Circuit protection is an essential part of modern electronics, not least as batteries with low internal resistance (and the ability to deliver huge and potentially damaging currents) are increasingly used in applications from portable devices through to vehicles of the future.
Fuses have been around almost as long as electricity has been used, protecting simple early applications such as lighting. However, most fuses have been non-resettable meaning that they have to be physically replaced once actuated – which is inconvenient and often expensive if the fuse is difficult to access (remote location or sealed equipment, for example). Simple, wire-based fuses have a number of other drawbacks including a wide tolerance for the tripping current, slow to actuate and struggling to cope with the inrush current associated with complex loads.
Resettable fuses, including polymeric positive temperature coefficient (PPTC) devices removed the issue with replacement but still had a number of drawbacks. Most notably, this included the inability to reduce current to zero during a fault condition and the relatively slow actuation that may take several seconds.
Silicon eFuses are based upon a low-resistance solid state switch that has current measurement circuitry as the primary control. If a pre-set current threshold is exceeded then the switch is opened and the current flow stops, generally in less than a microsecond. Thus, this fuse provides excellent protection by removing all current faster than a traditional fuse or a resettable PPTC fuse.
Depending upon how the eFuse is configured, the current protection may be latching, so that the switch remains open until a reset signal is received – perhaps from a microcontroller, or a manual reset switch operated by a person. Non-latching eFuses will automatically retry by closing the switch and monitoring current, rapidly opening the switch if the fault is still present.
As eFuses are able to be controlled by logic-level signals, they are able to offer a number of additional features so that they become true circuit protection devices. They are able to provide inrush current protection, which can be fixed or adjustable depending upon the device.
As well as protecting against overcurrent, eFuses are also able to deal with voltage anomalies. Many include a voltage clamp that ensures the output voltage never exceeds a pre-set level, guarding against voltage rises due to light loads and providing additional protection for downstream circuitry.
If the voltage does not reach the correct level then transistors may not switch fully, giving the potential for damage - also, logic circuits may not operate correctly. To avoid these problems, eFuses can incorporate an undervoltage lockout (UVLO) that ensures the voltage is only connected when it is within specification.
Additional common eFuse features include an output to control an additional (external) switch for protection against reverse current and also a thermal shutdown.
Clearly, eFuses are able to offer a far wider range of protection than other fuses. To understand how these devices can be deployed quickly and easily, please download our comprehensive white paper