Led Design Guide Page 18 LED Design Guide

LED Lighting Design Guide 2015 Littelfuse LED Lighting Design Guide www.littelfuse.com Design Considerations in MOV Selection To properly select the right MOV for the AC circuitry of an LED luminaire, you normally need to know: 1. The maximum system RMS voltage. 2. The MOV continuous voltage at 10% above maximum system voltage. For example, for an LED luminaire rated 277Vac, a 320Vac MOV is recommended. 3. The worst-case transient energy that will need to be absorbed: the surge level (max. voltage/current) and number of surges. 4. Consideration of MOV degradation and indication. Especially in outdoor LED lighting applications, the MOV is continuously exposed to AC mains voltage transients which can cause degradation in the MOV. Once the MOV degrades, it becomes more conductive. Continuous degradation will reduce the resistance, and in turn increase the leakage current in the MOV to ultimately cause thermal runaway. In contrast, thermally protected varistors (TMOVs) protect against thermal runaway. As they heat up due to degradation, they have the ability to disconnect from the power source, and thereby avoid catastrophic failure that results in the creation of smoke or fire. In a typical outdoor lighting application, TMOVs or multiple TMOVs in parallel are placed across each of the conductive pairs, L-N, L-G and N-G ( Figure 9 ). This is the best case protection configuration for meeting surge suppression requirements. Fuse Line TMOV Varistor TMOV Neutral Ground 120VAC Integrated Thermal Element MOV Disk Figure 9. TMOV varistor offline protection scheme. Design Considerations in MOV Selection 18

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