Is it feasible to replace mechanical circuit breakers with SiC FET solid state circuit breakers?

【Introduction】There is a saying in engineering: “What moves will break.” We all know that mechanical parts are usually the first to fail, such as fans or relays, and in circuit systems, you need a set of forward-looking Procedures for sexual maintenance and replacement of these parts are provided “just in case”. The situation is worse when mechanical components have high stress levels during normal operation and then have to react reliably in emergency situations, such as contact circuit breakers in series with electric vehicle batteries.

Mechanical circuit breakers have low losses, but are slow and wear out. Solid-state circuit breakers with SiC FETs can solve these problems and their losses start to decrease.

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There’s a saying in engineering: “What moves will break.” We all know that mechanical parts are often the first to fail, like fans or relays, and in electrical systems, you need a suite of proactive maintenance and The procedure for replacing these parts is “just in case”. The situation is worse when mechanical components have high stress levels during normal operation and then have to react reliably in emergency situations, such as contact circuit breakers in series with electric vehicle batteries.

In this case, the operating current can be in the hundreds of amps, and in the case of a short circuit where the circuit breaker must cut, the current can be in the thousands of amps. The voltages are high, usually above 400V DC, and when the fault current is interrupted, the voltage peaks are even higher due to the connection inductance. Voltage can cause arcing, arcing can vaporize breaker contacts, and since it’s DC, arcing persists, and there’s no zero-crossing that eliminates arcing like AC. Switching on and off is also slow, taking tens of milliseconds, allowing damaging energy to pass through in a short-circuit situation. As the breaker ages, it also gets slower and loses more. All in all, high-current mechanical circuit breakers face many challenges, so they must be built robustly, and sometimes use exotic methods to clear the arc, such as creating multiple streams of compressed gas or using magnetic arc extinguishing coils.

Naturally, solid-state circuit breakers (SSCBs) have been devised as an alternative and fabricated using nearly every semiconductor technology available, from MOSFETs to IGBTs, SCRs and IGCTs. They solve arcing and mechanical wear problems well. Their serious downside is the voltage drop, in the case of an IGBT, it could have a 1.7V drop at 500A, resulting in a terrible 850W loss. IGCTs may have lower pressure drops, but are bulky. MOSFETs do not have the “knee” voltage of IGBTs, but have on-resistance. To improve upon the IGBT, this RDS(on) may need to be less than 3.4 milliohms and a voltage rating higher than 400V, which is not currently possible with a single MOSFET. Multiple MOSFETs in parallel can achieve this, but the cost will increase dramatically, and if you need bidirectional conduction capability, the cost will be doubled. Electromechanical circuit breakers are not cheap, but they still offer cost advantages.

Will SiC bring about a change?

Can magical new wide-bandgap semiconductor technology make up for the shortfalls? The on-resistance of a silicon carbide switch is about 10 times better than that of silicon for the same die area, and it has a much better thermal conductivity, allowing the heat to dissipate to handle double the maximum temperature. This allows enough dies to be paralleled in a small package to improve upon IGBTs that act as solid-state circuit breakers, and SiC FETs are an ideal candidate technology. The cascode structures of SiC JFETs and Si-MOSFETs are easy to drive and have RDS(on) x A performance characterizations that are outstanding in current switching technologies. A solid-state circuit breaker demonstrator, UnitedSiC parallels six self-produced 1200V dual-gate dies in a 1200V and 300A rated SOT-227 package, achieving a 2.2 milliohm resistance. In the test, the prototype safely interrupted the fault current of nearly 2000A, the waveform is shown in the figure.

[Figure 1. SiC FET solid-state circuit breaker safely interrupts currents approaching 2000A]

If the internal JFET gate is exposed and connected to a separate pin, it allows more direct control of the edge rate in fast switching applications and provides the high efficiency, selectable normally off or normally required in some applications such as solid state circuit breakers run. The ability to slightly forward shift the JFET gate will also slightly increase the on-resistance. But another feature will show up, and that is that above about positive 2V, the channel will be fully conductive and the gate will act as a forward biased diode. Now, if a fixed small current is injected, the actual knee voltage of the diode will be accurately related to the die temperature. This characteristic can be measured and used to perform rapid overtemperature detection, or even long-term health detection if temperature trends are recorded.

SiC FET Solid State Circuit Breakers Replacing Electromechanical Circuit Breakers Growing Trend

SiC FETs open the door to high-current solid-state circuit breaker applications with losses that will only decrease as technology advances. Paralleling devices has the potential to have final losses comparable to mechanical circuit breakers, and cost is not necessarily a hindrance, as die will develop and fewer die will be needed to achieve a given resistance. In the next few years, the cost of SiC wafers is bound to halve due to the economies of scale brought about by the expansion of the circuit breaker market due to sales of electric vehicles. Considering the maintenance and replacement costs of an electromechanical solution, this device is more attractive.

There’s also a saying in engineering: “If it ain’t broke, don’t fix it.” I would say, don’t wait for it to break, try a SiC FET solid state circuit breaker for a reassuring solution.

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