POWER & ENERGY EFFICIENCY HANDBOOK
Fusible resistors vs fuses Safety standards often dictate what component provides the best protection against overcurrents. Todd Phillips, Saad Lambaz • Littelfuse, Inc.
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people would probably agree that the time it takes to recharge their cell phone, gaming console, or tablet computer is both inconvenient and annoying. Faster charging requires a more powerful charger or power supply. The design of such supplies requires that designers emphasize device safety while also meeting cost, size, and efficiency constraints that are more stringent. As power levels rise, so too does the need for overcurrent protection. There are generally two different approaches to overcurrent protection: a conventional fuse or a fusible resistor. Fusible resistors have the benefit of combining overcurrent protection and inrush current protection in one component. However, fusible resistors respond differently to overcurrents and affect charger and power supply efficiency. A fusible resistor opens like a fuse when its current rating is exceeded. The component is generally a nichrome element with a melting temperature of around 1,400°C. Nichrome has a low thermal coefficient of resistance which allows the resistor to have a stable resistance over temperature. The 1,400°C melting temperature heats up surrounding components and the PCB during an overcurrent condition. Fuses are generally copper or silver elements with a melting temperature typically between 962°C and 1,083°C. Fuses also have a high thermal coefficient of resistance, at least a factor of 10 higher than a nichrome fusible resistor. Thus, the temperature of a fuse will rise faster during an overcurrent condition. The fuse resistance rises to bring the fuse to its melting point sooner. A fuse will prevent a heat build-up that happens when a fusible resistor experiences an overcurrent condition. The higher heat generated by the fusible resistor can damage other components and potentially lead to the ignition of nearby combustible components. The primary benefit of a fusible resistor is that its resistance limits inrush current. A fusible resistor that serves as the main overcurrent component in a power supply or charger can have
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DESIGN WORLD — EE NETWORK
10 • 2020
Examples of fusible resistors and fuses. Top, the Vishay ACxx-CS Series fusible resistor. Bottom, the Littelfuse 443 Series fuse.
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