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Gauging LED lifetime in optocouplers

Sept. 20, 2012
In LEDs heat and high currents hasten the dimming process which can lead to unreliable operation in LED-powered optocouplers. Some simple and straight-forward analysis will keep their light shining bright.

Authored by:
Foo Chwan Jye
Product Manager, Isolation Products
Avago Technologies
San Jose, Calif.

Edited by Robert Repas
[email protected]

Key points:
• Light output of an LED drops with intrinsic aging.
• Heat and high forward currents destroy the crystalline structure of LED junctions.
• The current-transfer-ratio of most LEDs degrades no more than 10% over a normal lifespan.

Avago Technologies

Optocouplers primarily transmit information or data between two circuits that have different electrical potentials. To transmit this data without error, the optocoupler needs two essential specifications: high-voltage isolation and good electrical-noise rejection.

In addition, these devices are expected to provide many years of reliable operation. These demands all but dictate optocouplers employ high-reliability LEDs to fulfill mission-critical needs.

LEDs are considered a mature technology. The first practical LED was developed 50 years ago, though evidence suggests that it was discovered even earlier. LED vendors have continually enhanced the manufacturing process to improve and refine LED performance. High reliability optocouplers let vendors offer products for industrial, renewable energy, automotive, and even ultra-high mission-critical applications for aerospace and the military.

Despite the many harsh applications that employ optocouplers, concerns still linger regarding optocoupler operating life. Much of this concern centers on the output of light from the LED within the optocoupler. As the LED ages, its light output power drops from intrinsic aging. That light output loss could potentially degrade optocoupler performance.

These three graphs show the drop in CTR over a 30-yr span for the different types of LEDs currently used in optocouplers. Note that the average value for all three types is less than 10%, though individual devices can reach a 20% drop or more.To ease those concerns, optocoupler manufacturers project the amount of LED degradation based on the Black Model. The Black Model uses LED reliability stress data obtained under accelerated conditions. It is a generally accepted empirical model developed at the end of the 1960s by J.R. Black to estimate the mean-time-to-failure (MTTF) of a 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.

Optocouplers use the LED to transmit a beam of light carrying information across an isolation or insulation barrier. Many times the isolation barrier is 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. During the design phase, engineers select the proper value for the input current-limiting resistor that defines the recommended input drive current (IF) to the LED to produce the desired light output.

Problems arise when the quantum efficiency of the LED (the total photons emitted per electron of input current) drops over time from thermal and electrical stresses within the LED’s PN junction. Stress testing the LED can help determine reliability for periods of continuous operation up to 10,000 ƒhr. Unfortunately optocoupler vendors don’t always provide stress test data in the device datasheets.

Furthermore, the conditions under which each vendor conducts their stress-test may differ making it difficult to directly compare test results. For example, the IF used to test the LED can be set at any value ranging from a few milliamps to 20 ƒmA or more. Higher currents place more stress on the junction and could lead to widely varying reliability figures.

One of the stress tests performed by Avago Technologies, San Jose, is the high-temperature operating life (HTOL) test. This test takes place with the LED operating at a temperature of 125°C and a continuous IF of 20ƒ mA. The test monitors the current transfer ratio (CTR) that compares the ratio of the output collector current (IC) to the forward LED input current (IF) that generates the light. The amount of light hitting the photodiode determines collector current, so the drop in CTR over time plots the drop in light output as the LED degrades.

Even though the IF stays constant, the light output from the LED drops over time because of heat stress in the LED crystalline structure. Thus, the photosensitive device IC and CTR drops with time. At predetermined points in the stress test, such as at 168, 500, and 1,000 hr, IC is measured and the CTR calculated. Performance plots then show the change in CTR against the number of hours that the stress test ran. LEDs used in optocouplers are fabricated from either aluminum-gallium-arsenide (AlGaAs) Type 1 and 2 or gallium-arsenide-phosphide (GaAsP). Each vendor optimizes its optocouplers to leverage the LED technology that best suits the application. Additionally, optocoupler vendors have developed a wide variety of devices that have various types of interfaces to suit many different applications.

At Avago, for example, Type 1 AlGaAs LEDs are mainly used in optocoupler product families that include digital optocouplers, isolation amplifiers, gate drivers, and IPM drivers. Type 2 AlGaAs LEDs mainly go into Avago’s optocoupler families that handle high-speed digital signals and low-power 10-Mbps digital optocouplers. Lastly, the GaAsP LEDs find use in a broad range of optocouplers, from digital optocouplers, analog optocouplers, gate drivers for intelligent power management, and many other applications.

The LEDs are manufactured using different processes. One type might use a diffusion-type process while another type would use epitaxial growth. LEDs are also subject to different doping concentrations. This lets the LED designer customize the light output power of the LED at different current flows to address speed and power performance needs of the optocoupler.

Among the three different LED types, GaAsP-based LEDs are the most mature but have the lowest light output power. AlGaAs Type 1 offers the highest light output power that lets them serve in more-stringent isolation applications that need high creepage/clearance distances inside the optocoupler package. AlGaAs Type 2 has a performance level that falls between the other two LED types. It’s used in a wide range of applications that require speed or power performance. All three LED types have similar degradation qualities, typically experiencing less than a 10% loss from the original CTR value after 30 years of field operations.

Acceleration factor

Obviously, it’s not practical to test an LED for it’s full 50,000 hr-plus expected lifespan. An acceleration factor (AF) based on the Black Model can be used to correlate actual HTOL stress test data points to the expected lifetime based on the actual operating conditions. But like the stress test data, not all of the values needed to calculate the AF are on the data sheets. In many instances, the only way to obtain the necessary information is to contact the vendor.

where AF = acceleration factor; Jacc = accelerated current density (HTOL stress-input current); Jnorm = nominal operation-current density (application operating input current at 100% duty cycle); Ea = activation energy of 0.43 eV; K = Boltzmann’s constant of 8.62 × 10-5 eV/K; Tnorm = nominal operating temperature (application ambient operating temperature); Tacc = accelerated operating temperature (HTOL stress temperature); and N = model parameter of 2.

For the same CTR degradation performance, the field lifetime of the LED can be projected according to the following equation:

TF = AF × TS

where TF = field time of the LED in hours, AF = acceleration factor, and TS = stress time of the LED in hours.

Data from the stress test of an Avago optocoupler will illustrate the use of the AF as a multiplier. The stress data conditions include: IF = 20 mA, temperature = 125°C, and LED-type “AA.” At a stress test length of 1,000 hr, the CTR degradation is measured as 99.2%. If that optocoupler is used with actual application conditions of IF = 5 mA, assuming a 100% duty-cycle operation, and an ambient temperature of 60°C, the AF is calculated as:

The projected field lifetime for the LED becomes AF × TS, or 184.7 × 1,000 = 184,767 hr (or 21 years). With the AF value calculated, all data points of stress hours can map to the expected field lifetime of the LED.

Typically, at the end of 30 field years of operation, the CTR of most LED types degrades no more than 10%. Depending on the system-design-to-expected-lifespan usage, the degradation calculations give designers more flexibility in choosing the IF value. They can, thus, optimize their system designs to get the best combination of reliable operating lifetime and power consumption.

In general, there are three basic factors to consider in maximizing the LED operating lifetime: Operate the LED at a lower IF, at a lower duty cycle, and at an ambient temperature of less than 125°C.

Automotive use

Automotive-grade optocouplers must address all emerging automotive isolation applications, particularly in applications that demand reliable long term operation at ambient temperatures as high as 125°C. Tests show that the LEDs used in these applications must possess a CTR drop of no more than 20% at 125°C to meet automotive requirements. Commercially available consumer-grade and general-purpose industrial infrared LEDs typically experience a CTR drop of up to 60% at 125°C. As this is beyond the recommended temperature range for commercially available LEDs, it highlights the importance that only LEDs specially designed for high-temperature operation should go into automotive-grade optocouplers.

© 2012 Penton Media, Inc.

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