Fuses are usually invisible in cars, but when they burn out, drivers have to open the fuse box and replace them with small pliers. However, this cumbersome work may soon become a thing of the past. The latest electric vehicles, including Rivian and Tesla, are already introducing so-called 'digital fuses' instead of traditional blade fuses. This eFuse (electronic fuse) device integrates the functions of a circuit protection fuse and a switch relay into a single chip, and is a new technology that controls power more intelligently.

Limitations of Mechanical Fuses That Hold Back Power Distribution

Traditional automotive fuses are mechanical devices that are slow and inaccurate, and must be replaced once they have been tripped (blown). If the delay before the fuse blows in an overcurrent situation is long, it can cause damage to the circuit or device, so manufacturers design with a generous safety margin to prevent this. For example, the fuse capacity is often set so that it does not blow even at a current that is 50 to 80% higher than the normal current of the vehicle load. As a result, the circuit uses much thicker wires and larger components, which increases the weight and cost of the system. The response delay and low accuracy of the fuse ultimately indirectly pose a risk to the safety of the vehicle.

In addition, existing fuses do not monitor status or provide diagnostic information, and if overload is repeated, they deteriorate and blow without warning. Users can only find out about the problem after the fuse has blown, and they have to physically replace each one. Relays are also mechanical contacts, so they are at risk of wear and poor contact when used for a long time. In short, the fuse+relay-based power distribution structure is showing its limits in the precision control and remote monitoring required by modern vehicle electrical systems. An industry power semiconductor manager points out that “the power distribution architecture composed of mechanical fuses and relays used so far is no longer sufficient, so they should all be replaced with electronic fuses.”

The fundamental background of this change is the megatrend of the automobile industry. Semiconductor companies such as Infineon emphasize three trends: electrification/electronicization of vehicles, autonomous driving, and software-defined vehicles. In fact, in the development of new vehicles including electric vehicles, there is a strong demand to simplify and lighten the vehicle electrical design by reducing mechanical parts. In order to improve the driving range of electric vehicles, it is important to optimize the vehicle wiring harness and reduce its weight, and to this end, global automakers are introducing a centralized electrical structure such as a new zone architecture. In this environment, the conventional power distribution method consisting of dozens of fuses and relays is becoming an obstacle, and the transition to semiconductor-based electronic power distribution is naturally being promoted.

'Digital Fuse' Evolving into a Semiconductor Smart Switch

A solution that is gaining attention to overcome these limitations is smart power switch technology, also known as electronic fuse (eFuse). When a semiconductor device detects overcurrent in a circuit, it electronically cuts off the output, performing a fuse-like protective function. At the same time, it also functions as a relay, switching power supply or cutoff during normal times. Some automotive engineers have described eFuses as “a combination of a fuse and a relay, but with more intelligence.” In other words, they replace a fuse and relay in a single chip, but with a brain that monitors current flow in real time.

Unlike mechanical fuses with fixed melting characteristics, electronic fuses allow flexible operation settings. By adjusting the overcurrent threshold or the time curve until blocking (I²t characteristic) through software or hardware settings, they provide protection according to the situation. For example, a sophisticated protection curve can be implemented by temporarily flowing a high inrush current depending on the operating mode of the load and then blocking it after a certain time has passed. In addition, electronic fuses are easy to maintain because they can recover by themselves even after repeated overloads, and can also report real-time diagnostic/monitoring data to the system to detect abnormalities in advance. It is a reusable fuse that can be reactivated and used by an ECU command rather than simply being destroyed after being blocked due to overcurrent.

Above all, these semiconductor solutions improve circuit protection accuracy and enable optimization of wiring thickness. Since there is no need to take an overly conservative margin like a mechanical fuse, the overall harness weight can be reduced and power loss can be reduced. An industry insider explained, “If the existing mechanical power distribution consisting of fuses and relays is converted to a high-precision smart switch, the circuit size and wiring weight can be significantly reduced, which can improve the fuel efficiency and efficiency of the vehicle.” In short, the advantage of electronic fuses is that they can transform the vehicle power grid into a lighter, simpler, and more intelligent structure.

Smart Fuse Case Study with Infineon PROFET™ Wire Guard 12V

A practical example of this digital fuse technology is the PROFET™ Wire Guard 12V smart high-side switch from Infineon Technologies. This device is the successor to the existing Infineon PROFET series and was developed with several innovations to completely replace fuses. According to the interview, previous generations of smart switches had high standby current consumption (dark current) levels of several mA and did not support the I²t protection curve, making it difficult to completely replace fuses. This has been significantly improved with the new product. In fact, the Wire Guard 12V product family offers the following enhanced features:

- Accurate I²t protection characteristics: Built-in protection curves that take into account both overcurrent values and durations accurately simulate the current-thermal stress limits that wiring can withstand. Up to six curves can be selected and set according to the application, enabling much more precise overcurrent blocking than mechanical fuses. This allows a single device to accommodate a variety of wire thicknesses, optimizing wiring protection while replacing mechanical relays or fuses.

- High-speed overcurrent detection and blocking: The overcurrent threshold can be adjusted by setting an external resistance, and the circuit is immediately blocked when a sudden overload exceeds the threshold occurs to protect the upper system. By quickly isolating the faulty part, such as a sudden short circuit, it improves the stability of the entire vehicle power grid.

- Large-capacity load support mode: When there is a capacitive load such as a large input capacitor or motor at the output, the CLS mode is provided to safely charge the initial inrush current. This ensures system reliability by handling temporary transient currents generated from headlamps, electric motors, etc. without malfunction.

- Ultra-low power Idle mode: When the vehicle is parked or stopped for a long time, it automatically switches to low power mode, keeping the output turned on while reducing current consumption to approximately 50 μA. This drastically reduces dark current compared to existing products, preventing battery discharge, and allows it to be applied to power circuits that require output that is always connected, such as a fuse.

- Unified diagnostic interface: Sequential diagnostic function that sequentially outputs signals of up to 5 addresses through a separate diagnostic pin. This provides the system with information on overcurrent tripping (OCT), I²t warning/operation status, sensor output accuracy verification, and short-circuit/overheat fault diagnosis. It simplifies the interface with the vehicle ECU by multiplexing and transmitting various status information through a single pin, and supports self-diagnosis to meet functional safety requirements.

Sequential Diagnosis

- Supports high current and safety ratings: The product line currently covers a load current of up to 40A (15~27A depending on the rating), and a higher-end product supporting up to 36A is scheduled to be added by the end of 2025. It operates stably even in the harsh automotive environment with a high-reliability design including overheating protection, and is designed to satisfy the safety requirements of ISO 26262 ASIL-D level, so it can be applied to important electrical components.

According to Infineon, the PROFET Wire Guard 12V solution has already been adopted by some global automakers in their production vehicles. German premium brands and emerging Chinese electric vehicle OEMs have already applied this smart switch for wire protection and fuse replacement, and it is expected to be expanded to more vehicle models in the future. The target applications are various fields such as electric seat motors, water pumps/heat pumps, lighting control, and ECU power distribution. Previously, power relays and fuses had to be used in parallel/series for these loads, but now a single semiconductor device can perform the same role and even perform status monitoring.

Meanwhile, tools are also available to help development engineers easily incorporate these smart power distribution devices into their designs. In the case of Infineon, design tools such as Automotive Power Explorer allow designers to import their load profiles or wiring specifications and compare them with the protection performance of Wire Guard products. The tool simulates how effectively a selected eFuse device can protect under a given load/wiring environment, calculates resistance values for adjustable overcurrent thresholds, and guides users on which built-in I²t curve to choose. It also calculates sensing accuracy and power consumption of the device, which can be used for thermal design and reliability verification. Using dedicated tools and data provided by manufacturers in this way makes designing in new digital fuse devices much easier.

Future Outlook: “We need to gain experience and prepare for change”

The trend of electronic systems in automobiles is progressing more slowly than expected, but the industry consensus is that it is an inevitable change. Infineon’s manager Jong-min Lee said, “It is a fact that no one can deny that electronic systems are being implemented,” and emphasized the need for electronic component design companies to accumulate related experience from now on. For example, even if digital fuses are not applied to all vehicles right now, leading companies are acquiring know-how by applying these semiconductor protection devices on a trial basis to various new projects. This is because the ability to learn and utilize the characteristics of various types of semiconductor components will soon become design competitiveness.

Eventually, as automobiles become more electronic and intelligent to the point where they are called “computers on wheels,” even fuses, which are basic elements that handle electricity, are emerging as targets for digital innovation. Industry experts predict that semiconductor replacement of fuses will gradually spread to not only electric vehicles but also internal combustion engine vehicles. As the paradigm shift in automotive power distribution systems approaches, it is time for developers to actively embrace new technologies and prepare for the coming changes.