Tag Archives: Circuit Protection

New Circuit Protection Technologies

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.

Where Would You Apply Crowbar Protection?

Crowbar is an appliance typically used by construction workers. It is a heavy steel rod with one of its ends pointed and the other shaped like a spatula – both very useful for digging or breaking up construction rubble. Normally, one would not associate such a crude instrument for use by engineers dealing in electronics, were it not for one unusual property of the crowbar. Throw it across a power line, whether accidentally or with a purpose, and the power line trips – a fail-safe arrangement to protect the load in case of an emergency.

In electronics, a crowbar protection is generally an electronic circuitry placed across the outputs of a power supply. It activates to protect the load against overvoltage. When it activates, it shorts the output terminals – the crowbar action. This serves to blow the fuse, trip the circuit breaker or to shut down some part of the circuit so that power to the load is cut off. Most power supplies, whether low- or high-voltage, employ this kind of protection.

The crowbar protection circuit has a sensor that monitors the output voltage of the supply, comparing it against a preset value. When an overvoltage occurs, it triggers the crowbar circuit, which in turn short circuits the output terminals, thereby cutting off power to the load.

Crowbar devices typically use one of two types of components as their main protection. These are the Silicon Controlled Rectifier or SCR, and the Metal Oxide Semiconductor Field Effect Transistor or MOSFET. The design of the monitoring circuit of the crowbar depends on the sensitivity of the load circuit to be protected. For instance, the reaction time of the monitoring circuit depends on how long the protected circuit can survive the excess voltage without damage, and the response time of the main protection device.

Several fault-conditions may lead to possible over voltages. These include a fault in either the power supply or the load, and operator error. Present day electronics are sensitive and often operate at very low voltages with small margin. That makes it imperative to ensure that the safe voltages are not exceeded, and sensitive and expensive equipment remain undamaged.

Although blowing the fuse is a popular method of protecting a circuit, it has its disadvantages. Recovery is only possible by manually replacing the fuse, once the fault condition is repaired. This is a time consuming affair, and not helpful for low downtime appliances. Therefore, most engineers prefer a fold-back type of crowbar protection.

In a typical crowbar protection, the entire load current is diverted from the load and directed to the short circuit across the output terminals. This is constant current limiting and puts the fuse under tremendous stress, causing it to blow, thereby protecting the power supply and its load. In contrast, with the fold-back crowbar protection, the load current through the short circuit reduces once the crowbar has activated and shorted the outputs.

The short circuit current reduces to the extent that the power dissipated by the supply can remain within its safe operating area. This prevents the fuse from blowing, and at the same time, the power supply keeps the load circuit safe because of the crowbar action. As soon as the cause of the overvoltage is repaired, the power supply resumes automatically.