As the automotive world is becoming ever more electrified the power requirements are changing, so have the protection needs. Fuse manufacturers are continually developing designs to meet these ever-changing requirements.
Our experience in protecting semiconductor devices has proved invaluable as vehicle powertrain systems have moved to power-based converters for the variable speed motor drives and for auxiliary power conversion.
There is an increasing expectation for component manufacturers to respond to the challenging demands placed on them by leading car manufacturers. The unique and dynamic environment of the electric vehicle places additional and often unknown stresses on internal electrical components pushing industrial components beyond their capabilities. We will explore the continued design challenges we face as we continue to see flaws in the use of industrial fuse links in Hybrid and Electric Vehicles (HEV) applications.
Industrial vs EV applications
Electrical fuse links have been in use since the earliest days of electric telegraph and power distribution protection. Since their conception electrical fuse links have been subject to ongoing development to meet the ever-changing application uses, for example cable protection, transformer protection to switches, batteries, photovoltaic (PV) and rail systems. The arrival of HEV applications brings with it a new set of design challenges for fuse links, with each application having varying requirements, an in-depth understanding of the environmental parameters and typical drive cycle profile is key to selecting a suitable fuse link for such a demanding environment.
Industrial fuse links are designed and tested to known standards IEC 60269 and UL 248. The behavior of fuse links in conditions applicable to these industrial standards have been researched and understood drawing conclusion to derating considerations in environments where fuse links are subject to conditions differing from the standard. The challenge in EV applications is that the conditions are often outside the researched behaviors or even outside the requirements of the standard itself. Essential environmental and electrical conditions must be considered when selecting an appropriate fuse link for electric vehicle protection.
Voltage and current dimensioning
Traditional automotive batteries were mostly lead-acid batteries rated at 12 V d.c., 24 V d.c. or 42 V d.c. Today however, EV batteries are moving to Lithium-Ion and can range from 150 V d. c. to 800 V d.c. as car manufacturers strive to improve their vehicles power, range and charging time. In EV applications, electrical fault conditions can reach as high as 950 V d.c. and components must be able to operate safely at this voltage level. This is a particularly important requirement for a fuse link, which must be able to safely interrupt the maximum system voltage when a fault occurs. The voltage rating of an industrial fuse link is usually defined in AC RMS voltages, and few industrial fuse links have an assigned DC rating.
DC faults are notoriously more difficult to clear than AC faults, the sinusoidal nature of alternating current assists with extinguishing the arcs inside the fuse during operation, this is not the case within a DC system where the voltage remains constant. Two variables should be considered in a DC system:
- Fault circuit time constant (L/R)
- Minimum prospective short-circuit current
It is not possible to define one DC voltage rating to cover considerable varying fault conditions and therefore specialized fuse links and specific application testing become the only option under DC conditions. Typically, the time constant of the fault conditions is < 5ms limiting the complexity of design, however the short-circuit level is variable depending on the state of the battery during a fault and the minimum prospective short-circuit current level can often be very low.
The fuse link rated current is the RMS current it can continuously carry without degrading or exceeding the applicable temperature rise limits under well-defined and steady-state conditions. The well-defined conditions for industrial fuse links are stated in standard IEC 60269 for the following application conditions:
- Ambient temperature: Between 10o C and 30o C (lowest temperature conditions specified at -5o C)
- Current density of busbars: 1.3 A/mm2
- Open air
- Steady-state (no cyclic loading)
- Static conditions (no vibration)
In an automotive environment however these parameters tend to differ significantly, and as such the fuse’s rating needs to be reassessed for each specific application to ensure that the selected fuse link is not run beyond its current carrying capabilities; as this will lead to premature aging of the fuse and will cause it to nuisance operate. The following should be considered when selecting a fuse for your application.