The fuse that has some nous
01 April 2020
Some of the great scientists of history are inextricably linked with our engineering work at the laboratory bench. George Ohm, a German physicist, is held in fond memory as we handle resistors. Likewise, English scientist, Michael Faraday when we select our capacitors; and American scientist, Joseph Henry when dimensioning inductors.
This article was originally featured in the April 2020 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
And as Turadj Aliabadi, Senior Marketing Manager for Discrete Semiconductors at Toshiba Electronics Europe tells us, while the 19th century smiled kindly upon these three core electrical components, it seems interest was waning by the time anyone got around to quantifying the characteristics of the humble fuse...
If looking to propose anyone to honour with the development of the simple fuse, whose name stems from the Latin, fusus (melted), it should probably be Arthur C. Cockburn. Although some of his experimental accuracy was mocked by London’s newly formed Society of Telegraph Engineers at their 1888 meeting, it does show that he went to some effort to scientifically determine the factors that came together to create a reliable fuse. According to Electric Fuses, by Wright & Newbury, his work determined that they should be rated to blow at around 150% to 200% of the rated current of the circuit being protected. With telegraph workers needed to be protected from lighting strikes, and electric lighting in its infancy, the fuse was becoming a critical safety component for a fledgling industry.
Fuses: what they can do – and what they lack
Fuses are sacrificial devices based upon a slender section of wire that is designed to melt, and thus stop the flow of current, should excessive current flow into the application being protected. Most electrical engineers whose careers have spanned more than a decade have fuses to thank for some of that longevity. Fuses, unlike their singly-dimensioned fellow components, are dimensioned according to two units: current and time. The current limit defines the upper limit of allowable current flow before the fuse sacrifices itself. The time element allows naturally occurring current surges beyond the specified current limit to be accommodated, such as often occurs during the inrush of current when a product is powered-on.
It is the sacrificial part that is the key problem behind fuses, meaning that, when a fuse blows, someone, such as a maintenance technician, will need to check the reason for the fuse blowing, and then replace it if safe to do so. This is time consuming, causes delay, and is sometimes challenging and costly, depending on how the fuse is integrated and the accessibility of the equipment.
Figure 1. Example application circuit for an eFuse protection solution
Oftentimes, an overcurrent situation occurs due to user error, such as a short circuit when inserting a faulty USB device into a PC or laptop. Rather than use a sacrificial device, the power supplies for such devices often make use of polymeric positive temperature coefficient (PPTC) devices. A type of low-resistance resistor, their resistance rises rapidly thanks to heating during the excessive current flow conditions of a fault, essentially restricting the flow of current. Once the fault has been removed, the device cools down returning close to its original low-resistance. In today’s world of generally safe electrical products, PPTCs provide protection, while not requiring a service technician to get them working again after the most likely cause of a fault: user error.
It should be noted that neither device is particularly fast in executing its protection function. Fuses typically require a second to blow, while PPTCs respond faster, but can take seconds to attain their full current restriction. While fuses disconnect appliances from power, PPTCs still allow a small current to flow even once tripped. Both devices are also dependent on the ambient operating temperature, so a derating at higher operation temperatures must be factored into the design.
Introducing the ‘clever fuse’
Semiconductor technology has been used to enhance or replace a variety of components over the past decades, and most recently, eFuses have continued that trend by replacing fuses and PPTCs. Computer motherboards, specifically the PCB traces supplying SATA hard drives or USB ports, benefit from the improved protection eFuses provide and the ability to reset them once the fault has been removed using a simple logic interface.
eFuses make use of advanced silicon processes that implement low-resistance MOSFET switches, providing low current loss when electricity is flowing. Integrated analogue comparators are capable of accurately monitoring current flow, reacting in sub-microsecond timeframes to completely cut off the supply. In conjunction with a host processor, a decision can be made as to the cause of the fault, and when to electrically restore power via the eFuse’s interface.
Being a silicon product, they of course offer a range of other useful features. Over-temperature monitoring, over-voltage clamping, under-voltage lockout, and reverse current protection are just some of the valuable extras these new devices provide.
Figure 2. Protection of a USB charging outlet using a TCKE805NL eFuse
Integrating eFuse technology
Of course, any application that provides power to user-fitted add-on sub-modules, such powered oscilloscope probes or programmable logic controllers (PLC), can benefit from eFuse technology. Solutions such as the TCKE8xxx series from Toshiba are easily integrated thanks to their compact 3.0 x 3.0 x 0.7 mm WSON10B packaging (figure 1). The devices offer a short-circuit trip current of 5.0 A to an accuracy of ±11%, while also providing either an auto-retry or latched response, depending on the device selected. Thanks to the integrated fast-trip comparator, the devices remove power under fault conditions within 150 ns. The series is also certified to IEC 62368, significantly easing the path for those looking to comply with single-unit failure modes.
The on-resistance (RON) of the integrated switch is a highly respectable 28 mO, while slew rate, to control inrush current, and the under-voltage lockout, can be set using external components. Internal temperature monitoring also provides protection and, once 160°C has been reached, automatically shuts off the output. Depending on the eFuse chosen, this protection may be latched, requiring a reset via the EN enable pin, or the switch will reengage power once it has cooled by around 20°C. The overvoltage clamping offered is determined by the precise device chosen.
An eFuse provides a perfect, compact solution in USB chargers and battery packs, protecting the charging outlet (figure 2). The TCKE805NL provides the optimal fit, offering latched protection together with an over-voltage clamp fixed at 6.04 V. A 75kO resistor connected to ILIM serves to limit current to 1.5 A, while a 2 nF capacitor provides a turn-on ramp time of 4 ms. Input and output capacitors of 1.0 µF located close to the VIN and OUT pins reduce voltage overshoot and undershoot during sudden changes in current draw. If required, an N-Channel FET can also be integrated to protect against reverse currents.
It seems a shame that the humble fuse has never achieved the same status as its fellow electrical components, despite being scientifically studied at around the same period of time. For sure, fuses and PPTCs have been an essential part of our safety tooling for many years. However, the type of protection required today is often against human-induced faults, rather than complete system failure. Configurable eFuses provide applications with reliable, but resettable protection, helping to extend the life of applications, and reducing the need for support from a technician in many cases.
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