Maximum efficiency

A maximum efficiency rating of 98.5 per cent for photovoltaic inverters has been achieved by the Fraunhofer Institute for Solar Energy Systems ISE in a test using prototype silicon carbide-based MOSFETs manufactured by Cree.

These are claimed to be the highest efficiency results reported for photovoltaic inverters to date. Fraunhofer researchers succeeded in reducing the power dissipation of conventional inverters by 30 to 50 percent when compared with results obtained with traditional silicon-based transistors. They are the first researchers worldwide to test the new semiconductor material for this application.

Inverters transform direct current generated by photovoltaic systems into alternating current and feed it into the public grid. The higher the efficiency rating of the inverter, the greater is the energy yield of the entire photovoltaic system. To achieve the highest energy output over time, the efficiency rating should also remain high over a wide range of power levels.

During testing, the inverter with Cree SiC components also set a new performance precedent across a wide range of power output values.

'We are thrilled to have achieved this record level of efficiency. Silicon carbide components switch faster and have a smaller forward bias power loss than traditional silicon-based transistors,' said Dr Bruno Burger, head of the Power Electronics Group at Fraunhofer ISE.

Fraunhofer ISE is the first organisation known to test silicon carbide MOSFETs in this application. 'Our work involved characterising the components and integrating them into existing inverters. If, in a further step, the inverter circuitry is optimised specifically for silicon carbide, then even greater efficiencies could certainly be achieved,' he added.

Currently, the semiconductor silicon carbide (SiC) has mainly been used in white LEDs. SiC diodes have been available for quite a while. However, the MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) which are necessary for the power stage in inverters have not been available until recently.

The Fraunhofer team achieved the efficiency rating record with a single-phase inverter with their patent-pending HERIC topology and a nominal power rating of 5kW. They also increased the efficiency rating of a three-phase inverter with a nominal power rating of 7kW from 95.1 to 97.5 percent.