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Efficient power electronics is key to widespread HEV adoption

Posted: 09 Feb 2012 ?? ?Print Version ?Bookmark and Share

Keywords:electric vehicles? power electronics? FPGA? DSP?

The widespread adoption of electric cars is dependent on power electronics optimized for use in hybrid and electric vehicles. Specifically, optimization 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, it refers to the use of a FPGA for time-critical functions, a DSP and/or microcontroller 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 optimized for use in hybrid and electric vehicles. Figure 1 describes the principal components of SKAI converters.

Figure 1: Components of a SKAI power converter for hybrid and electric vehicles.

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.

Figure 2: The time curves for motor voltage V, motor current I and power factor cos(phi) during a vehicle acceleration cycle.

For users this means better converter load cycling capability at no compromise to service life. Figure 2 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 figure 3 (in which Ttr stands for junction temperature IGBT, Td for junction temperature diode, and Ts for heat-sink temperature).

Figure 3: The load cycle described generates this temperature cycle.

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