Short description of the doctoral thesis:

"Enhancement of Efficiency and Reduction of Volume of Solar Inverters by Utilizing Modern Silicon Carbide Power Devices"

To assure the increasing demand of energy in times of simultaneously occurring decrease of fossil fuels, the use of alternative energies such as sun, wind and water is inevitable. Especially in case of solar energy, which can be transformed into electric energy via solar cells, the demand for high efficient converters is prominent. The converter's task is to alter the DC delivered by the solar modules into an alternating current for the national grid.

The intention of this PhD project is to further develop and to study innverters for solar plants. Thereby the enhancement of efficiency is predominant, while at the same time the construction volume, the weight, the life cycle costs and as possible the production costs are to be reduced. This is realized by the utilization of new power semiconductors made of silicon carbide (SiC). These power semiconductors are in the state of pre-commercial development.

SiC power semiconductors feature the ability to block considerably larger scales of voltage and cause less switching losses than comparable Si power semiconductors that are used so far. The small switching losses allow an increase of the switching frequency which permits a reduction of volume and weight of the passive components in the power section.

One focus of this PhD project is the analysis and the comparison of different types of new SiC power semiconductors (diode, JFET, Bipolartransistor and MOSFET) concerning their suitability for solar converters. Therefore the behavior of the switching losses and the conducting losses of the single types of power semiconductors is analyzed in regard to different operating temperatures, voltages and currents. Afterwards these insights provide the basis for the dimensioning of the power semiconductors and the cooling systems of the converter.

Another focus is the development of new concepts for the circuit topology and the circuit layout to completely exploit the advantages of SiC power semiconductors. A circuit topology is chosen that provides an optimal distribution of the device's power on the passive components and the SiC chip area. As well new circuit approaches are tested for their suitability for converters on solar modules. The circuit layout is optimized with regard to parasitic inductances on the conductors, because these lead to critical overvoltage at high switching frequencies and fast switching operations and therefore can destroy the power semiconductor. Besides, the electromagnetically radiation is to be reduced as far as possible.
A further focus is the research of the switching behavior of conventional gate drive units for controlling the components in combination with SiC power semiconductors and the development of adapted gate driver units. To drive the SiC power semiconductors efficiently and safe, fitting gate driver units for every type of power semiconductor are developed. These driver units provide fast switching edges for the gate and at the same time are safe from injection of electromagnetically disturbances caused by the fast current and voltage edges on the power semiconductor. Another request on the drivers is the certain protection of the power semiconductor against over current and over voltage.

To the operating converter fitting control concepts for the maximum power point tracking and for the input into the national grid are implemented. The intention is to maintain all relevant norms especially in regard to the behavior on the grid-side.