Knowledge How Long Do LED Lights Really Last?

Shine

Administrator
Staff member
One misconception of LED lighting is that it lasts forever. A majority of consumers and pseudo experts take the useful life of LED light sources as a proxy for the lifetime of an LED lighting system. The useful life of an LED package is often cited as the lumen maintenance life. The lumen maintenance life typically refers to the L70 life which is the time until initial light output has degraded to 70%. An LED light source is a semiconductor package which converts electrical energy to light as a result of electron-hole recombination. The nature allows LEDs to exhibit a very long lumen maintenance life. Depending on drive current and operating conditions, LEDs can have an L70 life of up to 200,000 hours.

However, equating the life of LEDs with the life of an LED lighting system (a lamp or luminaire) is either a marketing bullshit or sheer ignorance. While long lifespan is certainly a key advantage of LEDs over incandescent, fluorescent and high-intensity discharge (HID) lamps, an LED cannot operate on itself. LEDs are in fact very delicate light sources that perform well only in optimally controlled environments. Unlike many conventional sources such as incandescent bulbs that are voltage driven, LEDs are current driven devices. To produce flicker-free light and protect LEDs from damages caused by electrical overstresses, it's necessary to transform commonly available sources of alternating current (AC) electrical energy into a direct current (DC) supply that has outputs matched to the electrical characteristics of the array of LEDs. Furthermore, LEDs are self-heating devices. They lose around 50% of the electrical power input as heat that needs to be removed from the semiconductor dies. An LED will fail soon if the waste heat is allowed to accumulate.

Indeed the lumen maintenance life of LEDs is a questionable statistic because it's obtained from laboratory tests. Lumen maintenance measurements are taken with the LED packages operating continuously in a controlled environment—temperature, humidity and electrical conditions are defined by LM-80. Moreover, the lumen maintenance life is a projected statistic which is predicted using the LM-80 test data. There's usually not a good match between predictions based on laboratory test results and practical experiences with LED lamps and luminaires in real-world applications. Under constant current operation at room temperatures and with efficient heat dissipation, the service life of LEDs is impressively long. However, the junction temperature of LEDs in the field may be significantly higher than that of LEDs operated in the lab environment due to inefficient thermal management, high ambient temperatures, and/or LED overdriving. Beyond the maximum rated junction temperature, an LED can experience a 30% to 50% decrease in its useful life for every 10°C increase in temperature. This means the life expectancy of an LED that is rated for 60,000 hours when operated at a junction temperature of 85°C would be reduced to 40,000 hours when its junction temperature is raised to 95°C.

The reliability characteristics of LEDs deem that LED operation is interdependent upon drive electronics, the thermal management system, and environmental conditions. Therefore, a holistic approach is necessary to provide an acceptable LED lighting system. The design of an LED lamp or luminaire is a multidimensional engineering work. Failure of any part may lead to the whole system. It isn't just about the LED. Long life LEDs can be incorporated into poorly engineered systems. When LEDs are installed in a lamp or luminaire, many variables can affect the rate of lumen depreciation or lead to catastrophic failure. Poor system design and engineering can accelerate wide range of failure modes and mechanisms in LEDs. These include defect and dislocation generation and movement, die cracking, dopant diffusion, electromigration, electrical overstresses, electrical contact metallurgical interdiffusion, electrostatic discharge, carbonization of the encapsulant, delamination, encapsulant yellowing, lens cracking, phosphor degradation, and solder joint fatigue.

No LED lighting system today is designed and engineered without compromise. Due to cost sensitivities, the thermal management system and driver electronics are often traded for lower cost system designs. While thermal management is critically important to achieve a long lumen maintenance life and good color stability, it's not the contributing factor to catastrophical failures of LED lighting systems. The processes of temperature-induced LED failures such lumen depreciation and color shifts are slow and largely unnoticeable at early stages. What this means that poor thermal management may lead to irreversible reductions in LED efficiency and service life, but the LED could still operate for a period that is significantly longer than traditional light sources.

The life expectancy of LED lights is always a combination of gradual light degradation and abrupt failures. Typically, the weakest link in an LED system is the driver which can contribute to most of the abrupt failures. The design of LED driver circuits imposes special considerations in many aspects, ranging from current regulation, power factor correction (PFC), PWM/CCR dimming, and EMI filtering and screening to surge protection. As a semiconductor devices, LEDs are particularly susceptible to electrical overstresses which are often associated with catastrophic failures. Therefore all necessary precautions should be taken to protect the LEDs from EOS events such as an electrostatic discharge (ESD), in-rush current, or other types of transient electrical surge. The driver is the first component of an LED lighting system to fail. The key factor influencing the lifetime of a switching power supply is the electrolytic capacitor. The operating life of electrolytic capacitor is highly dependent its operation conditions such as voltage, current, frequency, and temperature. The rate at which the electrolyte evaporates depends on the temperature of the capacitor. Every 10º C drop in the operating temperature of an electrolytic capacitor causes its lifespan to be reduced by half. To ensure the lifetime of the driver will match that of the LED array, a long-life, high operating temperature capable electrolytic capacitors should be used.

The trade-off between cost and reliability causes the useful life of LED systems to be compromised at various levels. This is especially true in the LED replacement lamps (e.g. A19 light bulbs, T8 tubes) of which costs are dropping to commodity levels. Typically, LED lamps with integrated drivers have L70 lifetimes between 8,000 and 25,000 hours. It's not uncommon to see LED light bulbs that fail in their first year of use. Most of the claims that LED light bulbs can work for over 15 years is flat-out lying. Integrated LED fixtures such as troffers, downlights, high bay lights and street lights generally have lifetimes of 50,000 hours or beyond. This is because end-users of these products have higher expectations from LED lighting and are more willing to make value-based purchasing. These products are designed from scratch with the intention to provide an optimal operating environment for the LEDs. On the other side, the design of LED lamps is constrained by limitations of the legacy form factors which leave no adequate space for accommodating large heat sinks and complex driver circuits.
 
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