Wire Harness Protection Optimizes Zonal Architecture

­Modern vehicles are equipped with numerous systems that rely on proper wiring. During power distribution, power losses occur because of the current flowing through the wire, and the electrical resistance of the wire leads to heating. If the temperature becomes too high, insulation can be damaged, leading to a massive short-circuit risk. Protecting the wiring harness from overheating is crucial to prevent serious injury to the passengers and severe damage to the vehicle. For this purpose, fuses were usually used in the past and present as a proven and cost-effective solution to protect the cables in the vehicle power distribution from overheating.

As vehicles today become more feature-rich, and more driving assistance systems are introduced, reliable wire harness protection requirements are also evolving. Although fuses have been able to meet the requirements of hardware-based protection for various cables, configurability via software cannot be enabled with fuses and certain ASIL requirements require high implementation effort.

In addition, OEMs are optimizing wire harnesses for complexity and weight by developing zonal vehicle architectures. Although this provides benefits, there is still room for improvementin reducing the weight of the wire harness further. At the same time, meeting functional safety requirements for ADAS/AD applications and diagnostic data for software-defined vehicles potentially require additional safety signal paths, limiting the wire harness optimization for zone architectures.

No More Fuses

A problem with fuses is that their protection curve, which is based on the temperature coefficient of the material used, does not correlate with that of the wires in the wiring harness. In addition, their inaccuracy and aging, especially if the fuse is exposed to higher temperatures over a longer period of time, can result in an unintended opening of the fuse. This reduced reliability can lead to dangerous situations. Even without aged material, issues can occur: A certain amount of time is needed to heat the material inside the fuse, resulting in a delayed response to overload events. This causes enormous inaccuracies, making it necessary to take safety margins into account when dimensioning the wires (Fig. 1). As a result, the weight of the wire increases. Moreover, blown fuses must be replaced. For this reason, they must be easily accessible in the vehicle architecture, limiting further optimization of the zone architecture.

Protection Solutions Based on semiconductors

A major disadvantage of conventional fuses is the fact that they are disposable products and must be replaced after triggering. Wire protection solutions, which are based on semiconductors, do not. Once triggered these solutions can switch on again automatically, cyclically with a predetermined number of reset attempts. Others will stay off until the microcontroller reactivates the device. Either way, components no longer need replaced. In addition, semiconductor-based solutions react 100 times faster than their mechanical counterparts.

Infineon makes use of these properties in intelligent PROFET Guard high-side switches, which are based on semiconductors to enable extended self-protection and system protection.

PROFET Load Guard

PROFET Load Guard (Fig. 2) provides adjustable overcurrent limitation in combination with a capacitive load switching mode. This enables overcurrent protection for sensitive loads. Fault isolation to the power supply is also ensured. As a result, the power supply for safety-critical loads can be stabilized. In addition, the adjustable overcurrent limitation can be used for simple, hardware-based wire protection for simple load profiles like resistive loads with capacitive inrush. To achieve this, the overcurrent limitation is set to a value that is harmless for the wire connected.

As soon as the current reaches the threshold value, the device will limit the current until the internal temperature becomes too high. Once its overtemperature threshold is reached, the device will switch off. In this way, the wire is protected, as there is never more current flowing than the wire can withstand. Given that this is a linear function, this feature is limited to time-independent load profiles without inrush currents of up to around 8 A. However, the capacitive load switching mode (CLS mode) allows large capacitors to be charged within the safe operating area while the wire protection function remains active. The wire protection functionality is hardware-based and can be achieved via the adjustable overcurrent limitation, adjusted by an external resistor connected to the OCT pin of the device. This may also reduce the wiring harness, as the protection mechanism of PROFET Load Guard is far more predictable than that of melting fuses. Therefore, the safety margin could be reduced.

Since the wire protection of the PROFET Load Guard is hardware-based, the microcontroller is relieved of wire protection tasks. Therefore, the computing power can be used for other tasks instead. The product family also supports fast fault isolation using the adjustable overcurrent limitation to further enhance the system protection. Although being hardware-based, the protection functions can also be adjusted via software, using shift registers to adapt the resistors to changing requirements.

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Figure 2: Wire Protection PROFET Load Guard

Replacing Fuses

In contrast, PROFET Wire Guard (Fig. 3) enables the replacement of fuses in automotive power distribution for any kind of load profile. The devices were specifically designed to take over the role of the fuse in previous power distribution architectures and even the role of the microcontroller when it comes to wire protection with semiconductor devices. This enables complete optimization of the wire harness in terms of cost and weight, as big safety margins become obsolete.

With the integrated I²t wire protection, the device precisely calculates the load on the wire, taking the measured current into account. By feeding back the wire protection status to the microcontroller, the system behavior can be monitored precisely. Due to an integrated thermal model based on real I²t curves of widely used vehicle wires, the PROFET Wire Guard is very accurate and responsive. Such high responsiveness cannot be achieved with a microcontroller that samples the current and compares it with a pre-programmed thermal model in the microcontroller software, as the current sampling times are lower here.

With six selectable I²t protection curves per device, which scale with the corresponding ON resistance (R_DS(ON)), the wire protection can be optimized to the load profile. In addition, the product has a thermal model for common automotive wires, allowing precise comparison of the current present in the application with the maximum load on the wire. As soon as the maximum wire protection threshold is reached, the device switches off to protect the wire from overload. This shortens the time to market thanks to the specified and tested functionality at device level. In addition, the high precision of the solution means that the wire diameter can be optimized (Fig. 3), which in total can reduce the weight of the vehicle by several kilograms. In contrast to conventional fuses, the wire protection solution offers high accuracy and fast response times, which correlate directly with the wire and reset capability. A total of six I²t curves are implemented for adaptation to different wire curves and can be selected via an external resistor. Also, for PROFET Wire Guard, the protection functions can additionally be adjusted via software, using shift registers to adapt the resistors to changing requirements.

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Figure 3: Wire Protection with PROFET Wire Guard

The I²t wire protection feature is hardware-based and is independent of a microcontroller. In combination with an automatic idle mode, the PROFET Wire Guard offers wire protection functionality with a consumption of less than 60 µA – even in park mode and completely without microcontroller support. This in turn relieves the microcontroller and reduces the software effort on the implementation side. In addition, fast fault isolation is enabled by the adjustable overcurrent detection threshold. This supports the functional safety requirements for fault isolation. The functionality is similar to the adjustable overcurrent limitation of the PROFET Load Guard, but the component switches off immediately instead of limiting the current. This addresses applications with higher currents of up to 26 A.

To support the trend towards software-defined vehicles, the PROFET Wire Guard is equipped with sequential diagnostics. This allows up to seven values to be read from five different addresses. The diagnostics are carried out via a single pin, which also enables the integrity checks of the application, for example I²t setting, OCT setting and integrity of the pad connected to the IS pin. The device also provides real time information about the wire protection status which can be used to enable further design optimizations. In addition, the integrated capacitive load switching mode enables large capacitors to be charged quickly within the safe operating area. Moreover, the PROFET Wire Guard is ISO 26262-compliant, up to ASIL-D requirements, which simplifies its use in functional safety applications. As a result, this solution is the first product on the market that fully meets all the requirements for wire protection in modern power distribution and thus offers a way of permanently eliminating fuses from vehicles.

Moreover, the ISO26262 compliance development reduces the design in effort in ASIL applications significantly.

Infineon Technologies