Led Design Guide Page 23 LED Design Guide

2015 Littelfuse LED Lighting Design Guide www.littelfuse.com LED Lighting Design Guide Littelfuse technology provides a fully integrated, thermally self-protected MOV in its TMOV Varistor Series. These new devices use a patent-pending thermal element internal to the MOVs in direct contact with the metal oxide disks, allowing for optimum heat transfer. Because of the proximity of the thermal element to the MOV body, a higher opening temperature element can be used. This allows the thermally self-protected MOV to be wave soldered, and thereby simplifies OEM assembly processes. This construction method also allows a new device to perform to standard MOV ratings with regards to peak current, peak energy, voltage clamp levels, etc., while providing the safety of a thermally protected device. Figure 13 illustrates the integrated components of a TMOV. Fuse Line TMOV Varistor TMOV Neutral Ground 120VAC Integrated Thermal Element MOV Disk Figure 13. TMOV varistor offline protection scheme. Design Considerations in Fuse Selection to Withstand Surges Fuses are devices whose function is to provide protection for components and complete circuits by reliably and predictably melting under current overload conditions. They are intended to be the weak links in the electrical circuit. Lab tests are run on each fuse design to determine the amount of energy required to melt the fusing element; this is called the nominal I 2 t rating and is measured in A 2 s. Often, the nominal I 2 t method of fuse selection is meant for applications in which the fuse must endure large current pulses of short duration. Surge immunity testing for LED lighting applications requires complying with 820s combination waveforms. Different fuse constructions may not react the same way to a surge, even if their nominal I 2 t rating exceeds that of the waveform energy. Electrical surge pulses produce thermal cycling that can fatigue the fuse mechanically and shorten its life. Design Considerations in Fuse Selection to Withstand Surges 23

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