Charging technologies Thermal challenges in EV charging applications

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Improving the charging infrastructure for electric vehicles will require advancements in various areas, with thermal management being a key field that demands technological development.

Although the concept of electric vehicles (EVs) has existed as long as that of gasoline vehicles, they have only become widely adopted in recent years. This surge in popularity is due to significant advances in EV technology, coupled with substantial government support. The decision of the European Union to ban combustion engine vehicles by 2035 and to require fast charging stations for electric vehicles every 60 km by 2025 is a clear indication of the anticipated surge in demand.

As electric vehicles establish themselves as the dominant mode of transport, factors such as battery range and even faster charging times will play a crucial role in the survival of the global economy. Improving the charging infrastructure for electric vehicles will require advancements in various areas, with thermal management being a key area that necessitates technological developments.

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AC and DC EV chargers—what is the difference?

As the demand for faster charging solutions increases, there are both incremental and transformative changes in approach. A notable change is the growing use of direct current chargers - a term that may initially seem ambiguous, as all battery systems inherently operate on direct current. The crucial difference, however, lies in where the conversion from alternating current to direct current occurs in these systems.

The conventional alternating current charger, typically found in households, primarily serves as a sophisticated interface responsible for communication, filtering, and regulating the flow of alternating current to the vehicle. Subsequently, a direct current charger built into the vehicle rectifies this current and charges the batteries. In contrast, with a direct current charger, the current is rectified before delivery to the vehicle and passed on as high-voltage direct current.

The main advantage of direct current chargers is that they eliminate many constraints related to weight and size by moving the power conditioning components from the vehicle to an external structure.

By eliminating weight and size constraints, DC chargers can seamlessly integrate additional components to increase both the current throughput and the operating voltage. These chargers use advanced semiconductor components for rectifying the current as well as filters and power resistors that generate significant heat during operation. While the contributions of filters and resistors to heat dissipation are noteworthy, the predominant heat emitter in an EV charging system is the IGBT, a semiconductor component that has become increasingly common in recent decades. This robust component has opened numerous opportunities in the field of charging, yet ensuring proper cooling remains a major concern.

Addressing the thermal challenges

An IGBT is essentially a mix of a field-effect transistor (FET) and a bipolar junction transistor (BJT). IGBTs are known for handling high voltages, having minimal on-state resistance, switching quickly, and exhibiting remarkable thermal resilience. Therefore, they are ideally suited for high-power scenarios such as EV chargers.

In EV charging circuits where IGBTs serve as rectifiers or inverters, their frequent switching operations lead to significant heat generation. The greatest thermal challenge currently lies in the substantial increase in heat dissipation by IGBTs. Over the past three decades, heat dissipation has more than increased tenfold, from 1.2 to 12.5 kW, and forecasts indicate a further increase. Figure 2 illustrates this trend in terms of power per unit area.

In comparison, modern CPUs achieve a power output of about 0.18 kW, which corresponds to a modest 7 kW/cm². This huge discrepancy highlights the immense challenges for the thermal management of IGBTs in high-power applications.

Two factors play an important role in improving the cooling of IGBTs. First, the surface area of IGBTs is about twice as large as that of CPUs. Second, IGBTs can withstand higher operating temperatures of up to 170 °C, while modern CPUs typically operate at only 105 °C.

The most effective method for managing thermal conditions is a combination of heat sinks and forced air circulation. Semiconductor components, such as IGBTs, generally have an extremely low internal thermal resistance, while the thermal resistance between the component and the ambient air is comparatively high. By incorporating a heat sink, the available surface area for heat dissipation to the ambient air is significantly increased, thereby reducing the thermal resistance. Furthermore, directing the airflow over the heat sink further increases its efficiency. Since the interface between the device and the air represents the greatest thermal resistance in the system, minimizing it is crucial. The advantage of this straightforward approach lies in the reliability of passive heat sinks and the proven technology of fans.

CUI Devices offers custom heatsinks specifically for EV charging applications with dimensions of up to 950 mm x 350 mm x 75 mm. These heatsinks are capable of handling less demanding requirements passively or more challenging scenarios actively with forced air.

In addition to air cooling, liquid cooling offers an alternative for dissipating heat from high-power components like IGBTs. Water cooling systems are attractive because they achieve the lowest thermal resistances. However, they come with higher costs and greater complexity compared to air cooling solutions. Even with water cooling, heat sinks and fans remain indispensable components for effectively removing heat from the system.

Given the associated costs and complexity, direct cooling of IGBTs with heat sinks and fans remains the preferred approach. Ongoing research focuses on improving air cooling technologies specifically tailored for IGBT applications. This active research aims to optimize heat dissipation while minimizing the costs and system complexity associated with liquid cooling methods.

Considerations for thermal system design

The effectiveness of any cooling system heavily depends on the strategic placement of components to optimize airflow and improve heat distribution. Insufficient spacing between components can impede airflow and limit the size of usable heat sinks. Therefore, it is crucial to strategically position the critical heat-generating components throughout the system to enable efficient cooling.

In addition to the placement of components, the positioning of temperature sensors is also critical. In large systems like DC EV chargers, real-time temperature monitoring using control systems plays a crucial role in active thermal management. Automatic adjustments of cooling mechanisms based on temperature readings can optimize system performance and prevent overheating by regulating power output or adjusting fan speed. However, the accuracy of these automatic adjustments depends on the quality and precision of the temperature sensors. Poor sensor placement can lead to inaccurate temperature readings and consequently ineffective system responses. Therefore, the placement of temperature sensors must be carefully considered to ensure the accuracy and reliability of temperature monitoring and control.

Ambient conditions

Electric vehicle charging stations are often installed outdoors and exposed to various weather conditions. Therefore, the design of weatherproof enclosures with adequate ventilation and protection against environmental factors such as rain and extreme temperatures is essential to ensure optimal thermal performance. It is crucial that the airflow paths and ventilation systems are designed to prevent water ingress while ensuring unobstructed airflow.

In addition to external factors, heat from direct sunlight poses a major challenge as it leads to a significant increase in the internal ambient temperature of the charger. While this is a legitimate concern, the most efficient solution is relatively simple. Well-designed shading structures with adequate air circulation between the shading and the charger reduce heating from solar exposure, thus ensuring lower ambient temperatures within the charger.

What's next?

In recent years, the global proliferation of electric vehicles has gained remarkable momentum, with demand steadily and significantly increasing across various technological fronts. As the number of electric vehicles on the roads continues to rise, the rollout of charging infrastructure is expected to grow in tandem. The effective operation and efficiency of charging stations are of utmost importance for the development of this emerging charging infrastructure. Cost efficiency is also a crucial factor, as the speed with which individuals and businesses integrate these chargers into their homes and facilities depends on affordability.

In anticipation of the continued growth of electric vehicles and chargers, the development of the underlying technologies must be considered. This means that potential advancements in charging power and capacity must be taken into account, software and hardware standards need to be further developed, and room for unforeseen innovations should be left. This proactive approach ensures that thermal management systems can adapt to changing demands over time.

Fundamentally, electric vehicle chargers face similar thermal management issues as other compact, high-power electronic devices. However, the power density of IGBTs used in EV chargers, combined with the increasing demands placed on them, presents a unique challenge. As charging speeds and battery capacities continue to rise, the need to design chargers effectively and safely becomes more stringent, requiring more from thermal management development teams than ever before.

CUI Devices offers a comprehensive range of thermal management components along with industry-leading thermal design services to support the evolving needs of the electric vehicle charging ecosystem. (tk)

*Rolf Horn is an application engineer at DigiKey.

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