An outdated noticed typically uttered by {an electrical} engineer (or their hapless techs) when an digital system releases its magic smoke goes one thing like this: “The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 manner of claiming that typical fuses (and most sorts 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 on account of physics and economics: a standard fuse or thermal circuit breaker will take about 1,000 to 10,000 instances 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 far more costly gadgets, so as soon as once more this appears economically questionable.
To really shield the downstream semiconductor gadgets 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 doubtlessly 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 offered in six variants for 400–800 V battery methods.
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 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 manner of expressing the vitality required to rupture the fusible factor by leaving out the resistance, R, within the vitality 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 unattainable) with typical applied sciences.
Varied tips can be utilized to switch this present vs time conduct—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 steel to truly 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 likely be developed because the fusible hyperlink melts, prolonging the period of time that present will likely be allowed by way of 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 so on).
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 various instances of their operational life will make the most of the bimetallic strip immediately to maneuver the current-carrying contacts aside (through 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, somewhat 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 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 endure from some downsides that don’t afflict the less complicated fuse (or to not the identical diploma, anyway). For one factor, it is going to take far longer to reply to a extreme overload or short-circuit than a fuse of the identical nominal present score. For one more, the journey present (or the time delay earlier than tripping happens) will likely 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 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 may 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 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 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 could 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 cannot solely reply far quicker to an overcurrent fault, they’ll additionally implement present vs time behaviors that will be impractical (or unattainable) with typical applied sciences, in addition to shield in opposition to further hazards resembling overvoltage, over- and under-temperature, and so on, at little to no further value (i.e. with a couple of extra elements or further strains of code). Moreover, e-fuses could value lower than typical electromechanical circuit breakers of comparable present interrupt score (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 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 destructive 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 primarily based on a Corridor impact present sensor (or a competing know-how, resembling large magnetoresistance) will likely 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 manner, the output of the current-measuring circuit will likely be in comparison with a reference voltage by a comparator, which itself could be applied 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 lately, particularly if different protecting capabilities are to be applied. A key requirement for the comparator operate 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 prime 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 immediately drive the semiconductor swap, so some type of gate driver circuit or IC will likely 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 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 often used within the BMS for Li-ion batteries (which, in any case, incorporates all the capabilities 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 unattainable for the e-fuse’s swap to outlive a tough quick downstream.
At increased voltages, it’s extra normal to place the swap 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 destructive 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 accurately, have the next fusing vitality—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 unattainable for the e-fuse’s swap to outlive a tough quick downstream.
Observe that this latter requirement additionally applies to standard circuit breakers and fuses, however is way simpler to satisfy with gadgets that don’t depend on energetic semiconductors. This suggests that the swap(es) within the e-fuse will likely be beefier, and due to this fact costlier, 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 pace.
Lastly, e-fuses clearly require energy to function, and if the voltage of the upstream vitality 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 will even be wanted, including to the general value whereas lowering reliability. This additionally signifies that an e-fuse can’t immediately 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 be aware that there are some energy circuit topologies wherein no exterior fuse will be capable to shield its semiconductor gadgets from failure, no matter fuse know-how 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 immediately throughout the provision 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 state of affairs wherein 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) gadgets 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 selections and part decisions within the energy circuits they’re supposed to guard, however they may cut back general guarantee and technical assist prices, whereas presumably costing much less up entrance, so that they’re positively value contemplating.
This text first appeared in Challenge 69: July-September 2024 – Subscribe now.
