The Importance of Thermal Management in EV Charging Applications

Pink background, illustration of car being charged

Electric vehicles (EVs) have been around as long as gas vehicles, but it's only in recent years that they have made significant inroads in replacing traditional vehicles. There is an effort to make EVs the predominant vehicle on the roads as soon as reasonably possible. The EU specifically has mandated a ban on internal combustion vehicles by 2035, showing the earnest nature of their intent. With substantial advancements in EV technology and significant government support, the expectation is that demand for EVs will soar. As EVs become the predominant mode of transportation, enhancing battery range, and charging speed will be pivotal for the global economy. The evolution of EV charging systems will also necessitate advancements in various areas, including thermal management.

AC versus DC EV Chargers

As the need for faster chargers grows, there have been some notable shifts in approach. One significant change is the move towards DC chargers. While all battery systems operate with DC electricity, the key distinction lies in where the power rectification from AC to DC occurs. Traditional AC chargers, commonly found in residential or commercial settings, act as connectors that regulate and track the flow of AC power to the vehicle but then allow the rectification to take place aboard the EV itself. DC chargers, on the other hand, perform the rectification externally in a stationary charging unit, delivering high-voltage DC power directly to the vehicle. This shift eliminates many weight and size constraints by relocating power conditioning hardware outside the EV, as the charging circuitry will not be mobile nor an integral part of the vehicle.

Diagram for car charging points Home, Work & Charging Station

Figure 1: DC chargers are significantly faster but are more complex and generate more heat (Image source: CUI Devices)

With these constraints removed, DC chargers can be more complex, utilizing more and larger components, enhancing their current throughput and operating voltage. However, these chargers, dealing with larger amounts of power, generate substantial heat. While filters and resistors contribute to heat generation, the primary heat dissipator in EV charging systems is the Insulated-Gate Bipolar Transistor (IGBT), a semiconductor device increasingly used in recent decades. Effective cooling of IGBTs poses a significant challenge in EV charging systems.

Semiconductor Heat Generators and Thermal Management Solutions

The IGBT, a hybrid of a Field Effect Transistor (FET) and a Bipolar Junction Transistor (BJT), is highly favored in high-power applications like EV chargers. The construction of an IGBT makes it able to withstand high voltages while still exhibiting low on-resistance, fast switching speeds, and remarkable thermal robustness. However, while they can survive high temperatures of 170 Celsius, compared to only 105 Celsius for most modern computer central processing units (CPUs), they can still easily exceed 170C without the proper cooling. Their usage in most charging circuitry involves frequent switching of the IGBTs, which generates considerable heat. Indeed, the current thermal challenge stems from the exponential increase in heat dissipation of IGBTs, from 1.2 kW three decades ago to 12.5 kW today, with further increases anticipated. As you can see in this graphic, the power per square centimeter of surface increase has been dramatic and it may be best compared to the comparatively low power of modern CPUs at .18 kW, or only 7 kW per square centimeter.

Diagram of power per square centimeter of surface increase over years - 1990, 2012, 2020

Figure 2: IGBTs have increased in their power density by leaps and bounds (Image source: CUI Devices)

Effective cooling of IGBTs is aided by two factors: their larger surface area and their ability to operate at higher temperatures. This larger surface area makes them more suited to one of the most reliable thermal management solutions available - a combination of heat sinks and forced air. Heat sinks significantly reduce the thermal resistance between the IGBT and ambient air while forced air provided by a fan, versus natural convection, further reduces the thermal resistance. In some cases, liquid cooling is considered and, while liquid cooling options can achieve even lower thermal resistances, their higher cost and complexity make direct cooling with heat sinks and fans a more desirable approach. Most active research done by thermal engineers is focused on improving air cooling technologies for IGBTs. In fact, at CUI Devices, we have developed heat sinks measuring up to 950x350x75 mm designed specifically to address the thermal challenges of EV charging applications.

Product illustration

Figure 3: Heat sinks and fans are an effective heat management solution for IGBTs (Image source: CUI Devices)

Component and Thermal Monitoring Placement

Efficient cooling systems depend on strategically placing components to optimize airflow and maximize heat distribution. Inadequate spacing between components restricts airflow and limits the size of heat sinks that can be used. Therefore, critical heat-generating components should be strategically positioned within the system to facilitate efficient overall cooling. While it is important to focus on the main heat-generating components, the entire system should be included in analysis, particularly as many supporting semiconductor devices cannot survive extreme high temperatures as well as IGBTs.

Similarly, the placement of thermal sensors is crucial for effective thermal management. In large systems like DC EV chargers, real-time temperature monitoring is highly encouraged as it enables active thermal management. Automatic adjustments in cooling mechanisms such as fan-speed or even limiting the output of the charger are all based on temperature readings that optimize performance and prevent overheating. However, the accuracy of these adjustments depends on the quality of input data from properly placed temperature sensors.

External Factors and Environmental Considerations

On-the-go EV charging stations are often installed outdoors, thus, they must be designed to handle various environmental conditions. Weatherproof enclosures with adequate ventilation for optimized cooling while still maintaining protection against elements like rain and extreme temperatures are essential for protecting the charging stations from sub-optimal thermal conditions. Another factor to consider is the solar heating from sunlight beating down on charger enclosures. This poses a significant concern as it significantly increases internal ambient temperatures. Fortunately, effective shading with adequate airflow between the shading and the charging unit can substantially lower the ambient temperature of the charger and is a relatively simple way to address the problem.

Illustration of protecting chargers from direct sun

Figure 4: Protecting chargers from direct sun is an inexpensive and effective way to control temperatures (Image source: CUI Devices)

What the Future Holds

In recent years, the adoption of electric vehicles has surged globally, with demand continuing to rise alongside advancements in technology. As EVs become more commonly found on roadways, an accompanying surge of charging infrastructure is inevitable. As the past has shown, proper upfront investments in infrastructure are important as it can be difficult to make updates to that infrastructure in the future. The expectation is not only for the growth in the number of EVs and chargers but also for continuous improvements in the technologies underlying them. Considering potential increases in charging power and capacity, evolving software, and hardware standards, and anticipating unforeseen changes, thermal management systems must adapt to evolving demands over time. By properly designing high-quality, effective, easily upgradable, and energy efficient chargers, the solution to the growing EV charging solution will not just be solved today but for years to come.

Electric vehicle chargers share fundamental thermal management concerns with other dense, high-power electronic devices. The high-power density of IGBTs and the increasing demands placed on them present unique challenges. As charging speeds and battery capacities continue to rise, the need for effective and safe charger designs grows in turn, demanding innovation from both electrical engineers and thermal management engineers. CUI Devices’ range of thermal management components as well as industry-leading thermal design services are here to address these challenges head on!

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