The field of electric flight is completely new and there is still a wide field of fundamental research, e.g. concerning the influences of temperature, air density and cosmic radiation. Established technology has to be rethought - challenges are the demand for high reliability, high power density and maximum efficiency at the same time. The use of modern wide-bandgap semiconductors such as SiC and GaN can meet these requirements, but the challenges in application and validation are high. In addition, there are fundamental questions - what fuel will we fly on? With what kind of engines, with how many propulsion systems and what aircraft configurations? What will the energy transfer and conversion look like? We are dealing with all these things at IMAB.
Reliability of Semiconductors
In 2020, a new laboratory for assessing the reliability of power electronic components was established. The reliability laboratory is directly connected to two other power electronics laboratories, which provide a variety of equipment. Separate power and cooling water supply and a monitoring system enable 24-hour operation. A high-performance air conditioning system offers the best laboratory conditions.
The increasing required power density in mobile applications, the growing onboard voltages in electrical systems, the influence of fast-switching semiconductors and new fields of application with novel environmental conditions are significant challenges. With the help of the laboratory, the institute can evaluate the reliability of active and passive power electronic components.
The central unit of the modular test platform is a climatic cabinet, which allows the environmental parameters (humidity and temperature) to be adjusted. The climatic cabinet has a test chamber volume of 600 l, can regulate power losses of up to 3.5 kW (at -20 °C to 100 °C) and covers a wide climatic and temperature range. With the help of different load set-ups, creepage distances, dielectrics, insulations in windings of magnetic components, and capacitors can be selectively aged at an accelerated rate. A real-time controller (Ni cRIO) controls the self-designed load structures and records crucial parameters. In addition, the behaviour of the passive components can be captured with an impedance analyser and corresponding devices.
The active components and their assembly and connection structures are aged at an accelerated rate by the Power Cycler. The Power Cycler, a high-performance current source, has four channels of 600 A each. The cooling plate can precisely adjust the case temperature of the components. A temperature change is achieved via the current. The assembly and connection technology with its different expansion coefficients can thus be aged at an accelerated rate. Furthermore, the test stand can directly record measured variables of the ageing process. In addition, The power cycler can also determine the thermal network of active components.
DC/DC converters in DC grids
Some modern renewable energy systems like photovoltaic or storage systems provide DC energy. Several applications need DC energy directly or work with DC links. DC grids seems to be advantageously for these use cases. That is why current research dedicates DC grids. On the one side DC grids represent challenges for electrical safety and protection technologies. Classical failure detection and shut-off systems, which are provided for AC grids can not be applied in DC grids. On the other side DC/DC converters are required as connecting links between the different grid levels, feed-in systems and loads. In dependency on grid level or functionality these converters have to perform at high voltages and high power. These in grid systems until now unknown power electronic units can affect switching operation in the DC grid, which will be carried out for safety and protection purposes. Due to this beside functionality also safety, reliability and life time aspects are on the focus of research interests. Furthermore, methods to increase the power density are interesting for mobile application like vehicles, vessels or aircrafts.
Traction drives for alternative energy vehicles
Hybrid-electric vehicles or full electric vehicles need tractions drive systems at different HV voltage levels. In depency on voltage level GaN or SiC powers semiconductors contribute to higher efficiency and power density for the power electronics inside of the traction systems. From point of research challenges are in the cooling possibilities, assembly and connection technology, electromagnetic compatibility and filter design.
Some auxiliary units require high speed drives. Alternative topologies are in the research focus. Reliability and electromagnetic compatibility have to ensure and to investigate for these special applications.
Until now industrial drives represent on the most important application of power electronics. The available electric energy should be used most efficient as possible. That is why the topic "Energy efficiency" is one of the most important driving force for further research in the field of industrial drives. Under this aspect the potential of current fast switching SiC-MOSFET technologies are in the focus of the research. SiC-MOSFETs allow an efficient use during drive and brake operation. But especially drive systems with long cables benefit from SiC technologies. Such systems requires additional filter units between inverter and motor. Filter can be reduced in volume when higher switching frequencies are possible. Research questions for SiC inverters deal with the low-inductive design of the commutation circuits with fast switching power semiconductors, the electromagnetic compatibility, the interaction of inverter and filter components concerning losses and the relation of filter design and volume.
For on-board charger the reduction of size and weight is in the focus of the research investigations. For off-board systems the challenge is in the relatively high power demand. These basic requirement results in different topology approaches. On-board charger in hybrid or full electric vehicles up to approximately 7,2 kW benefit from resonant switched DC-DC converters. Topologies like LLC or DAB are in the focus of our theoretical and experimental investigations. For one-phase GaN power semiconductors promise really high switching frequencies up to the MHz limit (fS ≤1MHz). But thermal design of the active and also of the passive components and the electromagnetic compatibility have to be taken into the account. For three-phase chargers (more than 11kW) SiC power semiconductors allow the most compact design, especially for the passive components and with this for the overall power density.
System approaches with HF transformer are common because of the easier safety compliance. Otherwise transformatorless approaches promise an increasing of power density because of the absence of a transformer. Research focus of presentation of advantages and disadvantages of both approaches. Solutions concerning safety and electromagnetic compatibility have to take into the account.
Renewable energy systems
The use of photovoltaic and wind energy is the basis for the change in the electrical power generation from fossil fuels to renewable energy systems. One challenge is the fluctuating generation. Otherwise a high volume of new electrical loads like E-vehicles change the user behavior and increases the energy demand. Both aspects requires storage possibilities.
The grid-codes require the use of sine-filter for the most feed-in systems. From all filter types sine-filters are the most voluminous and heavy. A reduction can be achieved by higher switching frequencies. Due to this renewable energy systems benefit from new fast switching power semiconductors like SiC and GaN notably. The fast switching allows a significant increasing of switching frequencies at low losses in power semiconductors. This can also result in smaller filter components. Furthermore, the number of different sources and loads in an energy systems increases. For such systems the investigation of modular, efficient and cost-effective realizations over a broad power range is obviously. The aim is the extension of such a system with the increasing of the energy demand over the time.