Relays for Wall Boxes Charging Electric Vehicles Gets Cooler

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Omron Electronics Components Europe BV

Wuxi InfiMotion Technology Co., Ltd.

Thermal management is one of the biggest challenges in the development of wall boxes: size, shape, efficiency, reliability, and safety are determined by it. Advances in relay technology contribute to making new components discreet and easy to install while simultaneously ensuring more efficient charging.

The ongoing electrification promises a cleaner and more energy-efficient life. Consumers are driving the demand for more compact devices that integrate seamlessly and stylishly into their lives and living spaces while offering higher performance. These demands for smaller, slimmer, and faster devices apply to everything from accessories and small appliances to power supplies and chargers, including chargers for electric vehicles (EVSE—electric vehicle service equipment).

High-performance wall boxes for charging electric cars are now increasingly found in households and businesses. These support Mode 3 charging, which mandates integrated control and protection functions, and charge an electric vehicle with up to 22 kW in three phases from the AC power grid.

Such wall boxes provide convenient access to safe charging, allowing the charging process to occur overnight or during the workday. They also help to dispel concerns about charging time and range, which are often cited as the main reasons for drivers' reluctance to use electric vehicles in everyday life.

Wall Boxes: Why Efficient Thermal Management is Necessary

On the other hand, size and aesthetics are becoming important distinguishing features among manufacturers. However, integrating power electronics into the smallest possible housing poses challenges for thermal management. Therefore, efficient thermal management is crucial to ensure the long-term reliability of the EVSE and safety during use.

Wall boxes of this type are either operated continuously or used to charge multiple vehicles in quick succession, especially at workplaces. This means the wall boxes barely cool down between charging sessions. Under direct sunlight, their internal temperature can easily reach 70 to 80˚C (158 to 176°F) and fluctuate by 50 to 60˚C (122 to 140°F) within a few hours.

If chargers throttle the charging current via temperature sensors during overheating, this means slower charging times and thus less convenience for the end user. On an especially hot day, or if the wall box overheats due to a defect, the charger might not function at all.

Self-Heating Effects in Wall Boxes

Within the wall box, self-heating occurs in resistors, power transistors, coils, transformers, cables, and connectors due to power loss, which increases with the square of the flowing current (I²R).

A surprisingly high proportion of the I²R-related heat generation in a wallbox is associated with the contact resistance of the main switching device, typically an electromechanical relay or contactor. Unlike power transistors, which can be connected in parallel to distribute the load current, the parallel connection of electromechanical switches is impractical.

Since the entire load current must flow through the relay, the contact heats up significantly upon closing. An increase in contact resistance by just one milliohm can cause a temperature rise at the load of up to 18˚C (approx. 65°F, reference conditions for the thermal simulation: 32-A current load on three load terminals. T_ambient = 85℃ (185°F). Evaluation board: 2 layers with 10 mm wide (approx. 0.4 inches) and 0.3 mm deep (approx. 0.01 inches) copper traces).

Excessive heat dissipation within the housing is undesirable and also hinders a slimmer and more compact design of the wallbox. A smaller housing has a reduced surface area for heat dissipation, leading to a higher temperature increase. Additionally, a denser arrangement of components in a confined space can restrict airflow and result in local hotspots.

In many cases, additional thermal management with heat sinks, enhanced ventilation, or active fan cooling is necessary. However, this increases the cost and complexity of the wall box. Additionally, a fan can be noisy and may affect overall reliability.

Wall Boxes: Relay Evolution in Terms of Heat Generation

Reducing the heat load generated by electrical components, including relay contacts, can break this deadlock and provide developers with greater flexibility in downsizing the wallbox. Advances in relay design have led to compact new PCB relays that combine high current capacity with low contact resistance, resulting in lower power loss and reduced temperature rise.

In the past, PCB relays were suitable for applications up to around 25 A. For higher currents, contactors with screw terminals mounted on external DIN rails were often used. Modern PCB relays come with innovative features that allow them to handle current levels well above 25 A. This enables them to replace conventional contactors, for example, in high-performance industrial systems and power supplies.

New High-Performance Relays for Wall Boxes

Among these new high-performance relays is the G9KC series from Omron, with a guaranteed initial contact resistance of less than 6 milliohms—about half the resistance of comparable solutions on the market. It is specifically optimized for use in AC wallboxes. The extremely low resistance under full load (32 A per phase) not only improves the efficiency and performance of the charging process but also reduces hotspots caused by the current flow, thereby lowering the likelihood of current throttling.

The reliability and durability of the relays themselves, as well as the surrounding components, are also improved. It is crucial that the relay maintains a low contact resistance throughout its entire lifespan. This is especially important as wall boxes are typically designed to remain in operation for several years or longer.

The core of the G9KC is a specially designed, mechanically linked double-break contact that, thanks to an improved contact design (a so-called cradle structure), offers exceptional durability. This structure increases energy efficiency while simultaneously reducing heat dissipation. As a result, operating temperatures in a typical 22 kW, 32 A wallbox can be reduced by up to 10˚C with a well-designed charger. This not only enables faster and more efficient charging but also opens up new opportunities for developers to create more compact and robust wallbox designs.

With a main contact gap of 4 mm and a short-circuit resistance of 10 kA, tested according to IEC62955 (TÜV-certified), the relay exceeds the requirements of the upcoming standard for conductive charging systems for electric vehicles, IEC 61851 ED4. Additionally, the relay, with its compact 4-pole structure, can replace larger multipole contactors or up to four individual single-pole relays.

Comfort and Safety During Charging

While Mode 3 chargers offer comfort and safety to address range anxiety in electric vehicles, the charging infrastructure must be further improved to meet the demands of the rapidly growing EV market. By using thermally more efficient components, additional features can be integrated, such as extra sensors to monitor charging speed. Other innovations could include wireless charging.

On the other hand, thermal management remains one of the biggest challenges in the development of wall boxes. Recent developments in relay technology contribute to making new components discreet and easy to install while ensuring more efficient charging. (kr)

*Steve Drumm is Strategic Marketing Manager – Solutions in Energy at OMRON Electronic Components Europe B.V. in Hoofddorp, Netherlands.

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