An previous noticed typically uttered by {an electrical} engineer (or their hapless techs) when an digital system releases its magic smoke goes one thing like this: “The transistor bravely sacrificed itself to guard the fuse.”
This can be a succinct manner of claiming that standard 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 many others.
This regrettable shortcoming is usually attributable to physics and economics: a traditional 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. value far more than is economically justifiable). Specialised circuit breakers that mix a fast-acting magnetic journey together with the standard thermal one can enable for much less over-sizing of the downstream semiconductors, however they themselves are far more costly units, so as soon as once more this appears economically questionable.
To really defend the downstream semiconductor units from short-circuit faults with out affected by nuisance tripping throughout regular working situations requires an overcurrent protecting system 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 doubtlessly 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 offered in six variants for 400–800 V battery programs.
To raised perceive the professionals and cons of digital fuses, a assessment of standard fuse and circuit breaker expertise is so as. All fuses basically depend on the melting of a fusible component 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 component is the results of I2R losses, a doubling of present ought to end in 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 manner of expressing the power required to rupture the fusible component 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 will additionally implement present vs time behaviors that will be impractical (or inconceivable) with standard applied sciences.
Numerous tips can be utilized to switch this present vs time habits—notably in lengthening the time that reasonable overcurrents will 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 might be developed because the fusible hyperlink melts, prolonging the period of time that present might be allowed by way of it (albeit with a voltage drop throughout mentioned arc). This arcing downside turns into more and more tough 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 component with quartz sand, or spring loading it, and many others).
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 straight to maneuver the current-carrying contacts aside (by way of self-heating of the strip from load present), however extra strong designs—and particularly at increased currents—will use a separate heater coil across the strip which can itself actuate a toggling mechanism so that there’s a snap-action response upon reaching the essential journey temperature, relatively than a extra gradual motion which might encourage arcing. The toggle mechanism permits for both a bistable motion, that means it’s secure in both the closed or open (tripped) positions, or a monostable motion, that means it can routinely return to the closed place after the bimetallic strip has cooled down.
Whatever the particular design of a thermal circuit breaker, it can undergo from some downsides that don’t afflict the less complicated fuse (or to not the identical diploma, anyway). For one factor, it can take far longer to answer a extreme overload or short-circuit than a fuse of the identical nominal present ranking. For one more, the journey present (or the time delay earlier than tripping happens) might 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 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 way of a solenoid—that’s, a linear coil of wire round a transferring ferromagnetic plunger—which might independently function the toggling mechanism.
The magnetic drive developed by a solenoid is proportional to the present flowing by way of 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 towards. 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 standard fuse and circuit breaker designs will be addressed by going with an digital resolution—that’s, by utilizing a semiconductor change to interrupt the circuit within the occasion of a fault situation—giving rise to what’s popularly known 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 sooner to an overcurrent fault, they will additionally implement present vs time behaviors that will be impractical (or inconceivable) with standard applied sciences, in addition to defend towards extra hazards comparable to overvoltage, over- and under-temperature, and many others, at little to no additional value (i.e. with just a few extra elements or extra strains of code). Moreover, e-fuses might value lower than standard electromechanical circuit breakers of comparable present interrupt ranking (although, admittedly, they evaluate much less favorably to standard fuses on this regard).
The best 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 sometimes be a resistive shunt that’s wired in collection with the destructive line (aka the low facet), whose voltage drop is multiplied by an op-amp-based differential amplifier with a modest quantity of acquire (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) might be extra engaging, as a result of there is no such thing as a I2R loss as incurred by a shunt, and galvanic isolation is routinely offered. Both manner, the output of the current-measuring circuit might be in comparison with a reference voltage by a comparator, which itself will be applied 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 applied. A key requirement for the comparator operate is that it have hysteresis, or barely completely 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 prime voltage 800 V automotive purposes 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 change, so some type of gate driver circuit or IC might 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 circulation in a single course—i.e. solely discharge, from the angle 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 brief downstream.
At increased voltages, it’s extra standard to place the change within the excessive facet (i.e. the optimistic line) and use both an remoted or level-shifting gate driver to manage it, particularly when the destructive line have to be earth grounded for security causes. One key requirement for the semiconductor change is that it should have the ability to stand up to a better peak present (or, extra accurately, have a better fusing power—I2t once more) than the system 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 brief downstream.
Observe that this latter requirement additionally applies to standard circuit breakers and fuses, however is much simpler to satisfy with units that don’t depend on lively semiconductors. This means that the change(es) within the e-fuse might be beefier, and due to this fact costlier, 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 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 also be wanted, including to the general value whereas reducing reliability. This additionally signifies that an e-fuse can’t straight substitute an current standard fuse or circuit breaker, because it wants one other wire again to the ability supply to finish its provide circuit (although that is sometimes straightforward to retrofit).
You will need to word that there are some energy circuit topologies by which no exterior fuse will have the ability to defend its semiconductor units from failure, no matter fuse expertise or how briskly it may well 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 can 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 brief within the windings of the motor.
The one good resolution for safeguarding the switches that are straight fed by a voltage supply is lively current-limiting on the particular person change stage (aka pulse-by-pulse present limiting). One other scenario by which an e-fuse would possibly fare poorly—particularly compared to its standard 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 usually way more rugged than their digital counterparts (e-fuses). In conclusion, e-fuses aren’t a panacea, they usually gained’t prevent from unhealthy design selections and part selections within the energy circuits they’re supposed to guard, however they might scale back general guarantee and technical assist prices, whereas probably costing much less up entrance, so that they’re undoubtedly price contemplating.
This text first appeared in Situation 69: July-September 2024 – Subscribe now.
