Vehicle electrification – power electronics is the key to success
One of the key technologies needed to achieve widespread acceptance for electric vehicles is the use of power electronics optimised for use in hybrid and electric vehicles. Specifically, optimisation has to be achieved with regard to the efficiency of power electronic control of motors and auxiliary devices in electric vehicles, as well as to improvements in energy efficiency made possible by the connection of stationary vehicles to smart grids, which in turn enable intelligent connection between traction batteries and the grid as well as the efficient use of regenerative sources of energy. The lack of standard interfaces among component manufacturers, however, has led to the need for the development of product platforms that can be flexibly adapted to different requirements.
For 20 years Semikron has been developing and mass-producing power electronics for use in industrial electric vehicles, meaning the company is very familiar with the specific requirements of vehicle environments: power electronics used in electric vehicles have to deliver maximum power density and reliability, while being affordable and highly efficient at the same time. The interface for connection to the master controller in the vehicle must be flexible enough to facilitate adaptation to different conditions.
To achieve maximum power density, electric losses and thermal resistances must be kept to a minimum, and maximum component integration is required. At the same time, electromagnetic radiation has to be kept to within permissible limits. Low losses in turn mean high efficiency. The possible level of product reliability is determined, on the one hand, by the power semiconductor and DC link capacitor packaging, as well as the degree of electronic component integration and, on the other hand, by the thermal shocks that occur.
One way to achieve interface flexibility is to combine programmable elements. In other words, the use of a Field Programmable Gate Array (FPGA) for time-critical functions, a Digital Signal Processor (DSP) and/or microcontroller (µC) for the implementation of the control algorithms, as well as connection to the master control unit. The combination of various programmable elements also enables the implementation of safe designs with respect to its functionality, as well as the transparent allocation of design tasks in co-operation projects.
Converters from the SKAI product family from Semikron are already in their 2nd generation and have been optimised for use in hybrid and electric vehicles. The figure shown (Fig. 1) describes the principal components of SKAI converters.
Fig. 1: Components of a SKAI power converter for hybrid and electric vehicles. For higher resolution, click here.
The three-phase IGBT-based inverter SKAI2HV achieves, for example, a power density of up to 20 kVA/litre at a nominal efficiency of >98%. The EMC requirements stipulated in the European EMC Directive for registered vehicles are duly complied with. The power semiconductors used in the converters are sintered rather than soldered. The use of silver sintering results in higher power density and longer service life at the same time.
For users this means better converter load cycling capability at no compromise to service life. (Fig. 1a) shows, for example, the time curves for motor voltage V, motor current I and power factor cos(phi) during a vehicle acceleration cycle. The vehicle accelerates from 0 km/h to100 km/h in 10s, then braking within a period of 25s until coming to a standstill. This cycle is repeated every 90s. In the power semiconductors the load cycle described generates the temperature cycle shown in (Fig. 1b) (In which Ttr stands for junction temperature IGBT, Td for junction temperature diode, and Ts for heat-sink temperature).
Fig 1a. For better resolution, click here.
Fig. 1b. For better resolution, click here.
At an average temperature of 85°C, the power semiconductors are subjected to a temperature step of 51 Kelvin. For the aforementioned load, 760,000 cycles or 2.2 years of uninterrupted operation is possible. This is down to the improved load cycling capability achieved by the silver sintering process. The DC link capacitor is loaded with 67A IRMS due to the motor current shown in (Fig. 1a). The good thermal connection between the capacitor and the cooling system means that this current load is possible for up to 2.8 years (under continuous load). The life cycle of an SKAI2HV converter is thus limited by the service life of the power semiconductors.
The interface between control unit and master controller in the vehicle comes as either a CAN bus interface or, alternatively, as a switching signal interface. Also available (as a non-standard option) is the motor control software QUASAR, which is based on field-oriented control. QUASAR guarantees stable control of the electric motor up to the strong field-weakening range and thus enables the implementation of dynamic and efficient traction drives and generator solutions. QUASAR normally receives input from a central controller via CAN and accurately converts this into speed or torque.
The QUASAR software solution, developed in compliance with the MISRA-C standard and adapted to the SKAI hardware, is designed to control brushless DC, interior permanent-magnet and surface-mount permanent-magnet synchronous motors, as well as three-phase induction motors. Different types of motor can be parameterised using this software. QUASAR includes features such as temperature monitoring in motors and inverters, battery deep discharge protection, overvoltage protection for DC energy recovery, CAN communication in accordance with CANOpen, software update via CAN boot loader in the vehicle network, etc. Users can also, of course, choose to use their own individual control software if they prefer. What is more, the controller hardware can also be adapted to specific requirements.
Fig. 2: SKAI-2 power electronics unit.
The power interface features screw terminals; the wiring comes with either standard cable glands (Fig. 2) or reliable pre-configured lead-throughs ( Fig 2a) to the screw terminals.
Semikron helps users configure their power electronic for use in electric drives. Based on the electric power profile required, as shown, for example, in (Fig. 2b), the overall service life and the impact of period critical load variations on the service life can be determined. With this detailed data, optimised motor control is easily achievable. For those designing a total electric drive solution, Semikron offers a calculation service in conjunction with competent partners. The only data needed for this is the required torque and speed characteristics.
The power electronics platform will continue to be developed in strict compliance with the aforementioned requirements. Instrumental in this is the silver sintering process developed in-house at Semikron: in the next product generation sintered joints will replace not only the chip solder connections, but also the wire bonds presently used, plus the thermal paste used to connect the power semiconductor modules to the heat sink. This innovative step will eliminate all elements in power electronics that are prone to wear and thus detrimental to service life. (Fig. 3)
In future thick-wire bond connections will be replaced by a sintered flexible board, where the conductor paths routed across the underside of the board will conduct the current. The surface of this flex-board contains the electronic components, which, in combination with the ceramic and flexible PCB layout, optimise the switching properties of the power semiconductors and, consequently, the switching losses and EMC properties. Tests show that, thanks to the aforementioned measures, the next generation three-phase inverter can achieve a power density of 40kVA/litre – double the power density of its predecessor.
Given the current state of the art, the use of power semiconductors made of gallium nitride or silicon carbide rather than silicon, also in combination with the innovative silver sintering process, is technically feasible. These semiconductors can be used to increase power density further and even today are economically feasible in chargers, for example for power factor correction.
Returning to the German government’s ambitious target, it can be said that for this to be achieved, the Total Cost of Ownership (TCO) of an electric vehicle has to be on the level of that of a conventional vehicle. Developments will therefore be driven by the need to reduce the cost of mass-produced electrical energy storage devices. Since the TCO is largely influenced by system integration costs, a further challenge will be how to adapt power electronic control systems to the given energy storage device, motors and vehicle master controller, on the one hand, and to ensure that existing hardware and, in particular, software components are used where possible.
About the author: Dr.-Ing. Klaus Backhaus is Program Manager High Voltage Systems, Semikron GmbH