Fuji Electric Announces Smaller, Lighter Automotive IGBT Module

2014/06/05 21:08
Tadashi Nezu, Nikkei Electronics

Fuji Electric Co Ltd introduced directly-water-cooled IGBT power modules for electric vehicles including hybrid cars at PCIM Europe 2014.

This time, the company announced (1) the technologies used for a module employed for a commercially-available vehicle and (2) a next-generation module that is smaller and lighter than it.

Module employed for commercially-available vehicle
The most distinguished feature of the module employed for the commercially-available vehicle is that aluminum (Al) is used for its heat sink. Al is lighter, cheaper and easier to process than copper (Cu), which is usually used.

Though the Al-based heat sink has many advantages, it also causes a problem. Specifically, the difference in thermal expansion coefficient between Al and the ceramic substrate (Si3N4) on which a power semiconductor device is mounted becomes larger, compared with Cu. As a result, the stress applied at the time of thermal cycling becomes stronger, causing cracks in the solder used for connecting the ceramic substrate and heat sink.

Therefore, Fuji Electric developed a new solder material. This time, the company used an "improved" SnSb-based solder material in place of a SnAg-based material. With the new solder material, it ensured a reliability high enough for automotive applications while using an Al heat sink.

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Fuji Electric also made improvements to the water-cooled part. It controlled the flow of water so that multiple IGBTs in the power module can be efficiently cooled. The IGBT module for commercially-available vehicles comes with two inverter circuits and one converter circuit. So, the power module is equipped with multiple IGBT chips.

To reduce costs, all of the IGBT chips have the same size. However, because the inverter and converter use different voltages and currents, each IGBT chip has a different heat generation density. To deal with the difference in heat generation density, the company made improvements to the internal structure of the water jacket so that the flow of water can be controlled.

Next-generation Module
The next-generation module is about 45% smaller in volume and about 30% lighter in weight than the company's module employed for a commercially-available vehicle. Specifically, the dimensions and weight of the next-generation module are 280 x 155 x 40mm and 2.4kg, respectively. The smaller size and lighter weight were realized by integrating the fins for the water jacket with those for the heat sink.

Aluminum (Al) can be more easily processed than copper (Cu), making it easier to form this structure. Also, the maximum junction temperature of IGBT chip was raised from 150 to 175°C.

The next-generation module has not been employed for any actual electric vehicle. And Fuji Electric defines it as a prototype for research and development. The company has been engaged in the research and development, aiming to have a small, lightweight next-generation power module employed for vehicles to be mass-produced in 2018, it said.