Led Design Guide Page 6 LED Design Guide

LED Lighting Design Guide 2015 Littelfuse LED Lighting Design Guide www.littelfuse.com Littelfuse offers complete AC line interface protection as well as transient protection on the DC side. Its LED Lighting Design video discusses reliability and safety issues that need to be addressed in selecting protective devices for LED retrofit bulbs and luminaires to meet industry standards. Refer to Figure 3 for an overview of the various components and systems within the power and control circuits inside an LED lighting assembly. As shown in Figure 3 , a Littelfuse AC fuse in series with the line will provide safety protection against short circuit and overload conditions. They are available in a wide range of form factors, amperage ratings, voltage ratings, breaking capacity and mounting options to provide design flexibility to the design engineer. The AC/DC power conversion circuit can use either isolated or non-isolated topologies. Transient protection is not only critical for the DC side electronics but more importantly the LED load. The ICs can be protected by nature of isolated bias windings and filtering elements. However, insufficient spacing on the PCB, flashover of physically small components, or unintentional coupling through transformers and opto couplers can fail due to inadequate transient protection. A well designed surge protection scheme will limit the peak voltage and current envelopes to avoid coupling through unintentional paths. DC-DC Converter Line Rectifier Controller / Opto Couplers EMI Filter AC Input SPD L N G High Voltage DC Fuse Constant Current LED String Driver LEDs Iso / Step Down Osc. A fuse with adequate I 2 T rating is necessary to pass Energy Star ring-wave tests according to IEEE C.62.41 A metal oxide varistor (MOV) can help protect the LED bulb from overvoltage surges and ring-wave effectes by clamping short-duration voltage impulses. A TVS Diode on the dc side can be used to protect against transients before entry into converter circuit. Open circuit LED protection device TVS Diode Transient protection for energy let through to the LED circuit MOV Surge protection device with high surge withstand MOVs (e.g., 25 - 34mm dia. Sizes) A positive temperature coefficient resistor can help protect LEDs from over-current and over-temperature. Equipment that is directly connected to AC mains (100 to 240VAC) can be damaged by short circuit overload conditions, lightning surges or load switching transients that create voltage spikes or ring waves Click/toggle the green boxes below to view our product lines. Figure 3. Typical LED luminaire driver circuit with transient and surge energy protection devices Safety and Reliability of LED Bulbs (continued) 6 PLED PTC TVS Diode MOV AC Line Fuse

Previous Page
Next Page