Znak Politechniki Warszawskiej

Wide band gap power devices in power electronics

How to convert electrical energy with a little power losses? Researchers at the Department of Industrial Electronics Warsaw University of Technology are working on use for this purpose transistors made from wide band gap semiconductors - silicon carbide (SiC) and gallium nitride (GaN).

Silicon power losses

In modern societies, most of the electricity is converted on the road between the producer and the customer. Part of the energy is lost with each conversion. Very often a system that adjust the form of electrical energy to meet the needs of the customer a power electronic converter.

It is possible to indicate here a whole range of examples from billions of small chargers for smartphones working on the power of single watts to industrial electric drives rated at megawatts. The heart of each power electronic converter are switching power transistors, which usually generate a significant part of the power losses. Therefore, the way to achieve savings in the conversion of electrical energy, without any doubt, leads through the development and use of better semiconductor devices.

For decades these elements are produced in the base of silicon (Si). However, there are semiconductor materials with significantly better physical properties such as, for example, silicon carbide (SiC). Semiconductor devices made of such a material show much better parameters in comparison to their silicon counterparts. Since the production of silicon carbide and its further processing are more difficult, it took many years to develop production technologies of the power devices that meet the requirements of power electronics. The breakthrough came at the end of the first decade of the XXI century, when the first transistors (JFETs), which are able to operate at voltages above kilovolt and currents of several tens of amperes, were offered on the market. And as expected they were significantly better than the widely used silicon components.

Silicon Carbide = new possibilities

Researchers from the Department of Industrial Electronics Warsaw University of Technology, lead by prof. Roman Barlik were working on the applications of SiC Schottky diodes since 2005. In 2008 they gained access to the new technology of SiC JFET transistors. The cost of these items prototype was many times higher than comparable silicon elements, but the first tests in the laboratory showed that they were worth the money. They could switch between the conduction and blocking states within tens of nanoseconds, what is impossible to do with Si transistors with similar characteristics. Fast switching reduces the amount of energy dissipated during each switching. As part of a research project led by Jacek Rąbkowski a new transistors were applied in power electronic converters. First in Poland and one of the first in the world "fully-SiC" three-phase converter was developed in the ZEP laboratory in 2009. High speed of the SiC transistors was used in this case to increase the switching frequency of transistors by ten times, to 100 kHz, which allowed to significantly reduce the size and weight associated passive components - inductors and capacitors.

The new SiC components offer possibility to build a much better power converters - a couple of years it took researchers from a number of research teams in the world to convince the industrial partners. For this purpose many demonstrators has been built showing the possibilities of new power devices in various applications. Thus, for example, a group of researchers from KTH in Stockholm, which included Jacek Rąbkowski, has developed a drive inverter with an output of 40 kVA and record energy efficiency. The inverter based on SiC transistors for use in electric traction dissipated only approx. 0.3% power losses (efficiency up to 99.7%), so little that it was necessary to develop a new measuring method based on electro-thermal measurements. Standard methods were not accurate enough- the level of losses in relation to silicon was 10-times lower.

1% of power losses

If we remember that the inductor is by definition element which stores the electrical energy in the magnetic field, the fact that sometimes it can be simultaneously capacitor can surprise us. This happens at a very rapid voltage changes in the order of tens of volts per nanosecond, which takes place during a very fast switching of the SiC transistors. A small parasitic capacitance existing between the coils of the inductor and accumulated the electric charge can be a problem to modern power electronic converters, causing, among others, increase of the power loss and the reduction of the energy quality. Mariusz Zdanowski is dealing with these problems in the Foundation for Polish Science project VENTURES, developing high-performance converters with SiC elements. Designed and built prototype is expected to work with a battery of photovoltaic panels and is characterized by a much better performance in relation to Si based systems. On the one hand, due to the very high frequency of the current (0.5 MHz), at the nominal power level 6 kW occupies a volume of only 3/4 liters, on the other hand shows only 1% power loss.

 

High frequency, efficiency or maybe increased junction temperature of the transistor - SiC components provide impressive possibilities in the design of power electronic converters. In this situation, finding the best options is not easy and that is why Simon Piasecki in his doctoral thesis is working on multi-parameter optimization procedure. It will replace the designer in most tasks, who is expected to provide only basic input parameters and define their requirements, for instance a system with high efficiency. A genetic algorithm will check a number of possibilities based on the elements database and propose the optimal set of design parameters of the system. The first effects can already be seen - the grid AC / DC converter expected to work with renewable energy sources designed and built in the laboratory reached efficiency close to 99%.
Almost ten years of experience with the latest technology of semiconductor power devices is not only a scientific success and recognition of Department of Industrial Electronics (Faculty of Electrical Engineering) team in the world. It is also important that domestic companies from the power electronics sector have direct access to the latest developments at the same time as their competitors from countries such as the US, Japan and Germany. With financing joint projects by NCBiR SiC transistors will soon help to improve performance power converters made in Poland.