An outdated noticed usually uttered by {an electrical} engineer (or their hapless techs) when an digital gadget releases its magic smoke goes one thing like this: “The transistor bravely sacrificed itself to guard the fuse.”
This can be a succinct method of claiming that standard fuses (and most varieties of circuit breakers) shouldn’t be counted on to guard a downstream load from full destruction within the occasion of a fault, however solely to (hopefully!) forestall a hearth from overheated wiring, sparking, and many others.
This regrettable shortcoming is generally on account of physics and economics: a standard fuse or thermal circuit breaker will take about 1,000 to 10,000 occasions too lengthy to guard most semiconductor gadgets from overcurrent even when the latter are grossly over-sized (i.e. value far more than is economically justifiable). Specialised circuit breakers that mix a fast-acting magnetic journey together with the standard thermal one can permit for much less over-sizing of the downstream semiconductors, however they themselves are rather more costly gadgets, so as soon as once more this appears economically questionable.
To actually shield the downstream semiconductor gadgets from short-circuit faults with out affected by nuisance tripping throughout regular working situations requires an overcurrent protecting gadget that responds to extreme overcurrents in 1 to 10 microseconds whereas nonetheless sustaining a extra standard fuse response to reasonable overcurrents, which is exactly what an digital fuse can do—and probably at a aggressive value to the specialised magnetic-thermal circuit breaker, particularly if it permits the usage of extra rationally-rated semiconductors.
Microchip’s Silicon Carbide E-Fuse demonstrator is on the market in six variants for 400–800 V battery programs.
To higher perceive the professionals and cons of digital fuses, a evaluation of standard fuse and circuit breaker expertise is so as. All fuses basically depend on the melting of a fusible factor to interrupt the circuit throughout overcurrent situations. The period of time required is inversely proportional to the diploma of overcurrent. For the reason that heating of the fusible factor is the results of I2R losses, a doubling of present ought to lead to a quartering of the time to blow (that’s, following a sq. regulation). Certainly, fuses are most frequently characterised by I2t curves as a shorthand method of expressing the power required to rupture the fusible factor by leaving out the resistance, R, within the power equation of I2R over a given time, t.
E-fuses can’t solely reply far quicker to an overcurrent fault, they’ll additionally implement present vs time behaviors that may be impractical (or unimaginable) with standard applied sciences.
Varied tips can be utilized to change this present vs time habits—notably in lengthening the time that reasonable overcurrents may be tolerated with out unduly compromising the response to shorts—however at greater overcurrents the period of time required for a fuse to open tends to an asymptotic minimal (as a result of want for the molten metallic to really fall far sufficient away to interrupt the circuit).
Moreover, if the fuse should break a DC circuit above 50 V or so, an arc will probably be developed because the fusible hyperlink melts, prolonging the period of time that present will probably be allowed by it (albeit with a voltage drop throughout mentioned arc). This arcing drawback turns into more and more troublesome to handle with open-circuit DC voltage (it’s much less of a problem for AC, due to the periodic returning to 0 V), to the purpose that some type of compelled arc quenching is required above 300 VDC or so (by surrounding the fusible factor with quartz sand, or spring loading it, and many others).
Thermal circuit breakers additionally depend on I2R heating to journey, however this happens through the heating of a bimetallic strip that causes it to bend. Cheap thermal circuit breakers that aren’t anticipated to journey quite a lot of occasions of their operational life will make the most of the bimetallic strip straight to maneuver the current-carrying contacts aside (through self-heating of the strip from load present), however extra strong designs—and particularly at greater currents—will use a separate heater coil across the strip which is able to itself actuate a toggling mechanism so that there’s a snap-action response upon reaching the vital journey temperature, slightly than a extra gradual motion which might encourage arcing. The toggle mechanism permits for both a bistable motion, that means it’s steady in both the closed or open (tripped) positions, or a monostable motion, that means it would mechanically return to the closed place after the bimetallic strip has cooled down.
Whatever the particular design of a thermal circuit breaker, it would endure from some downsides that don’t afflict the less complicated fuse (or to not the identical diploma, anyway). For one factor, it would take far longer to answer a extreme overload or short-circuit than a fuse of the identical nominal present score. For an additional, the journey present (or the time delay earlier than tripping happens) will probably be extra delicate to the ambient temperature, as a result of the temperature rise wanted to function the bimetallic strip should essentially be restricted in order to not create the very hearth hazard it’s supposed to stop.
Thermal-magnetic circuit breakers tackle the extreme delay in responding to extreme overloads that’s attribute of purely thermal circuit breakers by passing the load present by a solenoid—that’s, a linear coil of wire round a shifting ferromagnetic plunger—which might independently function the toggling mechanism.
The magnetic power developed by a solenoid is proportional to the present flowing by it, so the upper the load present, the extra forcefully (and subsequently quicker) its plunger will transfer. The magnetic-hydraulic circuit breaker addresses the sensitivity of journey present to ambient temperature by changing the bimetallic strip with a viscous fluid that the solenoid plunger should function in opposition to. This preserves the comparatively gradual response to reasonable overloads typical of the thermal circuit breaker with the quicker response to short-circuits achieved by the magnetically-actuated mechanism.
The aforementioned shortcomings of standard fuse and circuit breaker designs may be addressed by going with an digital answer—that’s, through the use of a semiconductor swap to interrupt the circuit within the occasion of a fault situation—giving rise to what’s popularly referred to as an digital fuse, or e-fuse for brief (although it might be extra correct to explain it as an digital circuit breaker). E-fuses can’t solely reply far quicker to an overcurrent fault, they’ll additionally implement present vs time behaviors that may be impractical (or unimaginable) with standard applied sciences, in addition to shield in opposition to further hazards resembling overvoltage, over- and under-temperature, and many others, at little to no further value (i.e. with a number of extra parts or further traces of code). Moreover, e-fuses might value lower than standard electromechanical circuit breakers of comparable present interrupt score (although, admittedly, they examine much less favorably to traditional fuses on this regard).
The best implementation of an e-fuse is a current-measuring circuit feeding a comparator that controls a semiconductor swap. At working voltages as much as about 50 V and most currents as much as about 20 A, the present measuring circuit will usually be a resistive shunt that’s wired in collection with the adverse line (aka the low facet), whose voltage drop is multiplied by an op-amp-based differential amplifier with a modest quantity of achieve (2x to 10x is typical right here) in order to not incur too excessive of an influence loss within the shunt. At greater voltages and/or currents, a present measuring circuit based mostly on a Corridor impact present sensor (or a competing expertise, resembling big magnetoresistance) will probably be extra engaging, as a result of there isn’t a I2R loss as incurred by a shunt, and galvanic isolation is mechanically supplied. Both method, the output of the current-measuring circuit will probably be in comparison with a reference voltage by a comparator, which itself may be carried out with an analog IC (such because the venerable LM331 from Texas Devices), although it’s in all probability extra frequent to make use of a microcontroller IC lately, particularly if different protecting features are to be carried out. A key requirement for the comparator perform is that it have hysteresis, or barely totally different turn-on and turn-off thresholds, to stop oscillation when the load present is close to the overcurrent journey level.
An e-fuse proof of idea for top voltage 800 V automotive functions developed by Asahi Kasei Microdevices Company and Silicon Austria Labs GmbH.
The output of the comparator will seemingly be unable to straight drive the semiconductor swap, so some type of gate driver circuit or IC will probably be required, however this might merely be a complementary pair of bipolar transistors wired within the basic push-pull configuration. The semiconductor swap is mostly both a single low-side MOSFET (for blocking present movement in a single route—i.e. solely discharge, from the angle of a battery), or two MOSFETs wired source-to-source (for bidirectional blocking). This latter configuration is often used within the BMS for Li-ion batteries (which, in spite of everything, incorporates the entire features of an e-fuse, along with its cell monitoring and balancing duties).
A much less apparent requirement for the semiconductor swap is that the impedance of the upstream energy supply not be so low as to make it unimaginable for the e-fuse’s swap to outlive a tough brief downstream.
At greater voltages, it’s extra traditional to place the swap within the excessive facet (i.e. the constructive line) and use both an remoted or level-shifting gate driver to regulate it, particularly when the adverse line should be earth grounded for security causes. One key requirement for the semiconductor swap is that it should be capable to stand up to the next peak present (or, extra appropriately, have the next fusing power—I2t once more) than the gadget it’s defending; a much less apparent requirement is that the impedance of the upstream energy supply not be so low as to make it unimaginable for the e-fuse’s swap to outlive a tough brief downstream.
Observe that this latter requirement additionally applies to traditional circuit breakers and fuses, however is way simpler to fulfill with gadgets that don’t depend on energetic semiconductors. This suggests that the swap(es) within the e-fuse will probably be beefier, and subsequently dearer, than the semiconductor gadgets it’s supposed to guard, which might be economically questionable if not for the e-fuse’s prolonged safety capabilities and far quicker response velocity.
Lastly, e-fuses clearly require energy to function, and if the voltage of the upstream power supply isn’t straight usable by the e-fuse circuitry (within the vary of three V to 30 V, say) then some kind of DC-DC converter may even be wanted, including to the general value whereas lowering reliability. This additionally implies that an e-fuse can’t straight exchange an present standard fuse or circuit breaker, because it wants one other wire again to the facility supply to finish its provide circuit (although that is usually straightforward to retrofit).
It is very important be aware that there are some energy circuit topologies by which no exterior fuse will be capable to shield its semiconductor gadgets from failure, no matter fuse expertise or how briskly it may possibly open. The commonest instance of such are the switches in a 3-phase inverter, as they’re instantly preceded by a capacitor wired straight throughout the provision rails. This capacitor basically presents a really low-impedance voltage supply to the switches, which implies it would ship a really excessive peak present to them within the occasion of a fault resembling overlapping conduction of each switches in a bridge leg or a turn-to-turn brief within the windings of the motor.
The one good answer for safeguarding the switches that are straight fed by a voltage supply is energetic current-limiting on the particular person swap degree (aka pulse-by-pulse present limiting). One other scenario by which an e-fuse may fare poorly—particularly compared to its standard counterparts—is when subjected to a extreme voltage spike, resembling from a close-by oblique lightning strike. That is primarily as a result of passive electrical (fuses) or electromechanical (circuit breakers) gadgets are typically way more rugged than their digital counterparts (e-fuses). In conclusion, e-fuses aren’t a panacea, and so they received’t prevent from unhealthy design choices and element decisions within the energy circuits they’re supposed to guard, however they may scale back total guarantee and technical help prices, whereas presumably costing much less up entrance, in order that they’re positively price contemplating.
This text first appeared in Situation 69: July-September 2024 – Subscribe now.
