Double-Sided Cooling for MOSFETs

Emission regulations for the automotive industry are increasingly tightening. To meet these demands, the industry is moving rapidly towards the electrification of vehicles. Primarily, they are making use of batteries and electric motors for the purpose. However, they also must use power electronics for controlling the performance of hybrid and electric vehicles.

In this context, European companies are leading the way with their innovative technologies. This is especially so in the development of power components and modules, and specifically in the compound semiconductor materials field.

ICs used for handling electrical power are now increasingly using gallium nitride (GaN) and silicon carbide (SiC). Most of these devices are wide-bandwidth devices, and work at high temperatures and voltages, but with the high efficiency that is typically demanded of them in automotive applications.

Silicon Carbide is particularly appealing to the automotive industry because of its physical properties. While silicon can withstand an electrical field of 0.3 MV/cm before it breaks down, SiC can withstand 2.8 MV/cm. Additionally, SiC offers an internal resistance 100 times lower than that of silicon. These parameters imply that a smaller chip of SiC can handle the same level of current while operating at a higher voltage level. This allows smaller systems if made of SiC.

Apart from functioning more efficiently at elevated temperatures, a full SiC MOSFET module can reduce switching losses by 64%, when operating at a chip temperature of 125 °C. Power control units for controlling traction motors in hybrid electric vehicles must operate from engine compartments, and this places additional thermal loads on them.

Manufacturers are now exploring various solutions for improving the efficiency, durability, and reliability of SiC MOSFETs under the above operating conditions. One of these is to reduce the amount of wire bonding by using double-sided cooling structures. This cools the power semiconductor chips more effectively. Therefore, overmolded modules with double side cooling are rapidly becoming more popular, especially for mid-power and low-cost applications.

As a result of the research at the North Carolina State University, researchers have developed a prototype inverter using SiC MOSFETs that can transfer 99% of the input energy to the motor. This is about 2% higher than silicon-based inverters under regular conditions.

While an electric vehicle could achieve only 4.1 kW/L in the year 2010, new SiC-based inverters can deliver about 12.1 kW/L of power. This is very close to the goal of 13.4 kW/L that the US Department of Energy has set for inverters to be achieved by 2020.

With the new power component using double-sided cooling, it is capable of dissipating more heat effectively in comparison to earlier versions. These double-sided air-cooled inverters can operate up to 35 kW, easily eliminating the need for heavy and bulky liquid cooling systems.

The power modules use FREEDM Power Chip on Bus MOSFET devices to reduce parasitic inductance. The integrated power interconnect structure helps achieve this. With the power chips attached directly to the busbar, their thermal performance improves further. Air, as dielectric fluid, provides the necessary electrical isolation, while the busbar also doubles as an integrated heatsink. Thermal resistance for the power module can reach about 0.5 °C/w.