Calculate The LED Lifetime Performance In Optocouplers To Predict Reliability White Paper

Avago optocouplers employ high-reliability LEDs to fulfil the critical system reliability requirements. LED technology is a mature technology and over the 35 years since it was introduced, Avago has continually enhanced the manufacturing process to improve and refine LED performance. This allows Avago’s optocouplers to find homes in industrial, renewable energy, automotive and even ultra-high mission critical applications, such as military and aerospace applications.

Despite the many harsh applications that employ optocouplers, there are still concerns regarding theoptocoupler operating lifetime since LED light output power decreases over time. That optical power reduction could potentially result in the optocoupler performing improperly and unreliably. This article addresses the performance concerns by showing how Avago, an industry leader in optocouplers, uses LED reliability stress data under accelerated conditions to project expected LED degradation performance based on Black Model (a generally accepted empirical model developed at the end of the 1960s by J.R Black to estimate the mean-time-to-failure of wire associated with electromigration). The analysis gives designers greater confidence and design flexibility so they can specify the most appropriate LED forward input current for their application.

LED Reliability Stress Tests

Optocouplers use a LED to transmit digital or analog information across an isolation (or insulation) barrier (often just an air gap). On the other side of the barrier is a phototransistor or other light-sensing device that converts the optical signal back into an electrical signal. Designers can set an input current-limiting resistor that defines a recommended input drive current to the LED to produce the desired light output. However, the optocoupler’s LED quantum efficiency (total photons per electron of input current) decreases over time due to thermal and electrical stressing of the LED PN junction. Avago performs stress testing to determine LED degradation for periods of continuous operation up to 10,000 hours for the various LED types used in different models of their optocouplers. One of the stress tests, a High Temperature Operating Life test is performed with the LED operating at 125° C and a continuous IF of 20 mA.

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