OVERVIEW
eFuse, or electronic fuse, is an IC (integrated circuit) that can replace conventional fuses or other protection devices like re-settable polymer fuses. Unlike conventional circuit protections, the eFuse offers much faster and more accurate intervention, without requiring replacement after the actuation. This is the biggest advantage of eFuse over the conventional fuses. They can be used to protect a system/device/PCB against overvoltage, short circuit, inrush currents and reverse polarity events.
How does eFuse work?
Although both thermal and eFuses contain the word “fuse,” their operating principles are quite different. The principle of the conventional thermal fuse is that if current gets too large, the current heats its fusible link via I2R dissipation and melts it down. The speed of this melting action and opening of the current path depends on how much excess current flows and for how long. Whereas , in the eFuse, current to the load passes through a FET while a dedicated current sensor measures that current by monitoring the voltage across a sense resistor. When the sensed voltage exceeds a threshold limit, the FET is turned off and the current flow stops (a minor FET leakage current may still flow) or limits the output current to a safe value defined by the user. If the abnormal overload condition persists, the device goes into an open state, disconnecting the load from the power supply. Typically, the overload current limit can be programmed by means of an external resistor.
Block diagram of a typical eFuse IC is given below:
TPS259271DRCR and TPS26631PWPR from texas instruments, NIS5820MT1TXG from ON semiconductor, STEF01FTR from STMicroelectronics, TCKE800NL from Toshiba Semiconductor are some of the popular eFuse ICs available on the market.
Advantages
- The overcurrent protection value is fairly accurate, and depending on the eFuse, is user-set via external resistors; this is more precise than thermal fuses.
- No need to replace fuses after fault condition. Unlike a basic fusible-link, the standard eFuse can automatically re-establish current flow after the overload condition subsides.
- Cost reduction and miniaturization: By using eFuse as a single-package for various protection functions such as overvoltage protection function, inrush current suppression function, overheat protection etc.
- Very fast short-circuit protection: the ultra-high-speed short-circuit protection technique provides a basic fusing role in milliseconds or even microseconds, much faster than a thermal fuse.
- It can also prevent excessive voltage from being applied to the load by clamping the outputs for instantaneous voltage raises.
Applications
Some potential applications of eFuses include:
- Applications that require overcurrent protection: When an over current event happens, protection for the downstream circuit from excessive current is essential to avoid any potential circuit damage or fire. eFuses are one of the best alternatives for this job.
- Inrush Current Control: Inrush current is the instantaneous high input current drawn by a power supply or electrical equipment at turn-on. For example, this condition arises due to the high initial currents required to charge the capacitors and inductors. Inrush currents are a major concern in especially DC power systems. This huge inrush current causes voltage sag which could affect operation of other connected loads. This problem can be solved using eFuses.
- Short-Circuit Protection: eFuse can be replaced with conventional fuses and is able to respond faster for short circuit faults at the output and disconnects the circuit.
- Reverse Current Protection: Reverse currents can flow through the system when the voltage at the output is higher than that of input. A series diode can block reverse currents but results in significant power loss in the system which could lower the overall system efficiency. In this context, an eFuse is a more efficient option and also offers flexibility in current support.
- Overvoltage Protection: Voltage transients or inductive switching can cause over-voltage in the system which lasts anywhere from a few hundred microseconds to some milliseconds. An eFuse with output voltage-cut off guards the voltage sensitive circuits from overvoltage by turning OFF the internal FET.
- Input Reverse Polarity Protection: The standard solution for reverse polarity protection is to use either a blocking diode or a combination of fuse and TVS diode, where the fuse blows-off and protects the load against reverse polarity. But after the fault, the fuse needs to be replaced. Alternatively, an eFuse offers a more efficient and flexible solution for reverse polarity conditions using its internal FET to block the reverse current path.
Conclusion
eFuses are IC based protection devices that are used to limit circuit currents, voltages to safe levels during fault conditions. When connected in series to the power line, it operates like a standard fuse with the ability of detecting and quickly reacting to overcurrent and overvoltage conditions. In addition to the overcurrent and voltage protection function, various other protection functions such as, inrush current suppression, overheat protection, and the reverse current prevention function, which cannot be realized by conventional fuses, can be packaged in one package. This reduces the number of components, saves mounting space on the PCBs, thus contributing to lower costs and downsizing.