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Toyota gears up HEMTs for hybrid electric vehicles
To improve the efficiency of power switching in hybrid electric vehicles, Toyota is developing a pair of gallium nitride transistors.
Toyota is developing vertical HEMTs for high power switching in hybrid electric vehicles (HEVs), and lateral equivalents for lower power applications in cars.
That was the main message of the talk given by Tetsu Kachi from Toyota Central R&D Labs, Japan, to delegates at the ninth International Conference on Nitride Semiconductors (ICNS) in Glasgow, UK.
Kachi began by pointing out that the silicon electronics used today to convert DC power from the battery to an AC form to power the motor has an efficiency of 90-95 percent. The energy that's wasted is dissipated as heat, and managing this requires a water-cooling system.
The amount of energy lost could increase because the trend is towards higher and higher powers in HEVs. The first generation of Prius used a 30 kW motor, but second and third generations have increased this figure to 50 kW and 70 kW. Even higher figures are being used by Lexus, which has a motor with a power of more than 150 kW in its model LS600h.
Losses in the DC-to-AC conversion process occur because the switching is imperfect. Ideally, no current would be drawn in the off-state, in the on-state there would be no resistance and switching speeds would be instantaneous.
Kachi explained that switching losses promise to fall by a factor of about six by replacing silicon switches with those made from GaN. What's more, the electronics would no longer need its own dedicated water cooling system. Instead, it could tap into the one used for the engine, and one day it might just require air cooling.
The representative of Toyota also explained that smaller power modules, operating at a few kilowatts, are also needed in HEVs for air-conditioning, and emergency and outdoor applications.
“Each loss is low, but the total loss is not low,” added Kachi, who pointed out that every efficiency saving boosts the mileage of the HEV.
Toyota are developing two types of GaN HEMT for the HEV: vertical devices for high-powers, which have the merits of high current density, high breakdown voltage and a high on-resistance; and lateral transistors for lower powers, which combine high frequencies with a low resistance, and high breakdown voltage and low cost.
The vertical devices feature a novel U-shaped trench that is formed by dry etching with an inductively coupled plasma, followed by wet etching for 60 minutes at 85 degrees C. The sidewall forms a MOS channel that works well up to 300 degrees C. Normally-off operation is realised with this device, which has a breakdown of 180V. The goal is to increase this to 600V.
Efforts on the lateral devices have created devices that are normally off above 3V, and have issues related to current collapse and reliability of the gate insulator. Engineers at Toyota will work to improve this device and its vertical cousin.
