An electronic fuse (eFuse) is an active, silicon-based protection integrated circuit that uses an internal power MOSFET to monitor and limit current, voltage, and temperature in DC circuits. Unlike a passive glass or ceramic thermal fuse that melts and must be discarded, an eFuse actively manages fault conditions, disconnects the load via semiconductor switching, and can automatically retry or latch off depending on your logic configuration.
For bench builders and PCB designers, the eFuse replaces the traditional fuse-and-relay combination, offering microsecond fault clearing, programmable inrush current limiting, and precise I²t (thermal energy) modeling. Below is the exact framework for selecting, wiring, and testing these components.
What Is an Electronic Fuse? (The Solid-State Shift)
At its core, an eFuse places a low-resistance N-channel or P-channel MOSFET in series with your power rail. A built-in charge pump drives the gate, while analog monitoring circuitry continuously measures the voltage drop across the MOSFET (which correlates to current) and the silicon die temperature.
Do not treat electronic fuses and mechanical DC breakers as interchangeable without looking at the trip curve. A mechanical breaker relies on a bimetallic strip for thermal overload (slow, seconds to minutes) and a solenoid for short-circuit magnetic tripping (fast, milliseconds). An eFuse uses a silicon thermal model that calculates I²t in real-time. It can clear a dead short in microseconds, long before a mechanical breaker's contacts even begin to physically separate, preventing catastrophic wire insulation meltdown in lightweight AWG harnesses.
eFuse Ratings: Mapping "Coil" and "Contact" to Silicon
Because electronic fuses are solid-state ICs, they do not contain physical electromagnetic coils or mechanical contacts. However, to bridge the gap between traditional electromechanical relay logic and modern silicon protection, we map the "coil" to the control/enable gate circuitry and the "contact" to the internal power MOSFET channel. Here is how the standard rating table translates to an eFuse IC like the TI TPS2596.
| Electromechanical Term | eFuse Silicon Equivalent | Example Value (TI TPS2596) | What It Governs |
|---|---|---|---|
| Coil Voltage | Control/Enable Pin Logic (VEN, VUVLO) | 2.7V to 19V Logic Thresholds | Turn-on/turn-off logic thresholds and undervoltage lockout |
| Contact Rating | Power MOSFET Channel (VDS, IMAX, RDS(on)) | 20V Max, 5A Cont., 28mΩ RDS(on) | Steady-state load capacity and conduction heat generation |
| Breaking Capacity | Short-Circuit Limit (ISC) & Clamping Energy | 12A Limit, Fast Trip, 40mJ Avalanche | Fault survival and maximum let-through energy during a dead short |
Wiring the eFuse: Control Logic vs. Power Path
Wiring an eFuse requires separating your PCB layout into two distinct domains: the high-current power path (the "contact" side) and the low-current logic path (the "coil" side).
The Power Path (Contact Side)
The IN and OUT pins carry the main load. Because the internal MOSFET has an RDS(on) (typically 15mΩ to 50mΩ), you must use heavy copper pours (2 oz copper minimum for >3A loads) to act as heatsinks. The silicon die transfers heat directly through the exposed thermal pad into the PCB ground plane.
The Control Logic (Coil Side)
The Enable (EN), Power Good (PG), and Fault (FLT) pins interface with your microcontroller or logic switches. These pins operate on microamp currents. Route them away from high dV/dt switching nodes to prevent capacitive coupling from accidentally toggling the eFuse off.
When wiring the "coil" logic to turn off an inductive load (like a DC motor, solenoid, or relay coil), the eFuse's internal MOSFET will interrupt the current in microseconds. This rapid di/dt generates a massive inductive voltage spike (V = L × di/dt). If you do not place an external TVS diode (e.g., SMAJ15A) or a freewheeling Schottky diode across the load, this spike will exceed the eFuse's internal avalanche energy rating, permanently shorting the silicon. Never rely on the eFuse's internal body diode to absorb repeated inductive kickback.
Selection Decision Path by Load Type
Which rating column governs your specific load? Use this decision tree to terminate your selection process with a concrete part number. For deeper architectural references, consult the STMicroelectronics eFuse design guides.
| Load Type | Governing Rating Column | Required eFuse Feature | Concrete Part Pick |
|---|---|---|---|
| Resistive (Heaters, LED strips) | Contact Rating (IMAX & RDS(on)) | Standard OCP, low quiescent current | ST STEF12 (12V rail, 4.5A limit) |
| Inductive (Solenoids, Valve coils) | Breaking Capacity (Avalanche Energy) | High repetitive avalanche rating + external TVS | TI TPS25921 (Robust clamp, 1.5A) |
| Motor / High Capacitive (High inrush) | Contact Rating + I²t Curve | Programmable startup ramp (dV/dt control) to prevent nuisance tripping | TI TPS25944 (Soft-start, 5A, 2.7-18V) |
Testing, Troubleshooting, and Replacement
Diagnosing a blown eFuse requires different techniques than checking a standard glass fuse. Because the protection element is a semiconductor, it can fail "short" (gate oxide puncture) or "open" (bond wire melt).
How to Test It Dead (Unpowered)
- Set your multimeter to Diode Test mode.
- Measure IN to OUT: A healthy, unpowered eFuse will typically read a forward voltage drop of ~0.3V to 0.5V (the internal MOSFET body diode). If it reads 0.00V (dead short), the silicon has suffered thermal runaway and failed shorted.
- Measure OUT to IN: Should read "OL" (Open Loop) if the IC features integrated reverse-current blocking. If it reads a diode drop, the IC is a standard unidirectional type (which is normal, but requires an external series diode if reverse battery protection is needed).
How to Test It Live (Powered Under Load)
Do not just check for voltage at the output. To verify the health of the internal "contact" (MOSFET channel), measure the voltage differential between the IN and OUT pins while the circuit is drawing its normal steady-state current. Use Ohm's law: R = ΔV / I. If your calculated resistance is more than 20% higher than the RDS(on) listed in the datasheet, the internal MOSFET is degrading and generating excess heat. Scope the FLT (Fault) pin; if it is pulling low periodically, you are experiencing micro-thermal shutdowns due to inadequate PCB heatsinking.
When to Repair vs. Replace
Never attempt to "repair" an electronic fuse. It is a monolithic silicon die encased in epoxy; it cannot be reset like a breaker or patched like a trace.
Replace the IC immediately if it fails testing.
Repair the PCB only if the eFuse failed short-circuit and the resulting thermal event delaminated the copper ground plane or burned the input traces. Cut away the charred FR4 fiberglass, expose clean copper, and solder a heavy-gauge jumper wire before installing the replacement IC.
The Default Recommendation
If you are designing a custom DC distribution board, a robotics power tree, or an automotive accessory harness and need a single, reliable baseline component: Use the Texas Instruments TPS2596.
It covers 2.7V to 19V rails, handles up to 5A continuous current, features programmable soft-start to handle capacitive inrush, and requires only two external 0402 resistors to set the current limit and undervoltage lockout. At roughly $1.80 per unit on Mouser, it provides the exact breaking capacity and I²t curve modeling required to protect downstream AWG 18-22 wiring without the cost and footprint of a dedicated microcontroller monitoring a shunt resistor.
