Power Electronics

Power electronics is already an integral part of modern motor vehicles, and its role is set to grow further in the coming years. This applies not only to electric and hybrid vehicles with their electric drive motors, but also to conventional vehicles with internal combustion engines, in which auxiliary units are increasingly being driven decentrally by electric motors in order to improve system efficiency and thereby save fuel. Key power electronics components in current motor vehicles are control units for driving and regulating electric machines. This includes inverters and rectifiers for rotating-field and direct-current machines, as used in a wide range of applications within motor vehicles. Examples include drives for pumps, fans and positioning tasks. Furthermore, DC voltage converters are increasingly being integrated into motor vehicles today to enable the operation of loads with different voltage levels within the vehicle’s electrical power network.

Due to the rising number of electrical loads in motor vehicles, the vehicle’s electrical power network must be designed to handle ever-higher power levels. Rapid switching on and off of high-power loads generates large current pulses, which can lead to voltage fluctuations in the vehicle’s electrical power network. The alternator cannot compensate for these voltage fluctuations. Its power output must not be varied arbitrarily quickly in order to limit the impact on the drive torque of the internal combustion engine and, consequently, the vehicle’s handling. In current electrical power systems, the battery must therefore absorb these large pulsating current peaks. Of particular critical importance in this context are regenerative braking – i.e. the recovery of energy during vehicle deceleration – and the start-stop operation of the internal combustion engine.

To meet these increasing demands, new architectures for electrical power networks in motor vehicles are required. Systems designed both to stabilise the mains voltage and to reduce the pulsating current draw from high-power consumers are being discussed and increasingly implemented. At the same time, redundancies must be maintained within the power network to ensure that safety-critical consumers can continue to operate reliably in the event of a power failure. Power electronics is therefore a key technology for implementing the new power network structures in motor vehicles. In this context, cost, reliability and the harsh environmental conditions inside the vehicle play a major role in the development of optimal systems and components.

Within the research focus area ‘Optimisation of power electronic converters and systems for automotive applications’, various research projects have been carried out by several staff members at the Laboratory for Electrical Machines and Power Electronics, headed by Prof. Dr Johannes Pforr, for a number of years. A brief overview of the ongoing projects is provided below.

Initial work on optimising high-power voltage converters began as early as 2004 at Ingolstadt University of Technology in cooperation with AUDI AG. The aim was to achieve extremely high efficiency – and thus low losses – in these converters, whilst simultaneously reducing costs, volume and weight. Thanks to their reduced volume and low losses, these converters can be optimally integrated into vehicles.

To reduce the volume of the converters, multi-phase converters with coupled inductors were developed. With appropriate control, the required inductive and capacitive filter components in these converters can be selected to be very small without inducing large alternating currents in the filter components. This reduces power dissipation in the converter across the entire operating range and achieves very high dynamic performance. The optimised converters are ideally suited for relieving the load on and stabilising the electrical power network within the motor vehicle. Fig. 1 shows a typical 6-phase DC-DC converter with a coupled filter inductor.

The technology developed has been successfully implemented in recent years in collaboration with the automotive supply industry. Current activities in this area of research focus on further reducing volume and power dissipation through the application of innovative control methods, optimising the switching behaviour of the semiconductors used, and investigating innovative converter topologies for electric, hybrid and internal combustion engine vehicles.

The aim of research in this area is to optimise the control of rotating-field machines for various applications in hybrid, electric and internal combustion vehicles.

In an initial research project, the power electronic control system for an electric power steering system was optimised. The focus of this project was on dynamically decoupling the power output of the electricity grid from the power consumption of a selected load. This prevents the occurrence of peak currents in the electrical power grid and reduces the load on the battery. Using the example of electric power steering, various methods were systematically investigated and evaluated. The results led to the development of an innovative control method for rotating-field machines, in which the inverter is simultaneously used as a step-up converter to generate a higher and variable DC link voltage. This ‘integrated converter’ makes optimum use of the energy stored in the DC link capacitor for power decoupling, thereby minimising the size of the required energy storage device. Fig. 2 shows the circuit diagram of the integrated converter with a rotating-field machine. The theoretical and experimental results of the research demonstrate the advantages of this technology: in addition to avoiding a positive-shaped current draw, this technology also achieves, for example, a higher DC link voltage in the inverter and enables the implementation of a redundant power supply, thereby ensuring greater operational reliability in the event of a mains voltage failure. Both asynchronous and synchronous machines were considered as rotating-field machines.

Subsequent projects focus on the control of rotating-field machines for traction drives in hybrid and electric vehicles.

The aim of research in this area is to develop and optimise innovative assembly technology for automotive power electronics, with a view to improving system performance and reducing system costs.

In an initial research project, a concept was developed for the control and connection of spatially distributed light-emitting diodes (LEDs) in three-dimensional lighting systems for motor vehicles; this concept encompasses the power electronics for controlling the LEDs, the distribution of energy to the individual LED chips and, in high-power systems, the dissipation of the resulting power loss. The compact size of light-emitting diodes offers new creative freedom in the design of motor vehicle lighting. For an LED lamp, a large number of individual LEDs can be interconnected and integrated into the vehicle’s design. Furthermore, light-emitting diodes achieve an extremely long service life and high reliability. Their service life significantly exceeds the operating hours of a motor vehicle, thereby offering further advantages for use in motor vehicles. In order to fully utilise the potential of LED technology in motor vehicles, various methods of electrical control and connection of spatially distributed LEDs have been investigated and optimised. Fig. 3 shows an exploded view of a typical automotive rear light.

Subsequent projects will focus on the assembly technology of high-performance electronics in modern hybrid and electric vehicles.

Fig. 3 Exploded view of a typical automotive rear light

The demands placed on electrical power networks in motor vehicles will continue to increase due to new electrical loads, some of which are safety-critical. As a result of this development, the required average electrical power will continue to rise, and the range of electrical loads will become even more diverse in terms of average power, peak power and duty cycle. Changing power requirements complicate the stable operation of the power network and lead to increasingly complex power network conditions. This development calls for new power systems that can cope with growing cost pressures and operate reliably whilst delivering ever-greater functionality. Through the use of innovative power electronics, the architecture and effectiveness of the power grid can be redefined. Power electronics is therefore a key technology for optimising energy systems.

The aim of research in this area is to optimise the electrical power grids in electric, hybrid and internal combustion engine vehicles through the use of modern power electronics. The focus is on voltage stability and efficiency in energy transmission. Through a combination of simulation-based energy efficiency, reliability and cost analysis, evaluation methods for power electronic devices and components in motor vehicles are being developed. This creates an opportunity for preventive product optimisation.

In an initial research project, a converter was specifically optimised to stabilise the power supply voltage for sensitive loads. The ‘floating’ converter consists of a full bridge with a double-layer capacitor as an energy storage device, Fig. 4. It is connected in series with the selected loads and, unlike conventional converter topologies, is not connected to the vehicle ground. This results in a particularly cost-effective and compact converter with very high current-carrying capacity and very high efficiency. The ‘floating’ operation of the converter allows the use of semiconductors with very low turn-off voltages, Fig. 2. The ‘floating’ converter operates in the power supply network like a controllable voltage source connected in series with the loads and can therefore maintain a constant voltage across the loads. The energy stored in the double-layer capacitor can be used to compensate for short-term voltage dips and surges, such as those that occur during start-stop operation and energy recovery.

Subsequent research projects will investigate 1) the interaction and thus the mutual influence of different power electronic components in electrical power grids and 2) the efficiency of recuperation, i.e. the recovery of braking energy, in modern motor vehicles.

Fig. 4 Floating converter

The importance of power electronics in modern motor vehicles continues to grow. A key motivation for the use of an increasing number of power electronic components and systems is the growing demand to reduce fuel consumption whilst simultaneously enhancing driving comfort. This development is leading to the decentralisation of auxiliary units, which are no longer connected directly to the internal combustion engine via a belt, but are instead driven individually by separate electric motors. Power is supplied to these auxiliary units only when it is actually required. In the research area presented here, ‘Optimisation of power electronic converters and systems for automotive applications’, new concepts and architectures featuring innovative power electronic components are being developed and optimised for future automotive electrical power networks. The projects presented are being carried out in collaboration with the automotive and automotive supplier industries.

Special thanks go to AUDI AG for its ongoing support of research in the field of automotive power electronics. In this context, thanks are also due to Flextronics Automotive GmbH & Co. KG for its generous support of the research project “Optimisation of high-power automotive converters”. I would also like to take this opportunity to thank all other collaborating companies for their financial support of individual sub-projects.

• M. Wechsler, M. Simon, S. Edler, J. Pforr, "A simple technique to measure semiconductor switching and conduction losses of inverters at specified chip temperatures", IEEE ECCE 2015, Canada, 2015
• M. Mürken, M. Simon, J. Pforr, T. Hackner, "Application of a floating H-Bridge converter to stabilize the automotive Energy Net", IEEE ECCE 2015, Canada, 2015
• W. Thomas, "Integrated Automotive High-Power LED Lighting Systems in 3D-MID Technology", PhD-Thesis, TU Delft, 2014
• M. Simon, M. Mürken, C. Augustin, J. Pforr, "Multi-port converter with bidirectional energy flow for automotive energy net applications", Proc. of IEEE EPE 2014, Finland, 2014
• T. Hackner, "Multifunctional Converter Drive for Automotive Electric Steering System", PhD-Thesis, TU Deft, 2013
• Stadler M., Utz S., Pforr J.,"Optimierung mehrphasiger automobiler Gleichspannungswandler mit gekoppelten Induktivitäten", Haus der Technik, München, Germany, 23-24 April, 2012
• Hackner T., Pforr J., "Fault Tolerant Electric Power Steering System with Multi-Funktional Converter Drive Using Two-Phase Operation", PCIM, Nürnberg, Germany 2012
• Utz S., Pforr J., "Impact of Input and Output Voltage Perturbations on the Behaviour of Automotive Multi-Phase Converters with Coupled Inductors", IEEE ECCE 2011, USA, 2011
• Utz S., Hackner T., Pforr J., "A novel tri-state driver to improve the switching performance in automotive converters", IEEE EPE-PEMC 2010, Macedonia, 2010
• Thomas W., Pforr J., "A novel current sharing method for automotive LED-Lighting system", IEEE EPE, 2009
• Stadler M., Utz S., Pforr J., "Filter Optimization for multi-phase DC-DC converter in automotive energy backup system", 24th IEEE APEC 2009, USA, 2009
• Stadler M., Pforr J., "Two-Phase Boost Converter for 14V/42V automotive applications using coupled tapped inductors" PCIM, Nürnberg, Germany, May 27-29, 2008
• Stadler M., Pforr J., "Zero-Voltage Switched Multi-Phase Converter utilizing Non-Linear and coupled Inductors", Proc. of 22th IEEE APEC 2007, pp. 1038 - 1042, USA, 2007