How to integrate your hot swap with an eFuse

When working on a power design, the question of protection often arises. How much protection do you need, and how do you implement it? If you are still using a fuse for protection, be sure to check out my colleague’s blog on upgrading your fuse. But if instead you’re using a hot-swap controller with an external FET for protection, read on to learn how to save space with an eFuse.

The benefits of using a hot-swap controller

The main difference between an eFuse and a hot-swap controller is that the hot swap is a controller, meaning that it drives an external FET, as shown in Figure 1. To regulate the power supplied to the load, this FET switches on and off based on the control logic inside the hot-swap controller. This switching mitigates conditions such as overvoltage or overcurrent by turning the FET off when the sense resistor detects these faults (R_SENSE in Figure 1). Some controllers will also report faults through a fault (FLT) pin to the microcontroller.

Figure 1: Typical hot-swap controller block diagram

Because the hot swap has an external FET, you have control over which FET you would like to use in your system. Being able to replace the external FET and “tune” each design provides three main advantages:

• A flexible R_DSON that you can modify for each project.
• No upper boundary for the current limit, as you can always place a larger FET to increase the maximum current.
• You can use the same controller across multiple projects (by swapping the external FET as needed).

The advantages of an integrated eFuse

On the other hand, an eFuse offers higher integration than a hot-swap controller, and can save precious board space. As shown in Figure 2, an eFuse integrates both the sense element (R_SENSE) as well as the external FET.

Figure 2: Typical eFuse block diagram

Most of the components required by eFuses are resistors and capacitors, which enable you to customize the current limit and the soft start/slew rate. Figure 3 shows the suggested placements for the TPS25926 (R1, R2 and C_dVdT are optional).

Figure 3: TPS25926 eFuse application schematic

UL-recognized eFuses facilitate UL certification

Another distinct advantage that eFuses bring to the table is UL recognition. For products requiring UL certification, using UL-recognized eFuses (via UL 2367) expedites system-level UL certification. Industrial system designers can also leverage eFuses to create low-power circuits for UL 60730 (IEC 60335-1) compliance.

Many eFuses are also UL 60950 recognized, meaning that if the current-limit resistor shorts or is otherwise disconnected from the eFuse, it will still be able to function. A “default” low current-limit setting programmed into the eFuse ensures that it will fail safely.

So the next time you ask yourself how to protect your space-constrained power design, remember to use a TI eFuse. 

Additional resources

• Visit Texas Instruments’ broad portfolio of eFuses
• Read these E2E™ Community blog posts:
  • • Get out of the Dark: upgrade your fuse!
    • “How to save time and money with low power circuits.”