Hybrid Innovation by Toyota
Thermal Characterization in Automotive Hybrid Inverter Power Modules
By Boris Marovic, Product Marketing, Mentor Graphics
The general principal of a hybrid is to reduce the fuel consumption by leveraging both the electric motor and combustion engine, depending on which part of the drive cycle you are in. Usually the electric motor is used to accelerate the vehicle at lower driving speeds until a certain speed is reached, when the combustion engine kicks in and takes over. Hybrid models also provide the functionality to drive using the electric power stored in the battery, only switching to combustion to generate the electric power when the battery runs low, or runs on either battery or combustion engine solely or even both in some type of boost mode. Either way, it is not a full electric vehicle. The electric range is limited and only used to reduce the overall emissions of the vehicle.
Batteries are charged either by recuperating braking energy or from a generator driven by the combustion engine. in order to charge the battery the generated electric power has to be transformed from alternating current (AC) into the direct current (DC) needed for the battery. As soon as the battery is used to drive the vehicle, the current has to be inverted from DC to AC to drive the electric motor. Such tasks are handled by power electronics systems that incorporate several different components such as insulated-gate Bipolar Transistors (igBTs). igBTs can run at hundreds of amperes and act as electronic switches that can run at incredibly high switching rates and therefore create a lot of heat.
The name Toyota is not only synonymous with environmentally conscious vehicles, the most well-known could arguably be the Prius, but also with innovation. Hybrid Synergy Drive or HSD is the term designated to the technology Toyota has developed for their range of full hybrid vehicles. (Figures 1 &2) Toyota's effort in hybrid electric vehicle development is well known with the first hybrids on the road in the late 90´s. Since then the total production has reached millions globally, and is set to grow in the future.
Hisao Nishimori is an Engineering Manager at Toyota's Hybrid Vehicle Development Group. In a presentation given at Mentor Graphics' Tech Design Forum 2012 in Shinagawa, Japan in September, Nishimori discussed the work Toyota has done to characterize and optimize the thermal characteristics of power modules for Hybrids and Electric Vehicles. IGBTs are power electronic components that need to be optimized to have the ideal thermal resistance to better dissipate the generated heat of the component to the cooling system it is attached to. Applying Mentor Graphics' T3Ster® Transient Thermal Characterization system with the appropriate high-current boosters enables hybrids to reach the required power to heat the component, record the thermal response and derive the structure function of the component.
Prior to implementing T3Ster functionality, Toyota's design process began with the design of the module that was then built into a prototype and tested under different conditions such as switching frequency or grease thickness for the IGBT etc. Measurements were then taken with a thermal camera and the results evaluated. An unsatisfactory or inaccurate result required the team to go back and change the design and repeat the steps. Nishimori says, "When the design changes again, the re-evaluation of a new prototype took a lot of lead time, effort and cost."
Toyota's goal is to optimize the process to achieve highly accurate and repeatable results in a shorter time and therefore be more efficient. "T3Ster gave us a detailed insight into the structure of the component and its heat flow path. It shows the difference in thermal resistance in the vicinity of the tested element clearly and the measurements can be verified before and after durability tests to identify any changes in the heat dissipation characteristics." Toyota was also able to detect details in the structure, even in molded components when measuring all elements of the module at once. "We were even able to measure the bonding material under the element that matches the design value." With the help of T3Ster, Toyota is able to quickly feedback highly accurate results to the designers in order to further improve the design. Everything from grease thickness to bonding state could be optimized. Previously Toyota was unable to verify the status of the product and its heat dissipation in the vicinity of the molded element.
In summary Nishimori says, "By delivering the structure function from the measurement results by T3Ster, the comparison and verification of the design values can be done in more detail. The application of this measurement in an endurance test has helped us to uncover weaknesses in the design affecting its lifetime, and a general front-loading is now made possible because the design values could now be measured. And we can reduce the prototyping costs by reducing the rework on our designs." (Figure 3) Nishimori expressed his view of the design and evaluation process perfectly in an automotive metaphor: "Evaluation and Design are the two 'Wheels' of quality assurance and the important 'Engine' gives the driving force to the wheels. With the right combination of CAD, CAE and automated measurements the wheels will bring the combined power of 'Design Quality'onto the road." (Figure 4)