Led Design Guide Page 21 LED Design Guide

2015 Littelfuse LED Lighting Design Guide www.littelfuse.com LED Lighting Design Guide A certain amount of impedance between the primary SPD and the driver can be beneficial; perhaps a few microHenries will help ensure proper coordination. For example, a longer length of cable between the primary SPD and the driver may be sufficient due to the characteristic impedance of the wire. On the other hand, lead wires on the input side of the SPD should be minimized to prevent increased clamping voltage in the SPD module due to the characteristic impedance of those wires. The following steps summarize a design process that helps ensure enough inductance is present between the SPD MOVs and the one in the driver: 1. MOV1 and MOV2 need to be coordinated so that most of surge current/energy flows through MOV1. 2. Select MOVs with V M of MOV1 V M of MOV2, where V M is the Maximum Continuous Operating Voltage. 3. Select MOVs with V C of MOV1 V C of MOV2, where V C is the Maximum Clamping Voltage. 4. V MOV1 = V MOV2 + L (di/dt) 5. Inductance L may be added in series with (ahead of) the SPD. Increasing inductance L will result in better coordination as MOV1 absorbs the higher surge energy 6. Refer to the embedded links in the SPD blocks of Figure 11 for MOV technical information. See the Littelfuse Varistor Ordering Information Diagram for details on constructing a part number. The Littelfuse Application Team has years of expertise in such coordination requirements and has helped many customers optimize their circuit reliability while meeting surge immunity requirements. Surge Protection Modules and MOV Coordination (continued) 21

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