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.”
It is a succinct method of claiming that typical fuses (and most kinds 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 fireplace from overheated wiring, sparking, and so on.
This regrettable shortcoming is usually on account of physics and economics: a traditional fuse or thermal circuit breaker will take about 1,000 to 10,000 instances 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 traditional thermal one can permit for much less over-sizing of the downstream semiconductors, however they themselves are rather 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 circumstances requires an overcurrent protecting system 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 probably at a aggressive value to the specialised magnetic-thermal circuit breaker, particularly if it permits using extra rationally-rated semiconductors.
Microchip’s Silicon Carbide E-Fuse demonstrator is obtainable in six variants for 400–800 V battery methods.
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 factor to interrupt the circuit throughout overcurrent circumstances. 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 cannot solely reply far quicker to an overcurrent fault, they’ll additionally implement present vs time behaviors that will be impractical (or inconceivable) with typical applied sciences.
Varied tips can be utilized to switch this present vs time conduct—significantly 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 steel 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 by it (albeit with a voltage drop throughout mentioned arc). This arcing drawback 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 factor 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 instances of their operational life will make the most of the bimetallic strip instantly 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 greater 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, fairly 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’ll mechanically return to the closed place after the bimetallic strip has cooled down.
Whatever the particular design of a thermal circuit breaker, it’ll endure from some downsides that don’t afflict the easier 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 ranking. For one more, 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 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 transferring ferromagnetic plunger—which may 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 towards. This preserves the comparatively sluggish 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 typical fuse and circuit breaker designs may be addressed by going with an digital resolution—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 could be extra correct to explain it as an digital circuit breaker). E-fuses cannot solely reply far quicker to an overcurrent fault, they’ll additionally implement present vs time behaviors that will be impractical (or inconceivable) with typical applied sciences, in addition to defend towards further hazards reminiscent of overvoltage, over- and under-temperature, and so on, 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 typical electromechanical circuit breakers of comparable present interrupt ranking (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 sometimes be a resistive shunt that’s wired in collection with the unfavourable line (aka the low aspect), 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 greater voltages and/or currents, a present measuring circuit primarily based on a Corridor impact present sensor (or a competing expertise, reminiscent of big magnetoresistance) shall be extra enticing, as a result of there isn’t a I2R loss as incurred by a shunt, and galvanic isolation is mechanically offered. Both method, the output of the current-measuring circuit shall be in comparison with a reference voltage by a comparator, which itself may 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 today, 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 top voltage 800 V automotive purposes developed by Asahi Kasei Microdevices Company and Silicon Austria Labs GmbH.
The output of the comparator will possible be unable to instantly drive the semiconductor swap, 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 swap is mostly both a single low-side MOSFET (for blocking present circulate 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 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 inconceivable for the e-fuse’s swap to outlive a tough brief downstream.
At greater voltages, it’s extra standard to place the swap within the excessive aspect (i.e. the optimistic line) and use both an remoted or level-shifting gate driver to regulate it, particularly when the unfavourable line should be earth grounded for security causes. One key requirement for the semiconductor swap 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 swap to outlive a tough brief downstream.
Notice that this latter requirement additionally applies to traditional circuit breakers and fuses, however is much simpler to fulfill with units that don’t depend on energetic semiconductors. This means that the swap(es) within the e-fuse shall be beefier, and subsequently 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 quicker response velocity.
Lastly, e-fuses clearly require energy to function, and if the voltage of the upstream power supply isn’t instantly usable by the e-fuse circuitry (within the vary of three V to 30 V, say) then some sort of DC-DC converter may even be wanted, including to the general value whereas reducing reliability. This additionally signifies that an e-fuse can’t instantly exchange an current typical fuse or circuit breaker, because it wants one other wire again to the ability supply to finish its provide circuit (although that is sometimes simple to retrofit).
It is very important 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 will probably open. The commonest instance of such are the switches in a 3-phase inverter, as they’re instantly preceded by a capacitor wired instantly throughout the provision 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 reminiscent of 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 instantly fed by a voltage supply is energetic current-limiting on the particular person swap stage (aka pulse-by-pulse present limiting). One other scenario by which an e-fuse would possibly fare poorly—particularly compared to its typical counterparts—is when subjected to a extreme voltage spike, reminiscent of from a close-by oblique lightning strike. That is primarily as a result of passive electrical (fuses) or electromechanical (circuit breakers) units are usually much more rugged than their digital counterparts (e-fuses). In conclusion, e-fuses aren’t a panacea, they usually gained’t prevent from unhealthy design choices and part decisions within the energy circuits they’re supposed to guard, however they may cut back total guarantee and technical help prices, whereas probably costing much less up entrance, so that they’re undoubtedly value contemplating.
This text first appeared in Difficulty 69: July-September 2024 – Subscribe now.
