Thermal Modelling of Power Modules in a Hybrid Vehicle Application
Abstract: Hybrid electric and full electric vehicles have attracted growing attention during the last decade. This is a consequence of several factors, such as growing environmental concerns, increasing oil prices and a strive for oil independency. Hybrid vehicles have proven to have significant potential to improve fuel economy and at the same time enhance the performance of the vehicle. For the hybrid cars to really penetrate the passenger car market, it is of vital importance that the cost of hybridization can be kept as low as possible and that the increased cost will be paid back within a reasonable time horizon. In addition to this, high reliability of the additional electrical drive system (EDS) needs to be ensured. The power electronics (PE) of the EDS is often packaged into some kind of standalone housing. An alternative solution is to integrate the power electronic inverter and the electric machine into one unit, sharing the same cooling system. This will reduce the packaging volume and at the same time omit the need for the expensive connectors and cables. The integration has other benefits such as, modularity and no separate housing for the inverter is required. All of the above mentioned benefits of the system integration lead to reduced costs of the system and in the long run a reduced price of the vehicle. However, integrating the PE and ETM into one unit makes the thermal design more difficult. This thesis focuses on determining the required cooling capacity of the cooling system, which is an important task, both from a reliability and cost perspective. Long lifetime or high reliability is important for customer acceptance. Performing simulations of power inverter systems, where both the electrical and thermal response is incorporated, often referred to as electro-thermal simulations, is a difficult task due to the different time scales of the two disciplines. If every switching instant is to be simulated, simulation times in the range of microseconds have to be used. Simulating inverter systems in a hybrid vehicle application, where driving cycles lasting for thousands of seconds normally is used, requires unreasonable simulation times. The thesis presents a method for solving this problem. Several electro-thermal simulations are carried out in order to determine the cooling requirements and its effect on module reliability of the inverter. The foundation for these simulations is a simulation model of the electrical drive system, where the main focus is on the thermal model of the inverter itself. The thesis presents a thermal model development procedure to derive simplified thermal models based on thermal impedances. Thermal models for a range of power modules, including both Si and SiC based modules, are presented. Different types of assemblies with single-sided cooling are thoroughly examined in the thesis. In addition to the different single-sided cooled module assemblies, one assembly with double-sided cooling is studied. The thesis shows that the type of layout and assembly greatly affects the thermal behaviour, and as a consequence the lifetime, of the power module. In addition to different module assemblies and cooling options, the system is evaluated for different driving cycles and cooling medium temperatures, together with a comparison of using a fixed and variable switching frequency. The thesis shows that using a variable switching frequency has a significant impact on the cooling requirements.
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