Exploring an Advanced Approach to Thermal Design for Dual-Sided Cooling of Power Semiconductor Modules in Electric and Hybrid Vehicles

In recent years, the automotive industry has witnessed a remarkable transformation driven by rapid advancements in technology and an increasing global emphasis on sustainable practices. Amidst this revolution, hybrid electric vehicles (HEVs) and fully electric vehicles (EVs) have emerged as pivotal players in shaping the future of transportation. The significance of these innovative vehicles extends far beyond mere modes of commuting; they signify an important shift towards a greener and more sustainable automotive landscape. Moreover, issues like mechanical noise, deterioration of fossil fuels, and excess generation of carbon dioxide can be solved by shifting our mode of transport to electric/hybrid vehicles. As a result, it has become important to increase the efficiency and service life of powertrains present in electric vehicles, and also solve prevalent issues like cost effectiveness, low reliability, high volume and weight, and low performance.

In an electric powertrain, the most critical component is the AC/DC inverter which controls power conversion from the battery to the motor or from a regenerative braking system to the battery. The current state-of-the-art inverter uses Free Wheeling Dioide and Insulated Gate Bipolar Transistors made up of silicon which is housed by protection devices, isolation layer and multiple devices to increase reliability, service life and power. The IGBT module is the most susceptible component in the power system of an EV/HEV as it is prone to forced short circuits and frequent high-loading conditions with its unusual intrinsic properties and assembly technology.

Ⅰ. Investigating an Innovative Automotive Power Module with Increased Efficiency and taking a closer look at its Thermal Design.

In order to overcome the challenges faced by the IGBT module, Dual-Sided Cooled(DSC) and Direct Liquid Cooled(DLC) module integrations have been some of the most promising solutions as they possess superior thermal, electrical, reliability and mechanical properties. DLC module has the ability to reduce thermal resistance to the heat sink by 50 percent when compared to traditional indirect cooling, moreover, it also eliminates external heat sink and Thermal Interface Material as it integrates the pin fin heat sink. However, the main disadvantage of DLC is the difficulty in manufacturing and integrating pin fin arrangement with the cooler along with issues regarding high electrical parasitics. On the other hand, the DSC module has superior intrinsic qualities and has an upper hand in thermal, power and electrical performance along with low volume, weight and costs however, it is still at its nascent stage and requires study for proper manufacturing.

The design and schematic layout for the new DSC automotive power module is shown in Figure 1

Figure 1 Schematic Diagram for a DSC power module.

Conventionally two dies are interconnected using wires, however DSC bonds the power chip's two facets onto isolation-equipped planar components, which is further improved in terms of stability by strategically affixing metal spacers to bolster isolation and circuit topology. The resulting bonding interfaces, formed through soldering or silver sintering processes, serve as robust foundations. Standard substrates play an important role in achieving isolation on both die sides, facilitating module connection to external heat sinks for efficient cooling. This setup enables bidirectional heat dissipation, significantly reducing equivalent thermal resistance thereby increasing overall efficiency and power transmission with high service life. The selection and optimization of materials used in packaging are of utmost importance as they decide the reliability and performance of a power module Moreover, thermal mechanical stress is one of the prominent reasons for the failure and degradation of interconnected parts in a power module. Therefore DSC uses molybdenum and Active Metal Brazed AIN substrate spacer to achieve better thermal reliability and performance.

In a DCS module, the heat generated is dissipated through the bottom and top side substrates in order to have a  parallel arrangement so that the overall equivalent Rth is reduced drastically which is governed by equation 1.

Equation 1 Relationship between Equivalent junction thermal resistance to the case and thermal resistance to top and bottom.

Using this relationship, it is found that the DSC module has the lowest Rth when compared to DLC and other power modules as shown in Figure 2. Therefore, it is safe to say that the DSC module has the capability to produce maximum thermal reliability and power density and a junction temperature below the maximum junction temperature with maximum distribution.

Figure 2 A comparison between different power modules in terms of junction thermal resistance.

The prototype of a DSC module is built by removing the baseplate and bonding wires as shown in Figure 3, due to which the thermal reliability and power cycling are significantly increased, with a notable decrease in weight and and size. Moreover, the thermal impedance results show promising results when the module contacts the external liquid-cooled heat sinks.

Figure 3 Physical prototype of a DSC power module.

Ⅱ. Conclusion

In the realm of power electronics, the thermal design of a power module emerges as a paramount concern, exerting heavy influence on various important aspects covering overall performance, reliability, volume/weight considerations, and cost implications. On the other hand, the importance of the thermal design of automotive IGBT modules cannot be underestimated as these modules are crucial components within electric and hybrid vehicles, responsible for controlling power distribution and ensuring efficient operation. Effective thermal design is essential in order to prevent overheating, help in overall heat distribution and ensure the long-term reliability of the IGBT module.

To address these critical factors, the adoption of the DSC (Direct Substrate Cooling) module structure stands out as an exceptionally promising solution, offering optimization not only in terms of thermal performance but also in various other facets. Moreover, the junction thermal resistance measurement with the use of a mathematical model shows that the DSC power module has the least Rth when compared to the DLC(Direct Liquid-cooled) power module which makes DSC more efficient in thermal distribution. Moreover, DLC has various issues like high electrical parasitics and issues in manufacturing pin fins in the cooler which can be easily overcome by a DSC power module.