Edited by Robert Repas, [email protected]
The light-emitting-diode (LED) lighting market is growing quickly, thanks in part to aggressive global government mandates. LEDs now go into such critical safety-lighting applications as traffic signals and airport-runway lighting. One result: Concerns about quality and reliability now extend even to the least-expensive LEDs.
A problem for design engineers is that LEDs are fragile solid-state devices susceptible to damage by transients and surges. An LED is essentially a P-N junction diode built to emit light when forward biased. A major cause of electrical opens in an LED is thermomechanical stress on the wire bonds. Another cause of LED failure is electrostatic-discharge (ESD) events or surges induced by nearby lightning strikes. This is especially true in outdoor applications.
A typical LED installation consists of an LED driver that regulates the amount of current to a string of LEDs connected in series. However, like old-style Christmas lights, a single failed-open LED blocks the current to all LEDs in the string, extinguishing every LED. Instead of a single dark LED, the lighting device might see 10 to more “failed” LEDs from the single breakdown.
But new “open LED” protection technologies and devices can prevent an entire string of LEDs from going out if a single LED fails. They boost reliability where it is critical for safety or reducing maintenance.
Open LED protection takes the form of an electronic shunt that bypasses current around the open circuit resulting from a single LED failure. It is an internally triggered two-terminal device which automatically resets if the LED heals itself or is replaced. This protector is a voltage-triggered switch with current leakage of only a few microamps when off and a low-impedance path when triggered on. The low-leakage, low-impedance extremes minimize power consumption within the device itself. The device is placed in parallel with the LED and is normally at high-impedance. The voltage drop across a failed-open LED triggers the protector to the on-state, thus shunting string current around the defective LED. Built-in surge immunity helps protect the LED from voltage and current surges induced by nearby lightning strikes or ESD events.
A few special features make the Open LED protector versatile. First, it has an operating temperature extending from -40 to 150°C. Thus the devices need minimal derating over extreme conditions. Many brightness-controlled LEDs use duty-cycle changes in switched power as a means of intensity control. The Open LED device is compatible with LED switching speeds up to 10 kHz without randomly triggering the protector.
One final word of caution: As beneficial as open LED protection may be, it is only one part of total protection that may include MOVs and other protective devices.