An outdated noticed usually uttered by {an electrical} engineer (or their hapless techs) when an digital system releases its magic smoke goes one thing like this: “The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 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!) stop a hearth from overheated wiring, sparking, and so forth.
This regrettable shortcoming is generally as a result 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 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 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 situations 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 on the market in six variants for 400–800 V battery techniques.
To raised perceive the professionals and cons of digital fuses, a assessment of typical fuse and circuit breaker know-how is so as. All fuses basically depend on the melting of a fusible ingredient 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 ingredient 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 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 will additionally implement present vs time behaviors that will be impractical (or not possible) with typical applied sciences.
Numerous methods 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 greater overcurrents the period of time required for a fuse to open tends to an asymptotic minimal (because of the 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 stated arc). This arcing downside 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 pressured arc quenching is required above 300 VDC or so (by surrounding the fusible ingredient with quartz sand, or spring loading it, and so forth).
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 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 important journey temperature, moderately 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 is going to mechanically return to the closed place after the bimetallic strip has cooled down.
Whatever the particular design of a thermal circuit breaker, it is going to undergo from some downsides that don’t afflict the easier fuse (or to not the identical diploma, anyway). For one factor, it is going to take far longer to answer a extreme overload or short-circuit than a fuse of the identical nominal present ranking. For an additional, 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 hearth hazard it’s supposed to stop.
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 by way of 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 way of it, so the upper the load present, the extra forcefully (and subsequently 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 sluggish 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 will be addressed by going with an digital answer—that’s, by utilizing 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 sooner to an overcurrent fault, they will additionally implement present vs time behaviors that will be impractical (or not possible) with typical applied sciences, in addition to shield in opposition to extra hazards resembling overvoltage, over- and under-temperature, and so forth, at little to no additional value (i.e. with a couple of extra parts or extra 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 standard 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 sequence with the unfavorable 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 know-how, resembling big magnetoresistance) might 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 mechanically supplied. Both method, the output of the current-measuring circuit might be in comparison with a reference voltage by a comparator, which itself will be carried out with an analog IC (such because the venerable LM331 from Texas Devices), although it’s most likely extra widespread 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 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 functions developed by Asahi Kasei Microdevices Company and Silicon Austria Labs GmbH.
The output of the comparator will possible be unable to immediately drive the semiconductor swap, 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 basic push-pull configuration. The semiconductor swap is mostly both a single low-side MOSFET (for blocking present circulation 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 swap is that the impedance of the upstream energy supply not be so low as to make it not possible for the e-fuse’s swap to outlive a tough quick downstream.
At greater voltages, it’s extra regular 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 unfavorable line have to be earth grounded for security causes. One key requirement for the semiconductor swap is that it should be capable of face up to the next peak present (or, extra appropriately, have the next 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 not possible for the e-fuse’s swap to outlive a tough quick downstream.
Be aware 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 energetic semiconductors. This means that the swap(es) within the e-fuse might 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 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 can even be wanted, including to the general value whereas lowering reliability. This additionally signifies that an e-fuse can’t immediately substitute an current typical fuse or circuit breaker, because it wants one other wire again to the facility supply to finish its provide circuit (although that is sometimes straightforward to retrofit).
You will need to notice that there are some energy circuit topologies by which no exterior fuse will be capable of shield its semiconductor units from failure, no matter fuse know-how or how briskly it could 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 basically presents a really low-impedance voltage supply to the switches, which implies it is going to 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 quick within the windings of the motor.
The one good answer for shielding the switches that are immediately 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 typical 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) 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 dangerous design selections and element decisions within the energy circuits they’re supposed to guard, however they might scale back total guarantee and technical help prices, whereas presumably costing much less up entrance, so that they’re positively value contemplating.
This text first appeared in Subject 69: July-September 2024 – Subscribe now.
