On the Mechanical Design, Thermal Management, and Packaging of High Power Density Automotive Traction Inverters

Abstract

In order to reduce the impact of the adverse effects of climate change caused by excessive emission of greenhouse gases, it is important to move towards developing cleaner alternatives to technologies that have been relying on fossil fuels. This means electrification of transportation industry and the use of reusable Battery Energy Storage System (BESS) and relying more on solar, wind, hydroelectric and nuclear energy generation. At the heart of this electrification and shift to renewable energy lies advancement of Power Conversion Unit (PCU) and utilization of its abilities to full extent, specifically inverter technology. Inverters help in converting Direct Current (DC) power to Alternating Current (AC) power and vice versa depending on the application. In Battery Electric Vehicle (BEV) and Hybrid Electric Vehicle (HEV) the inverter is used to convert the DC power from the battery pack to AC power for the motor during driving and vice versa during regenerative braking. Similarly inverters are used in solar power generation to convert the AC power from the solar panels to AC power for the grid.

For wind, hydroelectric and nuclear power generation the inverters are used because even though the power output is AC it widely changes depending on the wind speed in the case of wind generators, water flow rate in hydroelectric generators and steam generation for nuclear power generation. The inverters take this widely varying power and convert it to smooth and consistent AC power that can be passed on to the grid. This widespread adaptation of cleaner and greener electrified technology has accelerated the development of Inverters and they have steadily grown in their performance. Inverters contain semiconductor switching devices that help in the AC/DC or DC/AC power conversion. Historically Silicon (Si) Insulated Gate Bipolar Transistor (IGBT), commonly known as IGBT switches have been used for most automotive traction inverter applications. Recently, in an effort to increase power density and efficiency Wide-Band gap (WBG) devices such as Silicon Carbide (SiC) and Gallium Nitride (GaN) Metal Oxide Semiconductor Field Effect Transistor (MOSFET) have started to replace IGBT switches. While the power electronics have improved in their power density and efficiency, in order to fully utilize these switching devices to their full potential, it is important to provide adequate thermal management. This is because, even though the WBG devices have higher efficiency in general, as the power increases the heat dissipated due to losses also increases, and performance of these switching devices is directly tied to their junction temperature among other factors. Thus the full efficiency cannot be realized if the switching devices are running hotter.

Therefore, it is important to also scale the efficiency and performance of the thermal management system that goes along with power electronics. Additionally, the switching devices and the thermal management needs to be contained inside a housing that can withstand the harsh operating conditions and provide protection from the environment and mechanical vibrations. This thesis aims to provide a detailed analysis on the development of a 120kW, and 250kW inverter with a cooling system integrated into the housing. In order to optimize for performance, reduce weight and volume, design tools such as Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) are used in tandem with analytical equation driven solutions. This approach helps in creating fast and accurate thermal models that can be implemented in real time rather than simply relying on just CFD analysis. The thesis also outlines some of the basic Design For Manufacturing And Assembly (DFMA) rules that should be followed to ensure that the design can be manufactured and assembled with minimum cost and complications. Lastly, a prototype of 120kW inverter is built considering automotive standards as it represents some of the strictest, safest and harshest environments in which an inverter is being used. The prototype is then used to further investigate and validate the effectiveness and reliability of the tools and techniques described in this thesis. The thermal management is integrated into the housing not only because the housing acts as heat sink that spreads and dissipates the heat away from the switching devices, but also because it provide mechanical structure and rigidity to the housing, protecting it from mechanical loads and vibrations.