Led Design Guide Page 7 LED Design Guide

2015 Littelfuse LED Lighting Design Guide www.littelfuse.com LED Lighting Design Guide In the case of non-isolated LED drivers which are prominent in many retrofit lamps and lower power applications, the LEDs themselves can be damaged by the surge energy as there is no transformer isolation between the AC input and the LEDs on the DC side. Isolated circuits, such as flyback converters, are more robust since their transformers provide some degree of isolation and surge protection. Nevertheless, protection is still required to limit higher surge events, especially in outdoor lamp and luminaire environments. High energy surge events, such as those caused by nearby lightning strikes can flash over from the primary to the secondary side. The MOV across the AC power input in Figure 3 is the key to providing surge clamping protection against lightning related transients. An MOV in this location is a cost-effective way to minimize transient energy that could make its way into downstream electronics. MOV selection is based on a number of parameters, including voltage rating, peak pulse current, energy rating, disk size and lead configuration. Littelfuse provides varistors with disk diameter down to 5mm (ZA series), which allows them to be used in the space- constrained power supply section of an LED lamp. Littelfuse Application Note 9767 provides a detailed explanation of MOV technology. The MOV and the fuse must be a coordinated solution so that energy needed to melt the fuse element (also referred to as "i 2 t" value) is high enough to allow the MOV to operate under surge conditions without causing the fuse to open (also known as nuisance opening). In other words, during clamping and dissipation of surge energy, the fuse should not open. It should only open when an overload or short circuit condition occurs. There will be instances when the circuit is not able to survive surge events, even with a varistor on the AC input. In these cases, a TVS diode can be added at the input of the power converter stage. Referring again to Figure 3 , the TVS diode provides secondary protection against transients. The "let through" energy from the varistor will be further clamped by the TVS diode to a level that the circuit can survive. Littelfuse has many options for fuses, MOVs and TVS diodes, based on form factor, space constraints, electrical parameters and cost. (See embedded links in the green boxes of Figure 3 .) Safety and Reliability of LED Bulbs (continued) 7

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