Led Design Guide Page 22 LED Design Guide

LED Lighting Design Guide 2015 Littelfuse LED Lighting Design Guide www.littelfuse.com Protecting Against Temporary Overvoltage (TOV, IEC 61643) and Limited Current Abnormal Overvoltage (UL1449) by Using Thermally Protected MOVs In AC line applications, the loss of a Neutral-Ground connection may occur in such a way that there exists a risk that a sustained over-voltage may be applied to an MOV that is rated for a much lower continuous voltage. In an unlimited current condition the MOV will first fail to a low impedance (a few Ohms), but due to the high amount of energy available, it most often ruptures instantaneously. If, however, there are loads tied to the AC line that limit current flow, the MOV can overheat and potentially cause the SPD device to overheat resulting in smoke, out-gassing and eventually fire. For example, in a standard U.S. 120V AC Line application, two 120V AC power lines (180 degrees out of phase) are commonly fed from a center-tapped 240V transformer ( Figure 12 ). Let's assume an MOV rated at 150V is present in the top 120V circuit, and some load exists on the bottom 120V circuit. Both the MOV and load share the center tap which is the Neutral- Ground Connection. If a break occurs on the center tap (X-X in Figure 12 ), then the load in the bottom phase acts as a current limiter and the line fuse may not clear. Fuse MOV rated for 150V rms continuous voltage Load 120V Figure 12. Possible fault conditions that could cause a limited current abnormal over-voltage event. In this scenario, the 150V rated MOV is subjected to 240V at a limited current, potentially resulting in thermal run away for the MOV. This potential condition is specifically identified and addressed in the UL1449 and IEC 61643 Surge Protective Device Standards. As mentioned earlier in this document, new TMOV technology has been developed that will aid the designer in protecting against thermal overload in varistors. This in turn provides protection against: Temporary over-voltage (TOV) conditions as described in IEC 61643 Sustained abnormal over-voltage with limited current as described in UL1449. Protecting Against Temporary Overvoltage 22

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