The PCU embraces two functions: An inverter and a converter. The inverter manages the current flow to and from the electric motor(s) of a hybrid electric vehicle. It continuously controls the rotation speed of this motor as well as its power output when in generator mode. The converter varies the battery voltage as required. Given these responsibilities, it is clear that the entire current of a HEV runs through the PCU. The losses in this unit are significant; the PCU's overall efficiency is typically only 75% whereas 80 percent of the losses are generated in the semiconductors.
Along with Toyota Central R&D Labs (Toyota CRDL) and automotive supplier Denso, Toyota has developed power semiconductors based on Silicon Carbide (SiC). Currently more or less all power semiconductor manufacturers are working on implementing SiC transistors, but so far this technology has not yet reached the maturity to meet automotive requirements on a broad basis.
Smaller wafers, but better efficiency: Standard silicon vs SiC wafer (right)
The new SiC transistors offer much better energy efficiency than conventional parts, enabling the carmaker to reduce the site of the Power Control Unit by as much as 80%. In particular space-filling parts such as the cooler and the capacitors which could be shrunk significantly. At the same time, the efficiency of the electric section of the power train has been improved in a way that the cars get along with 10% less fuel.
Toyota is presenting this technology at the Automotive Engineering Exposition in Yokohama that takes place from May 21 through 23. Recognising that semiconductors are a key technology to further improve the energy efficiency of the automotive power train, Toyota announced to crank up its R&D capacities to bring the new technology as fast as possible into serial production.