.

Having noticed those characteristics, many manufacturers and research institutes are conducting research and development of SiC. Nissan has already developed a SiC diode in collaboration with Rohm Co Ltd (See related article).

This time, employing SiC and a structure called "heterojunction diode," Nissan drastically enhanced the reliability and heat resistance while increasing the power efficiency. As a result, while the area occupied by diodes was narrowed by 70%, the energy efficiency of the inverter circuit was improved by 20%, the company said.

The size of the chip is 5 × 5mm. Because the cooling system can be simplified, the size and weight of the inverter can be reduced by 15 to 20%, the company said.

The newly developed SiC diode can be used for the electric vehicles and hybrid cars that Nissan is now developing. The company plans to further downsize the inverter by applying SiC elements to the transistor with which the inverter is used.