LED Technology
The first line of
defence
LEDs are being found in a lot more outdoor applications, however adverse environmental conditions pose high risks for surges and other sources of damage. It’s important, therefore, that design engineers think about proper protection for these sensitive products, as Bharat Shenoy explains
L
ED lighting does offer many key advantages, including an extremely long lifespan, high efficiency, and a
broad variety of different colours. Thanks in part also to falling prices and global government mandates to reduce energy consumption, high-brightness LEDs (those that currently feature sapphire or SiC substrates, and AlN and GaN substrates in the future) are already being used in many different lighting applications like outdoor display boards, traffic signals and airport runway lighting. And forecasters are predicting a further significant growth in the LED market in the coming years. Despite the fact that they are known for
their efficiency and reliability, LEDs are really extremely delicate devices. Thermo- mechanical stress on the wire bonds is considered to be one of the main causes of an electrical open circuit in an LED. Other possible causes include electrostatic discharge (ESD) events or surges induced by nearby lightning strikes. An open circuit or damaged LED inside a string can cause the entire string to go dark, and if the product is used in a mission-critical application, like an airport runway light, for instance, the results could be catastrophic.
Areas needing protection Until just recently, few solutions were
26 November 2013
available for protecting LEDs in harsh outdoor environments. To make matters worse, even experts are often unaware of all of the considerations involved in specifying circuit protection for LED strings. So, although a new group of devices known as open LED protectors are now available, they are not yet well understood, despite how important it is to provide circuit protection for the entire LED portion of a product design. This article looks to address this and to
provide a few design tips when it comes to incorporating techniques such as over- voltage suppression and open LED protectors in an LED design. In an LED lighting application, three
main areas require circuit protection: the AC portion of the circuit prior to DC rectification; the DC section which is also called the LED driver, and the LEDs themselves.
Protect the AC line
An LED lighting set-up usually has a switched-mode power supply (SMPS) on the front end and an SMPS is in even greater need of protection than linear power supplies - proper sizing of the AC line fuse is a good place to start. Exterior AC sources are highly susceptible to nearby environmental surges. Lightning
Components in Electronics
surges, for example, are particularly problematic and as a result the line fuses need to be sized appropriately so they do not nuisance trip during surge events. Typical surge current can vary from 3KA up to 10KA for the highest exposure environments.
The main criteria for selecting the line fuse for the AC input include the voltage, current, and the I2T rating. This last parameter is related to the amount of energy the fuse element can withstand instantaneously without opening. Increasing the current or time delay rating for a fuse naturally increases I2T. For these high exposure outdoor environments, a high I2T fuse is required to withstand the
lightning surge currents and keep the luminaire operational. Another requirement in protecting a SMPS is through the use of a metal-oxide varistor (MOV). A MOV suppresses an overvoltage event as a shunt in the input circuit and is place in parallel with the power line or that particular mode that is being protected. A surge causes the MOV to lower its resistance and divert the surge away from the power supply. The main difference between fuses,
which are overcurrent devices and MOVs, which are overvoltage devices, is that fuses are specified as safety requirements, while a MOV is considered a reliability device.
Open LED
www.cieonline.co.uk
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