We get this question a lot.
Why do the POWERCELL and inMOTION cells use fuses to protect the outputs? Some say that we’re already using smart MOSFETs, why not rely on them to protect against over-current on the output wires? Others ask, why not use circuit breakers? There are several important and practical reasons why we rely on a traditional fuse to protect the outputs on our Infinitybox hardware.
With respect to MOSFETs, they are tried and true technology. They are used in practically every application for switching and current control. The MOSFETs used in the POWERCELLs are automotive-grade and designed to carry up to 270-amps. The chances of them failing in a typical customer car are minute. However, if a MOSFET fails, it doesn’t fail gracefully. If a MOSFET were to fail, it fails resistive. That means that they are going to generate a significant amount of heat, quickly. No intelligence built into the circuitry can interrupt the current flow if the silicon die in the MOSFET package has become a resistor. In that case, the fuse is your last line of defense to protect the system from thermal runaway.
You can use the same logic for inMOTION. It has sensors on the board that monitor the total amount of current flowing out to the load. inMOTION already shuts off the relay coils when it sees too much current, why not rely on that alone without the fuses? The practical reality is that a common failure mode for relays is for the contacts to weld together, especially in inductive motor application. If this were to happen, no intelligence on the inMOTION board can open the circuit. Again, the fuse becomes the last line of defense.
With the above being said, you can see the need for some circuit protection component in the system if other components fail. That leads to the debate between fuses and circuit breakers. Circuit breakers can certainly do the job, however a fuse will out perform a breaker in all categories.
- Fuses are more cost effective than circuit breakers.
- Fuses are smaller and package better than circuit breakers.
- Fuses have better low-overload protection characteristics than circuit breakers. This is very important in cases of resistive shorts like a failed MOSFET.
- Fuses respond quicker than circuit breakers under short-circuit conditions.
Most importantly, a fuse drives better end user behavior than a circuit breaker. At the end of the day, there is a reason why a fuse opens or a breaker trips. A motor has failed. Insulation on a wire has shorted to ground. An electronics module has failed. In all these cases, you do not necessarily want to blindly reset a breaker and keep on driving. The cause of the over-current condition needs to be identified and fixed before resuming normal operation of the vehicle or equipment. Otherwise, more severe thermal events can occur in the wiring harness causing damage or injury.
See our blog for additional posts on proper fusing and other circuit protection lessons. If you have any questions, reach out to our power distribution experts at (847) 232-1991 or email our team at firstname.lastname@example.org. You can also click this link to contact our team directly.