A wide variety of vehicle models is entering the EV market these days. The demand is for decreased charging times and increased range. This is heightening not only the challenges towards electrical system performance but also towards better circuit protection.
For instance, decreasing the charging times requires systems using higher voltages and higher currents. This has necessitated the shift from the 400 V system to the 800 V, bringing with it major challenges to the design of circuit protection, especially on the battery side. That is because manufacturers must now consider increased fault currents that the protection components must handle.
With motor currents and power ramping up, circuit protection and switching devices also face higher stresses. They now need to withstand not only the higher operating currents but also the higher cycling requirements. Increased range means higher fault currents.
Therefore, circuit protection requirements are moving in several directions simultaneously. SiC MOSFET switches, acting as solid-state resettable transistor switches, address the high-voltage, low-current subsystems.
The power distribution box in the vehicle is still using the conventional system architecture of a coordinated fuse and contactor. Coordination between the two is necessary to ensure they cover the full range of possible faults from a range of underlying causes including different states of charge of the battery.
Another circuit protection technique is the pyrotechnic approach. This comes into play in events of a catastrophic nature, such as in crashes, when it is necessary to physically cut the busbar. These systems are mostly triggered by circuits that deploy the airbag and work to quickly isolate the battery from the rest of the vehicle. This helps to protect the driver, the passengers, and the first responders from fire and explosion from short circuits through the body of the vehicle.
The above are leading to the development of newer types of protection, such as with breaktors, fully coordinating circuit protection, and switching. Its design allows the breaktor to trigger passively or it can actively interrupt in case of power loss, thereby improving the functional safety of critical protection systems. Moreover, it has the ability to reset itself.
Another is an automotive precision bidirectional eFuse, which is increasingly becoming a common device in a vehicle. Traditional automotive fuses can be low in accuracy and slow to react. This can be a safety issue, as the safety of the system is indirectly proportional to the response time of a fuse. An eFuse not only has high accuracy but also a low response time, which increases the safety of the system.
However, there is a durability issue related to fuses and contactors that vehicle manufacturers use. The solution for this is the pyrotechnical switch. This is a protection device based on a trigger-able circuit similar to the functioning of an airbag. It produces a controlled explosion to sever a conducting busbar. Pyrotechnical switches, while solving the challenge of coordination, must rely on accurate triggering rather than on the passive reaction of fuses. Additional components are necessary to ensure a reliable triggering.
All the above protection systems require a trade-off between speed and durability. While a big fuse can be slow to operate, a smaller one may be faster but may suffer from a fatigue risk.
