Led Design Guide Page 20 LED Design Guide

LED Lighting Design Guide 2015 Littelfuse LED Lighting Design Guide www.littelfuse.com Surge Protection Modules and MOV Coordination Generally, most LED power supplies are a constant current type, and are often referred to as LED drivers. These can be purchased as off-the-shelf assemblies containing MOVs to meet lower level surge requirements. Typically, drivers are rated to handle surges in the range of 1-4kV. The varistor (MOV2 in Figure 11 ) is usually located after the fuse on the AC mains, and can range from 7-14mm in diameter. However, to provide higher level surge immunity, outdoor lighting OEMs may want to add surge protection devices (SPDs) on the AC input lines of their luminaires ahead of the LED driver. Typically, compliance testing of luminaires involves 20kV/10kA surges in North America and up to 10kV/5kA 1 in Europe. SPD modules are responsible for protecting luminaires against these high surge levels, which can occur in outdoor lighting environments. The SPDs use 3-4 parallel or series connected high-surge withstand (e.g. 25mm or 34mm diameter) MOVs across the AC lines, as shown in the green MOV1 block of Figure 11 . As shown in that figure, the MOVs are installed from line to ground, neutral to ground and line to neutral. For installations in regions with severe lightning exposure, it is common to use parallel connected MOVs for the line to neutral leg. This will increase differential mode surge capability and the reliability of a luminaire. When adding this supplemental protection in front of the LED driver, it is very important to select (MOV1) characteristics that coordinate with those of the existing (MOV2) device in the driver. The coordination criterion for MOV1 selection is to make sure these larger disc MOVs in the SPD module clamp first, thereby taking the brunt of the surge energy before the smaller (MOV2) disc turns on. This will avoid catastrophic current through the driver MOV and premature opening of the fuse, which happens if the driver MOV turns on first. Therefore, The MOVs in the SPD module should have a lower max continuous operating voltage rating than the MOV in the driver. 1 Although North American luminaire standards characterize immunity testing as a 10kV/10kA requirement, the test setup calls for a 2 source impedance, and it is actually a 20kV/10kA requirement. AC Input LED DRIVER SPD L N G MOV2 MOV1 L AC Input LED DRIVER SPD L N G MOV2 MOV1 Figure 11. SPD protection schemes that increase the surge immunity of an LED driver. Click the green boxes below for more information. Surge Protection Modules and MOV Coordination 20 UltraMOV 25S or TMOV25S Series UltraMOV 25S or TMOV25S Series

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