There is a growing need to replace melting fuses and relays with smart eFuses for intelligent power distribution. Considerations for selecting the appropriate smart eFuse include features such as programmable wire protection (I2T), capacitive charging, low-power mode, pin count for control and configuration, current and voltage sensing, and safety.

Load profile characteristics such as operating current, peak current, type of load, PAAT requirements and Automotive Safety Integrity Level rating (ASIL) will determine what features the selected switch needs. For supporting very high operating currents, consider a high-side switch controller that drives an external field-effect transistor (FET) to support any load requirement. An external FET solution is recommended to drive very high continuous current levels, 30A or greater for example, because this current level will increase the junction temperature of an integrated FET solution to an unsustainable level depending on the Rdson specification of the integrated FET.

In contrast, high side switches and smart eFuses excel in supporting lighter currents in both performance and cost. The PDB generally consists of high-side switch controllers instead of high-side switches because these devices need to support upwards of hundreds of amperes to support the power needed for all downstream zones. The load profile characteristics also indicate what the programmed wire characteristics of the smart eFuse should be so that the smart eFuse will shut off automatically upon detection of overcurrent without intervention from the local MCU. An example of an eFuse's programmable fuse characteristics is shown in Figure 8.

The type of load and peak current also determine whether it’s possible to use a smart eFuse capacitive charging method. Smart eFuse switches are typically equipped with capacitive inrush techniques to handle capacitive inrush and prevent damage to the metal-oxide semiconductor field-effect transistor (MOSFET). On the other hand, motorized inrush current from starting a brushed-DC motor will typically require a decrease in the smart eFuse drain-to-source on-resistance, or an external MOSFET in the absence of pulse-width modulation or current limiting.

It’s important to consider the smart eFuse’s safety mechanisms as well – more specifically its limp-home mode features. Limp-home mode is a programmable safe state the device enters if the failure conditions are met such as the high side switch losing SPI communication with its SPI controller which is typically an MCU. When designing limp-home states that preserve required vehicle functions, it’s important to consider whether the outputs should stay on and what the recovery method is, which are both programmable features of smart eFuses before they enter this failure state.