Circuit-protection devices guard against electrical transients (MAGAZINE)

Open LED protectors can cope with significant over-voltage transients and keep a series string lit when one LED fails as an open circuit, say Phillip Havens, Jim Colby, and Teddy To.

Sep 20th, 2010
LEDs are fragile devices subject to heat, mechanical shock, electrostatic discharge (ESD) threats, and lightning induced surges. The increasing use of LED strings for solid-state lighting (SSL) and backlit displays is requiring more attention by designers to LED string reliability. High brightness LEDs built on sapphire substrates are especially sensitive to electrical transients. Even in household applications, LED strings require ESD protective devices to assure long, reliable operation of the entire assembly. In the absence of such protection, if one LED in a series string fails and opens the circuit, all the other LEDs turn off. Designers should consider circuit protection options from the AC line through to the individual LEDs.

A wide variety of protective devices can be considered for the power supply and LED driver. The circuit in Fig. 1 is an example that illustrates switch mode power supply (SMPS) protection in an LED street-lighting system. In this circuit, the AC fuse provides basic fire protection against major system failures that could cause an overcurrent condition but must be able to tolerate between 3kA to upwards of 6kA surges without opening. The DC fuse is for fast acting overcurrent protection in the event of a downstream component failure in the DC-to-DC converter or LED driver circuit.

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This article was published in the September/October 2010 issue of LEDs Magazine. To read the full version of this article, please visit our magazine page, where you can download FREE electronic PDF versions of all issues of LEDs Magazine. You can also request a print copy of LEDs Magazine (available by paid subscription) and sign up for our free weekly email newsletter.

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