The power electronics activity focuses on research and development (R&D) for flexible, integrated, modular power inverters and electronics for power conditioning and control, including a power switch stage capable of running a variety of motors and loads. An inverter is needed to convert direct current (dc) power from a fuel cell or a battery to alternating current (AC) power for the motor. An automotive integrated power module (AIPM) has been developed that approaches the FreedomCAR and Fuel Partnership targets for weight and cost, but only if the coolant temperature is lower than desired. Further research will focus on the use of
- Silicon carbide semiconductors, which can be operated at much higher temperatures than current silicon semiconductors;
- Improved thermal management technologies; and
- Innovative power circuit topologies that have the potential for reducing the weight, volume, and cost of the system.
Capacitors account for a major fraction of the weight, volume, and cost of an inverter. Currently, electrolytic aluminum capacitors are used for application below 450 volts (V); but, in addition to being bulky, they cannot tolerate high temperatures; they tolerate very little ripple current; they have short lifetimes; and they can sometimes fail. Two promising alternatives to electrolytic aluminum are polymer-film capacitors and ceramic capacitors. Polymer-film capacitors are used for voltages above 450 V and are less bulky, but they also cannot tolerate sufficiently high temperatures. Research to date has identified several candidate polymers with higher temperature capabilities, and that research will continue on ceramic capacitors, which have the greatest potential for volume reduction and the ability to tolerate very high temperatures. The emphasis for ceramic capacitors will be on ensuring a benign failure mode and lowering the cost.
Current motor controller technology revolves around digital signal processors, but external circuitry is still required to accomplish all of the functions necessary for efficient motor control. A new R&D effort is being initiated to develop a system on a chip that will provide the opportunity for considerable cost reduction. Hybrid fuel cell vehicles will require a bidirectional dc/dc converter to interconnect the fuel cell power high-voltage bus and the low-voltage bus for vehicle auxiliary loads. Technical issues to be addressed include choice of topology, filtering requirements, switches, switching frequency, radio-frequency interference considerations, thermal management, and types of magnetic components. Cost, reliability, weight, and volume are critical factors.