Significant efficiency improvements in hybrids and other vehicles with electric power trains is what Toyota is expecting from the deployment of silicon carbide (SiC) power devices. The Japanese carmaker announced that it is launching road tests with prototypes as early as this year.

Toyota will equip a Camry prototype and a fuel cell bus with power control units (PCUs) based on SiC semiconductors. Controlling motor drive power in HEVs and EVs, PCUs are playing a crucial role in the use of electricity — they supply battery power to the motors during operation and recharge the battery during deceleration whereby they utilise the energy gained by recuperation.

According to Toyota, power semiconductor currently account for some 20 percent of a vehicle's total electrical losses. Therefore developers regard raising the efficiency of the power semiconductors as a promising way to increase the overall powertrain efficiency. Here is where the SiC technology steps in: In comparison with today's silicon power semiconductors, SiC devices exhibit less resistance in the on-state, which translates directly into lower losses.

The semiconductor technology at hand has been developed jointly by Toyota, Denso and the Toyota Central R&D Labs.

In the Camry hybrid prototype, Toyota is installing SiC transistors and diodes in the PCU's internal voltage step-up converter and the inverter that controls the motor. Data gathered in the trials will include PCU voltage and current as well as driving speeds, driving patterns, and external conditions including outside temperature. Toyota engineers will compare these data with comparable data from conventional semiconductors; on this basis, the developers plan to assess the efficiency improvements associated with the SiC devices. Road testing of the Camry is scheduled to begin during the current month and will continue for about a year.

In a related move, the carmaker has begun testing and collecting operating data from a fuel cell bus currently in commercial operation in Toyota City. The bus is equipped with a fuel cell step-up converter that utilizes SiC diodes. This converter is part of the circuitry that controls the voltage of the energy generated by the fuel cell stack.

Data from the tests will be fed into the development process. The goal is putting SiC power semiconductors into series production as soon as possible.