As vehicle electronic devices become increasingly complex, providing correct and adequate protection for all components in the system is crucial for safety and reliability. Vehicle manufacturers are gradually abandoning traditional blade fuses and instead favoring the advantages brought by electronic fuses (eFuses).

This article will introduce how to operate electronic fuses in a way similar to traditional fuses, and look forward to the future prospects of programming electronic fuses to simulate traditional fuses.

These programmable devices are designed to protect power cords from damage caused by overcurrent, overvoltage, and short circuit conditions. Traditional fuses physically cut off the circuit when a fault occurs, while electronic fuses are different in that they can be reset and reconfigured, making them a more flexible and reusable solution. Electronic fuses are commonly used in modern electronic devices such as smartphones, tablets, and laptops, where precise and reliable protection is crucial. Nowadays, electronic fuses are increasingly used in more challenging environments, including the automotive industry.

In fact, electronic fuses will soon become a critical component of all automotive systems, protecting devices and subsystems from overcurrent conditions, thereby avoiding additional costs due to damage and reliability.

Each electronic fuse has a jump curve that defines the way and timing at which the electronic fuse disconnects the load. Due to different application scenarios, the jump point needs to be adjusted, and the most common method of adjustment is to connect an external resistor to a dedicated pin. However, as this article will introduce, the jumping mode required for electronic fuses may be complex, and other factors need to be considered in addition to current.

In order to provide designers with greater flexibility when deploying electronic fuses, ON Semiconductor is developing a new generation of devices that will allow for digital modification of the shape and range of the jump curve. In order to better master the method of using electronic fuses in design, designers should have a deep understanding of the process to be followed when designing jump curves for electronic fuses.

Thermal impedance analysis: The first step is to understand the physical properties and deployment environment of electronic fuses. This is to ensure accurate assessment of thermal response in environments where conditions may fluctuate significantly. This is crucial because thermal stress exceeding the capacity of devices (including electronic fuses) is one of the most common failure modes in power systems. With the continuous miniaturization of geometric dimensions, the likelihood of such failures occurring will increase without comprehensive analysis.

The key to understanding thermal effects is the heat transfer ladder (Figure 1), which connects the semiconductor junction to ambient air through the layers and materials that make up the electronic fuse. Please also refer to the application manual AND9733- High Side SmartFET with Analog Current Detection (onsemi. com).

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Figure 1- Heat Transfer Ladder for General Electronic Fuse Applications

This helps to understand how high current pulses transfer thermal energy throughout the entire system. In short, the longer the pulse duration, the farther the heat propagation distance. Pulses with a duration of less than 10 ms will remain inside the package, while pulses with longer duration will propagate to the PCB and dissipate there. This is caused by the heat capacity of the device and surrounding components (such as PCB).

The structure of PCB will have a significant impact on its thermal performance due to its layout and stacking method. The number of layers, weight of copper layers, and the presence of power and ground layers can all affect thermal performance, as shown in the simulation results in Figure 2. These simulations depict the thermal resistance of TSSOP14-EP package under different thermal conditions:

Left 1s0p-miniCu: TSSOP14 is installed on a single signal layer PCB, with the smallest copper area and no power layer

Figure 1s0p_1InCu: TSSOP14 is installed on a single signal layer PCB with a copper area of 1 square inch and no power layer

Figure 2s2p_1InCu: TSSOP14 is installed on a dual signal layer PCB with a copper area of 1 square inch and 2 power layers

The first step is to determine the RDS (ON) of the electronic fuse based on the thermal impedance (oC/W), ambient temperature, and maximum junction temperature by analyzing the steady-state current. Based on this, designers will be able to calculate the working limit.

The second step is to simulate the thermal effects in the application of electronic fuses by applying various shorter duration and larger current pulses. Then, a relationship diagram between thermal impedance and current pulse duration can be drawn:

Clearly demonstrated how thermal impedance varies with pulse duration, with a significant decrease in thermal impedance under shorter pulses. The performance here is directly related to the cost of the PCB, such as by increasing the number of layers, using thicker copper layers, or adding heat dissipation pads on the shell. However, for shorter pulses, factors such as RDS (ON) and chip size can affect the shape of the curve, while for longer pulses, the impact of PCB is more significant.

This curve must be defined and understood separately for each application, which is crucial for selecting the appropriate electronic fuse for the application. This requires designers to understand the current characteristics of electronic fuses, especially the amplitude and duration of pulses.

Application thermal requirements: The thermal impedance curve reflects the relationship between thermal impedance and time, while fuses require the relationship between time and current. The thermal limit curve of an electronic fuse can be obtained by reversing its thermal impedance curve, but some assumptions are required, including RDS (ON) and? T (acceptable change in chip temperature).

The resulting curve shows the maximum current pulse duration that limits the rise in junction temperature (Tj) within the design standard. Usually, good design practices adopt absolute overcurrent protection and reserve a few degrees of temperature buffer.

Determine the relationship between I2t and current: I2t is an important parameter often mentioned in discussions related to electronic fuses. It is mainly related to the current in the wiring harness, and if the current is too high, it may cause damage. For traditional fuses, I2t is usually listed as a constant along with the nominal fuse current value. The blue line in Figure 5 shows a constant I2t value.

However, adopting this method means that the entire heat dissipation capacity of the system cannot be fully utilized, which may lead to performance degradation. In fact, the wiring harness does not require a constant I2t (straight line), as longer durations are feasible at lower currents.

Using a constant I2t will limit the load that can be connected to the electronic fuse, so it is important to set I2t as an approximate curve in the electronic fuse. In this way, the jump point will approach (but not exceed) the limit line of the electronic fuse.

If we take a look at the typical curve of blade fuses, we will find more clearly the limitations of constant I2t.

Although the lower part of the curve is mainly determined by I2t, if a simple (straight) method is used for I2t, the higher part of the curve (i.e. the part that allows for longer duration at lower currents) will not exist.

look into the future

With a deep understanding of the thermal factors, jump curves, and their relationship with non constant I2t that affect electronic fuses, Anson Mei is actively developing electronic fuse technology that can be programmed for specific applications.

By serial communication (I2C or SPI), the desired jump curve shape can be programmed into the electronic fuse. Although this is usually a one-time process, fuses can also be reprogrammed on-site to adapt to changes in system configuration (such as load changes, additions, or removals).

The new electronic fuse will include a series of trip curves that users can program through serial communication.

Ansenmei actively collaborates with industry designers to define curves that cover as many current and future fuse application cases as possible.

This is reported by Top Components, a leading supplier of electronic components in the semiconductor industry

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