SSL Reliability: Only as Good as the Weakest Link

10.10.2012 // Murray Slovick // New Technology

Compared to traditional luminaires, solid-state lighting (SSL) luminaires are best viewed not simply as user-replaceable or interchangeable lamps but as integrated lighting systems requiring expertise in a wide variety of technologies. That’s why lighting solutions offered by distributors such as TTI include more than the LEDs themselves: the optoelectronics parts roster also comprises LED drivers, passives, interconnects, lenses, circuit protection devices, thermal management products and a variety of additional lighting accessories.

LEDs may have an expected lifetime of more than 40,000 hours (compared to 1,500 hours for a standard incandescent bulb). Not only does this very long life virtually eliminate the need for re-lamping, all adjunct components for LED lighting design must be selected carefully to ensure that they meet the lifetime and energy-efficiency requirements of the LED light source. If any one of the complementary parts fails, the system fails, and the engineer’s design fails to provide a viable solution to the client.

Since most of the electricity in an LED becomes heat rather than light (typically about a 70%/30% heat to light ratio), if this heat is not removed the LEDs run at high temperatures, which not only lowers their efficiency, but also makes the part less reliable (failure rates of LEDs are most often related to junction temperature and drive current). Usually LEDs are encapsulated in a transparent resin, which is itself a poor thermal conductor. Heat generated from the P-N junction by electrical energy that was not converted to useful light is conducted to the outside from junction to solder point, then to the board, to a heat sink and finally to the atmosphere.

LEDs will decrease in relative flux output as junction temperature (Tj) rises. Most LED data sheets list typical luminous flux at junction temperature (Tj) = 25°C, while most LED applications use higher junction temperatures. An LED rated at 50,000 hours with a Tj of 25°C will survive only half as long with a junction temperature of 125°C. At 175°C, this LED can be expected to survive only about 100 hours. Therefore, sufficient control of this parameter is critical if the LED is to reach the part’s expected lifetime.

A packaged LED incorporates a number of different materials with different coefficients of thermal expansion. As a result, temperature swings also can introduce variations in material expansion, leading to mechanical stress that can fracture the wires and can cause catastrophic failure via an open circuit.

One of the secrets to a successful LED fixture design, then, is proper thermal management; conducting away the waste heat produced by LEDs. The most prevalent method to fix thermal issues is using passive cooling. This requires a heat-sink engineered into the design that provides a low resistance path for the heat to flow away from the LED. In general, heat sinks can dissipate power in three ways: conduction (heat transfer from one solid to another), convection (heat transfer from a solid to a moving fluid, for most LED applications the fluid is air), or radiation (heat transfer from two bodies of different surface temperatures through electromagnetic waves).

To ensure long LED life, a heat sink must be adequately dimensioned and consideration must be given to the conducting materials, often aluminum or copper. Alternatively, forced convection can be used, with air pushed through the heat sink using a fan or an air jet. Even a relatively low air velocity of 2 m/s can cut the overall thermal resistance of a heat sink by half. Using a mechanical air mover to cool the LED, however, is also a more costly solution than the standard extruded heat sink. Another downside is noise, and the mechanical construct can also eat into the reliability overhead. Similarly, liquid cooling via pipes can transfer heat away from the LED assembly but it, too, comes with a cost and weight penalty.

Temperature issues also impact the LED driver power supply where the life of an aluminum electrolytic capacitor can decreases by approximately 50% with every 10°C increase in temperature. At higher temperatures dissipation of the liquid electrolyte from the capacitor reduces the cathode connection, resulting in a decrease of capacitance. The reaction rate of the dielectric materials — a combination of a liquid and filler — will double for each 10°C increase in operating temperature. As the capacitor degrades, its ESR, or equivalent series resistance (the sum of electrolytic resistance, dielectric loss and electrode resistance) increases, leading to an increase in output current ripple, which affects the input current to the LED, and in turn the LED’s light output and efficacy.

With an intelligent microcontroller it is possible to compensate for LED ageing and correct the color profile to ensure the light quality is consistent over the LED’s lifespan. Also, the microcontroller can lower the light intensity or shut down LED strings if the temperature threshold is surpassed. Overall the MCU can monitor safe operation and remotely communicate any problems to ensure the lifespan of the LED is maximized.

LEDs are typically low voltage devices and require only a constant current source, unlike fluorescent and high-intensity discharge (HID) lamps that require a high-voltage ignition source. The nature of LEDs means that small changes in voltage result in large changes in current – as well as dramatic changes in brightness. The primary function of the driver module is supplying a controlled power level over the operating temperature range for the LED (or LEDs) in order to maintain a consistent light output. The topologies used in LED drivers are Buck, SEPIC (Single Ended Primary Inductance Converter), Boost, Flyback, Forward and Buck-Boost. These topologies are suitable for a range of power capacities and for different customer design requirements.

LEDs can be adversely affected by electrical transients, such as ESD or electrical spikes generated by the mains and electronic drivers. Fortunately, LEDs do not generate high inductive spikes or surges like magnetically ballasted HID lamps that often necessitate additional filtering devices to prevent problems for AC power distribution systems. Nevertheless, an LED driver module usually includes some level of protective circuitry to handle common faults that can occur in electronic systems. SSLs also can be exposed to extreme surge conditions such as power-line lightning strikes. Any power-line-coupled transients can reduce LED lifetimes. Electrical overstress (EOS, in general terms defined as any condition where one or more pins on an IC are subjected to current and/or voltage levels that exceed the Absolute Maximum Ratings per the IC data sheet) or ESD, a single fast, high current transfer of electrostatic charge between two objects at different electrostatic potentials, can result in an open circuit and immediate failure.

The solution to ESD and EOS events is relatively straightforward. A transient-voltage-suppression diode (TVS) with a breakdown voltage of around 50 V can help protect the device. TVS diodes are silicon avalanche devices typically chosen for their fast response time (low clamping voltage), lower capacitance and low leakage current. Often, the TVS diode can tolerate a 5,000-W peak pulse and has a response time of better than 1 ps. Addition of a capacitor to smooth the input signal is another appropriate corrective action to prevent EOS failures.

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Statements of fact and or opinions expressed in MarketEYE by its contributors are the responsibility of the authors alone and do not imply an opinion of the officers or the representatives of TTI, Inc.

Featured Contributor:
Murray Slovick

Murray Slovick

Murray Slovick is Editorial Director of Intelligent TechContent, an editorial services company that produces technical articles, white papers and social media posts for clients in the semiconductor/electronic design industry. Trained as an engineer, he has more than 20 years of experience as chief editor of award-winning publications covering various aspects of consumer electronics and semiconductor technology. He previously was Editorial Director at Hearst Business Media where he was responsible for the online and print content of Electronic Products, among other properties in the U.S. and China. He has also served as Executive Editor at CMP’s eeProductCenter and spent a decade as editor-in-chief of the IEEE flagship publication Spectrum.

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