INDUSTRY FOCUS - AUTOMOTIVE ELECTRONICS
connectors exhibit self-heating due to power dissipation that increases with the square of the current flowing (I2R). A surprising amount of I2R-related heat in the wallbox is associated with the contact resistance of the main switching device, which is usually an electromagnetic relay or contactor. Unlike power transistors, which can be connected in parallel to distribute load current, paralleling electromagnetic switches is impractical. As the full load current must pass through the relay, the contact resistance when closed has a considerable heating effect. Even a one milliohm increase in the relay contact resistance can equate to as much as *18˚C in increased load terminal temperature rise. Excessive heat dissipation within the enclosure is undesirable, of course, and also mitigates against achieving slimmer and more compact wallbox designs. For a given heat load, a small-sized enclosure has less surface area from which to dissipate heat and so will experience a higher temperature rise. In addition, packing components more tightly in the compact space can restrict airflow and lead to localised hotspots. Extra thermal management, possibly including heatsinks, increased ventilation, or active cooling such as a fan, may be required. However, this will increase the wallbox cost and complexity. Moreover, a fan can be noisy and compromises the overall reliability.
Relay Evolution
Lowering the heat load resulting from the electrical components, including the relay contacts, can break this impasse and allow designers more freedom to reduce the wallbox dimensions. Advancements from the cutting edge of relay design now enable compact new relays in PCB-mount form factors that combine high current capability with low contact resistance, resulting in lower dissipation and reduced temperature rise.
Historically, PCB relays have been best suited to applications up to about 25 A. For higher currents, designers have tended to specify contactors, which are typically off- board, DIN rail-mount components fitted with screw terminals. The latest PCB relays introduce new features to handle current significantly above 25 A and can replace traditional contactors in equipment such as high-power industrial systems and utility- grade power conditioners.
Among new high-power relays emerging as a result of this trend, the Omron G9KC series has a guaranteed initial contact resistance of less than 6 milliohms - roughly half that of any equivalent solution
High-power EVSE wallboxes are an increasingly common sight in today’s homes and businesses. These support Mode 3 charging, which stipulates built-in control and protection functions for safety, and can charge an EV at up to 22kW from a three- phase AC supply.
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on the market - and contains features optimised for AC wallbox applications. As well as improving charging efficiency and performance, the ultra-low resistance at full load (32A per live phase) significantly reduces hotspots caused by flowing current and reduces the likelihood of current throttling. The reliability and longevity of the relays themselves, as well as surrounding components are also improved. Crucially, the relay retains a low contact resistance throughout its lifetime. This is particularly important given that wallboxes are typically expected to remain in service for several years or more.
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At the heart of the G9KC is a purpose developed mechanically coupled double- break contact design providing class leading endurance due to its enhanced contact card (cradle) structure that significantly improves energy efficiency while reducing heat dissipation. As a result, with good overall charger design, operating temperatures in a typical 22 kW 32 A wallbox
can be reduced by as much as 10˚C. This not only facilitates faster, more efficient charging, but also unlocks new possibilities for wallbox designers to develop more compact and robust designs.
With a 4mm main contact gap and 10 kA short circuit capability, tested according to IEC62955 (TUV approved), the G9KC exceeds the requirements of the upcoming Electric vehicle conductive charging system Standard IEC 61851 ED4. Moreover, with its compact 4-pole structure, the G9KC can replace larger multi-pole contactors or up to four individual single-pole relays. This useful combination widens the scope of applications to include Behind the Meter (BTM), inside EV Charger and In Front of Meter (FTM).
Conclusion
While Mode-3 chargers offer convenience and safety to address EV range anxiety, charging infrastructure must continue improving to accommodate the rapidly growing EV market. Using more thermally efficient components could let designers add more functionality, such as extra sensors to monitor charging speeds, while further new developments could include wireless charging.
On the other hand, thermal management will continue to be one of the most critical design challenges governing wallbox size, shape, efficiency, reliability and safety. The latest developments in relay technology contribute towards enabling new designs to be unobtrusive and easily installed, as well as ensuring more efficient charging.
*Reference Thermal Simulation conditions: 32A carry current on three load terminals.
T.Amb.85°C. PCB evaluation board (2 layers of 10 mm wide, 0.3 mm depth copper foils)
OCTOBER 2025 | ELECTRONICS FOR ENGINEERS 33
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