Wave Hello To Improved Performance

The International Energy Agency (IEA) estimated that in 2019, approximately 33 percent of all transportation-related CO₂ emissions were generated by buses and light to heavy commercial vehicles. Transitioning to electric drives in this sector could clearly have a significant impact on reducing our emissions, but electrifying such demanding vehicles is not an easy task. Many of the concepts used in electrifying passenger cars can be applied to larger vehicles, but some commercial applications come with their own unique challenges.

On average during 2020, UPS was delivering about 24.7 million packages per day globally – and that is just one of the many major delivery companies! Imagine how many times a delivery truck has to stop and go throughout the day. Buses follow a similar schedule of frequent start-stop operations for extended periods of time, which places high demands on their power modules. The technical solution for such applications must be robust enough to manage these demands.

Heat sink systems for different electric vehicle applications

Today’s electric vehicles (EVs) typically use either closed or open liquid-cooled heat sink solutions. In closed systems, the power module is mounted directly on top of the heat sink with a layer of thermal grease between the two. While this clear separation allows each part to be adjusted and optimized individually, the overall performance can suffer. In the more efficient open system, the power module is in direct contact with the cooling liquid, for example, via a flat baseplate, a Pin Fin structure, or a wave structure.

Fig. 1: Top: standard closed heat sink with thermal grease between the power module and heat sink; Bottom: open heat sink with direct liquid cooling.

A structured baseplate can create turbulences and increase the surface area in touch with the cooling liquid, which is why it is preferred over a flat baseplate. So why not add a wave structure to the flat baseplate? Such a 3-dimensional structure is attached to the backside of the power module at the end of the manufacturing process. The wave structure significantly increases the total surface area exposed to the cooling liquid and likewise accommodates higher turbulence in the liquid. Because of this, power modules with this design result in better thermal transfer and reduced thermal resistance junction to fluid (R_{th_jf}).

Fig. 2: Typical appearance of the EconoDUAL 3 Wave for liquid-cooled applications, featuring a wave structure.

A powerful wave upgrade to a popular design

Our new EconoDUAL 3 Wave was designed with this concept in mind. The EconoDUAL 3 Wave upgrades Infineon’s EconoDUAL 3  package with a cooling structure on the backside to improve the thermal resistance without altering the other module features and characteristics.

But how much of a difference does this wave structure make? To find out, tests with the EconoDUAL 3 module with a half-bridge configuration and a nominal current of 900 Amp were performed. For a thorough comparison, the following three configurations were evaluated:

• a closed heat sink with thermal grease
• the EconoDUAL 3 module with a flat base plate (open heat sink)
• the EconoDUAL 3 Wave module with a wave structure (open heat sink)

By comparing the three side-by-side, we determined how their design variations impacted performance and lifetime, considering the different parameters needed to evaluate a closed heat sink (volume flow and resulting pressure drop Δ_p) compared to an open heat sink.

Fig. 3: Temperature measurement for the closed (left) and open heat sink with EconoDUAL 3 Wave (right) solution, both operating at 500 A I_rms and 15 l/min volume flow, using the same temperature scale.

In the application tests, the EconoDUAL 3 Wave operated at significantly lower temperatures (figure 3). At 500 A with volume flow of 15 l/min, the upgraded structure allowed for a temperature decrease of 25 K with much lower temperature ripples (figure 4). Lower temperature ripples are very attractive for heavy-duty vehicle applications where maximum temperature is often limited to avoid large swings. With lower temperature ripples, the modules can have more cycles and thus an extended lifetime. In addition to improved thermal performance, the wave structure makes it possible to increase output current by up to 30 percent, which offers an increase in power density as well.

Fig. 4: Temperature as measured with the thermal camera, for closed heat sink with 15 l/min and for open heat sink with ribbon-bonds at different flow rates.

Based on initial tests, the EconoDUAL 3 Wave’s upgrade can increase a package’s lifetime up to a factor of five at 500 A (figure 5). Alternatively, the output current can be increased by 20-30 percent depending on volume flow, without sacrificing lifetime.

Fig. 5: Relative lifetime, assuming only a single temperature ripple at the given I_rms. The inset shows the temperature ripple for the respective solution.

Considering the power module itself, along with its electrical configuration and gate driver board was not altered at all, this wave structure upgrade should be considered a very promising option – and a great way to wave hello to more power and longer lifetimes for your heavy-duty power modules.