Electric vehicles (EVs) need faster DC charging times to compete with traditional internal combustion engines, so EV charging systems are rapidly moving toward up-to-350-kW output power solutions to reduce charging times to less than 20 minutes. With such powers, voltage and current levels are leading designers into new engineering challenges.
In an interview with Power Electronics News, Jinsong Dai, senior global marketing manager focusing on electrification at Sensata Technologies, pointed out that the main challenges for DC fast charging are related to higher voltages and power levels. A migration to higher voltages and higher power levels causes some electrical protection issues.
“Many challenges for DC fast-charging applications ranging from fast-charging standards to charging infrastructure issues to thermal-efficiency management to protecting electrical systems from faults,” said Dai. “At Sensata, we focus on electrical protection for the DC fast-charging system. To shorten the charging time, many high-power chargers are migrating from 400 V to 1,000 V and from 50 kW to 350 kW or even higher. This brings a lot of challenges to the electrical protection components in the charging system — increases in voltage levels and charging power drive increased capability in high-voltage components such as contactors and fuses.”
Dai pointed out that the most obvious concern here is that higher voltages are more likely to generate an arc, which requires a greater separation distance before it can be extinguished. “This requires contactors and fuses to employ techniques to increase the separation distance to quickly quench any arcs,” he said. “Otherwise, the uninterrupted arc can cause a contactor or a fuse to explode violently. Another issue is the contactor and fuse pairing during the overcurrent condition. As the contactor begins to break a circuit, it can begin to dissipate some of the overcurrent within itself, preventing that current from being available to properly disrupt the fuse. The potential impact could be a catastrophic failure of the entire system. Sensata’s disconnect solution, where both the contactor and fuse are paired to work together, helps prevent that risk.”
Migrating to higher voltage
While higher voltage and current levels reduce charging times, they increase safety risks and system design challenges. High-voltage contactors provide safe circuit continuity, while fuses are required in tandem to protect the circuit in the event of a dangerous short-circuit event.
Sensata’s circuit protection solutions (Source: Sensata Technologies)
Traditional DC thermal fuse technology is designed for short-circuit situations, which means that in the event of a short, high current, a fuse uses the melting of the connection to break the circuit. Dai pointed out that the challenge for traditional DC thermal fuses is in overcurrent situations when the current is not high enough and it takes longer for the thermal fuse to melt.
“This creates a gray zone where current levels may overwhelm the contactor’s ability to interrupt the load without reaching the thermal point for a fuse to trigger,” he said. “This stretch of time before the thermal fuse can be activated while exceeding the breaking capability of the contactor is eliminated with the GigaFuse. The GigaFuse helps bridge the gap between what the contactors can do [normal operation] and when the fuse trips, helping provide both overcurrent and short-circuit protection.
“Contactors and fuses are mission-critical components within DC fast chargers,” Dai added. “Contactors provide safe circuit continuity during normal charging or separate the overcurrent, while fuses protect the charging system during hazardous short-circuit and overcurrent conditions. The key is to have contactors and fuses that can work in tandem to ensure seamless protection for the chargers during normal operating and overcurrent conditions.”
Contactors and fuses
Higher voltage levels and higher power charging for contactors means the products need to have increased breaking capability, and customers are asking for contactors rated to 1,000 V and 500 A, said Dai. In addition, another required functionality is the bidirectionality of the contactor that allows both the charging of EV batteries from the grid and vehicle-to-grid (V2G) system that would allow the exploitation of grid intelligence in an energy exchange market.
“In addition to V2G, dynamic power allocation is a technology trend that tailors charging to actual demand by combining or sharing power from multiple charging ports,” said Dai. “The bidirectional functionality of contactors enables chargers to allocate the power dynamically by allowing the current flow either forward or backward.”
The contactor technology used by Sensata Technologies is hermetically sealed and filled with gas, thus providing the necessary power to switch with the required robustness in a relatively small size compared with open-air contactors. Sensata continues its strategy in sealed switching technologies with the introduction of high-voltage fuse products. GigaFuse is a hermetically sealed electromechanical fuse designed for high-voltage and high-power fuse application requirements. “The GiagFuse series includes fuse products with both passive and passive/active combinations; it significantly increases system efficiency, eliminates thermal aging, and provides design flexibility for electrical protection,” said Dai.
The electromechanical triggering mechanism of Sensata’s GigaFuse increases system efficiency, eliminates thermal aging, and provides design flexibility for electrical protection. (Source: Sensata Technologies)
To address emerging technical challenges of high-power applications, Sensata provides new integrated technology for electrical protection. “A new solution that we will be sampling for customers ahead of full launch is the PyroTactor, the world’s first contactor with integrated pyro fuse,” said Dai. “The GFC PyroTactor combines the function of HV fuses and contactors in a single device, thereby eliminating the need to size each component separately. The GFC Series is bidirectional and capable of handling system voltages up to 1,500 V, and features include passive and active triggering. This new design significantly reduces the design complexity of contactor and fuse pairing, eliminates thermal aging, and significantly reduces install time, complexity, and cost.”
The article originally published at sister publication Power Electronics News.
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