An outdated noticed typically uttered by {an electrical} engineer (or their hapless techs) when an digital gadget releases its magic smoke goes one thing like this: “The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.”
It is a succinct method of claiming that typical fuses (and most forms 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 so on.
This regrettable shortcoming is usually resulting from 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 units from overcurrent even when the latter are grossly over-sized (i.e. price far more than is economically justifiable). Specialised circuit breakers that mix a fast-acting magnetic journey together with the traditional thermal one can permit for much less over-sizing of the downstream semiconductors, however they themselves are way more costly units, so as soon as once more this appears economically questionable.
To really shield the downstream semiconductor units from short-circuit faults with out affected by nuisance tripping throughout regular working circumstances requires an overcurrent protecting gadget that responds to extreme overcurrents in 1 to 10 microseconds whereas nonetheless sustaining a extra typical fuse response to reasonable overcurrents, which is exactly what an digital fuse can do—and doubtlessly at a aggressive price 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 out there in six variants for 400–800 V battery techniques.
To raised perceive the professionals and cons of digital fuses, a evaluate of typical fuse and circuit breaker expertise is so as. All fuses primarily depend on the melting of a fusible ingredient to interrupt the circuit throughout overcurrent circumstances. The period of time required is inversely proportional to the diploma of overcurrent. Because the heating of the fusible ingredient 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. legislation). Certainly, fuses are most frequently characterised by I2t curves as a shorthand method of expressing the power required to rupture the fusible ingredient by leaving out the resistance, R, within the power equation of I2R over a given time, t.
E-fuses can’t solely reply far sooner to an overcurrent fault, they’ll additionally implement present vs time behaviors that might be impractical (or inconceivable) with typical applied sciences.
Varied methods can be utilized to change this present vs time habits—significantly in lengthening the time that reasonable overcurrents could be tolerated with out unduly compromising the response to shorts—however at increased 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 shall be developed because the fusible hyperlink melts, prolonging the period of time that present shall be allowed via it (albeit with a voltage drop throughout mentioned arc). This arcing downside turns into more and more troublesome to handle with open-circuit DC voltage (it’s much less of a difficulty for AC, due to the periodic returning to 0 V), to the purpose that some type of pressured arc quenching is required above 300 VDC or so (by surrounding the fusible ingredient with quartz sand, or spring loading it, and so on).
Thermal circuit breakers additionally depend on I2R heating to journey, however this happens by way of the heating of a bimetallic strip that causes it to bend. Cheap thermal circuit breakers that aren’t anticipated to journey various occasions of their operational life will make the most of the bimetallic strip immediately to maneuver the current-carrying contacts aside (by way of self-heating of the strip from load present), however extra sturdy designs—and particularly at increased 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 important journey temperature, quite than a extra gradual motion which might encourage arcing. The toggle mechanism permits for both a bistable motion, which means it’s steady in both the closed or open (tripped) positions, or a monostable motion, which means it’ll routinely return to the closed place after the bimetallic strip has cooled down.
Whatever the particular design of a thermal circuit breaker, it’ll undergo from some downsides that don’t afflict the less complicated fuse (or to not the identical diploma, anyway). For one factor, it’ll 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) shall 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 fireplace hazard it’s supposed to forestall.
Thermal-magnetic circuit breakers handle the extreme delay in responding to extreme overloads that’s attribute of purely thermal circuit breakers by passing the load present via a solenoid—that’s, a linear coil of wire round a shifting ferromagnetic plunger—which might independently function the toggling mechanism.
The magnetic pressure developed by a solenoid is proportional to the present flowing via it, so the upper the load present, the extra forcefully (and due to this fact sooner) 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 sooner response to short-circuits achieved by the magnetically-actuated mechanism.
The aforementioned shortcomings of typical fuse and circuit breaker designs could be addressed by going with an digital resolution—that’s, through the use of a semiconductor change 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 could be extra correct to explain it as an digital circuit breaker). E-fuses can’t solely reply far sooner to an overcurrent fault, they’ll additionally implement present vs time behaviors that might be impractical (or inconceivable) with typical applied sciences, in addition to shield in opposition to further hazards comparable to overvoltage, over- and under-temperature, and so on, at little to no further price (i.e. with a couple of extra elements or further strains of code). Moreover, e-fuses might price lower than typical electromechanical circuit breakers of comparable present interrupt score (although, admittedly, they examine much less favorably to standard fuses on this regard).
The only implementation of an e-fuse is a current-measuring circuit feeding a comparator that controls a semiconductor change. 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 unfavourable 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 increased voltages and/or currents, a present measuring circuit based mostly on a Corridor impact present sensor (or a competing expertise, comparable to large magnetoresistance) shall be extra enticing, as a result of there is no such thing as a I2R loss as incurred by a shunt, and galvanic isolation is routinely supplied. Both method, the output of the current-measuring circuit shall be in comparison with a reference voltage by a comparator, which itself could 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 as of late, particularly if different protecting features are to be carried out. A key requirement for the comparator perform is that it have hysteresis, or barely completely different turn-on and turn-off thresholds, to forestall oscillation when the load present is close to the overcurrent journey level.
An e-fuse proof of idea for prime voltage 800 V automotive purposes developed by Asahi Kasei Microdevices Company and Silicon Austria Labs GmbH.
The output of the comparator will doubtless be unable to immediately drive the semiconductor change, so some type of gate driver circuit or IC shall be required, however this might merely be a complementary pair of bipolar transistors wired within the traditional push-pull configuration. The semiconductor change is mostly both a single low-side MOSFET (for blocking present circulate in a single course—i.e. solely discharge, from the attitude of a battery), or two MOSFETs wired source-to-source (for bidirectional blocking). This latter configuration is usually used within the BMS for Li-ion batteries (which, in any case, incorporates the entire features of an e-fuse, along with its cell monitoring and balancing duties).
A much less apparent requirement for the semiconductor change is that the impedance of the upstream energy supply not be so low as to make it inconceivable for the e-fuse’s change to outlive a tough quick downstream.
At increased voltages, it’s extra standard to place the change within the excessive facet (i.e. the constructive line) and use both an remoted or level-shifting gate driver to manage it, particularly when the unfavourable line have to be earth grounded for security causes. One key requirement for the semiconductor change is that it should be capable of face up to the next peak present (or, extra accurately, 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 inconceivable for the e-fuse’s change to outlive a tough quick downstream.
Notice that this latter requirement additionally applies to standard circuit breakers and fuses, however is much simpler to fulfill with units that don’t depend on energetic semiconductors. This means that the change(es) within the e-fuse shall be beefier, and due to this fact dearer, than the semiconductor units it’s supposed to guard, which might be economically questionable if not for the e-fuse’s prolonged safety capabilities and far sooner response velocity.
Lastly, e-fuses clearly require energy to function, and if the voltage of the upstream power supply isn’t immediately 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 price whereas lowering reliability. This additionally implies that an e-fuse can’t immediately exchange an present typical fuse or circuit breaker, because it wants one other wire again to the facility supply to finish its provide circuit (although that is usually simple to retrofit).
You will need to observe that there are some energy circuit topologies wherein no exterior fuse will be capable of shield its semiconductor units from failure, no matter fuse expertise or how briskly it might open. The commonest instance of such are the switches in a 3-phase inverter, as they’re instantly preceded by a capacitor wired immediately throughout the availability rails. This capacitor primarily presents a really low-impedance voltage supply to the switches, which suggests it’ll ship a really excessive peak present to them within the occasion of a fault comparable to overlapping conduction of each switches in a bridge leg or a turn-to-turn quick within the windings of the motor.
The one good resolution for shielding the switches that are immediately fed by a voltage supply is energetic current-limiting on the particular person change stage (aka pulse-by-pulse present limiting). One other scenario wherein an e-fuse may fare poorly—particularly compared to its typical counterparts—is when subjected to a extreme voltage spike, comparable to from a close-by oblique lightning strike. That is primarily as a result of passive electrical (fuses) or electromechanical (circuit breakers) units are typically way more rugged than their digital counterparts (e-fuses). In conclusion, e-fuses aren’t a panacea, they usually received’t prevent from unhealthy design choices and part selections within the energy circuits they’re supposed to guard, however they might cut back general guarantee and technical help prices, whereas presumably costing much less up entrance, so that they’re positively value contemplating.
This text first appeared in Situation 69: July-September 2024 – Subscribe now.
