Advanced thermal control ensures LED longevity
White light-emitting LEDs have proven to be a disruptive technology, challenging all old forms of light production. Its potential includes: 1) ultra-long life (>35,000 hours), 2) extremely high light efficiency (theoretically about 250 lumens/watt), and 3) low-temperature operation, which has taken the lighting market by storm. These huge expectations for the technology have prompted LED manufacturers and the industry as a whole to develop testing standards to ensure that lighting products embody the performance expected by consumers, whether they are end users or OEM manufacturers. Unlike regular filament incandescent lamps, LEDs do not "burn out" but may gradually experience a decrease in light output depending on operating conditions.
Why do LEDs degrade?
LEDs are complex solid-state devices. Figure 1 shows a cross-section of a typical device, showing the various structures that make up a packaged LED. For simplicity, we divide the components into 3 areas. First, a semiconductor device containing a p/n junction, in the case of a conventional white LED, actually produces blue light with a wavelength of approximately 450 nm. Next is a phosphor layer, which absorbs blue light and converts it into a broad band of colors that the eye perceives as white, in much the same way as a fluorescent tube. , with a series of transparent layers encapsulating the semiconductor and lenses that collimate the outgoing light. Each of these three regions may be involved in device degradation, albeit through different mechanisms.
semiconductor junction. LED manufacturers pay great attention to the process and composition of the semiconductors that make up the p/n junction of the diode. However, all current devices are composed of materials classified as III-V materials: Ga, In or Al in combination with N, P, As in the corresponding columns 3 and 5 of the periodic table. Virtually all commercial devices are heterojunctions, meaning they are a combination of different chemical compounds. Furthermore, they are single-crystal structures formed by epitaxial growth via chemical vapor deposition. These layers are grown on substrates such as sapphire or silicon carbide. They are typically complex layered structures, so-called "quantum wells," that allow electron processes to be carefully manipulated to maximize the conversion of charge into light. One consequence of these different material combinations is the creation of defects due to mismatches in atomic lattice size and thermal expansion coefficients between layers. The result of these defects are atomic defects in the crystal lattice structure, either in the bulk of the material or at the interface between different materials. To generate photoelectrons injected from majority carriers, the n-type doped layer recombines with holes injected from the p-type contact within the junction to form blue light. However, not all electrons and holes recombine to create light, otherwise we would have much higher performance! Nonradiative recombination of carriers may occur through a variety of mechanisms, but from a reliability perspective, the important mechanisms occur at these defects within the semiconductor. Because these defects have energy levels lower than the conduction and valence bands of the semiconductor, they act as a way for electrons and holes to recombine nonradiatively, thereby emitting heat instead of light. This energy can be very large, comparable to the energy of a chemical bond, creating more defects through the displacement or breakage of chemical bonds. This triggers a snowball effect that accelerates over time. Figure 2 is a simplified energy band diagram of a semiconductor showing various recombination processes. On the order of the energy of the chemical bonds, thereby creating more defects through the displacement or breakage of chemical bonds. This triggers a snowball effect that accelerates over time. Figure 2 is a simplified energy band diagram of a semiconductor showing various recombination processes. On the order of the energy of the chemical bonds, thereby creating more defects through the displacement or breakage of chemical bonds. This triggers a snowball effect that accelerates over time. Figure 2 is a simplified energy band diagram of a semiconductor showing various recombination processes.
Phosphor. Phosphors convert the 450 nm blue light emitted by LEDs into various colors of the visible spectrum, producing white light. They do this by absorbing blue light and losing part of the photon energy in a controlled manner, down-converting blue to red, green and blue over a broad band of wavelengths. These phosphors are typically complex rare earth silicates or oxides and can be doped to ensure specific emission wavelengths. Although these materials are polycrystalline and already contain many atomic defects, the recombination described in the previous section is also active here. In addition, chemical processes (such as reactions with water vapor or other compounds) may also cause degradation. Because these effects are highly dependent on the chemical composition of the phosphor, there can be considerable variation between LEDs since the phosphor used is part of a proprietary design. Even within a manufacturer's product line, different phosphors are often used, or they are applied in different ways, resulting in specific behaviors.
Lenses and Sealants. The transparent lenses and protective encapsulating materials used to collimate the light emitted from the phosphor structure of the semiconductor chip must maintain high transmittance throughout the lifetime of the LED. Since LEDs operate at elevated temperatures and humidity, performance degradation may occur here as well. Additionally, blue light emitted from LED phosphors may also play a role in the dimming process. Again, the specific chemical composition and structure of the lens will determine its behavior under normal and adverse circumstances and is highly dependent on process and composition.
in conclusion. The complex electrical and chemical processes that occur during LED operation cause a decrease in light output that is difficult to quantify through simple analytical expressions. While a mathematical description of one or both of these processes is possible, the complex overlap and interdependence of these processes makes it currently impossible.
Standard: LM-80 and TM-21
The Illuminating Engineering Society (IES) developed LED testing standards so that performance and reliability can be characterized in a consistent manner to evaluate their actual lifetime. LM-80 A standard describing a method for determining the "lumen maintenance" (ie, light output versus time) of an LED. The basic characteristics specified in the test protocol are:
- Ambient and LED housing temperature and orientation
- Drive voltage, current and waveforms -
The standard requires light output to be measured at an ambient air temperature of 25°C and an LED housing temperature of 55°C, 85°C, and other temperatures chosen by the manufacturer. The drive current is specified by the manufacturer because it changes with the LED chip area. The measurement time is at least 6000 hours (to 10,000 hours), and the interval is 1000 hours at most. A common practice among first-tier manufacturers is to use several different drive currents. As we will see later, this is very important in our approach to ensuring LED longevity.
Although the LM-80 provides a uniform method of measuring light output over time under standardized conditions, the actual operating temperature of the LED may differ significantly from the LM-80 value. And, since this is not an accelerated testing method, it takes a long time to draw reliability conclusions. Another newer IES standard, TM-21, helps resolve this dilemma. The standard is effectively an "ad hoc" model of LED degradation, allowing time-temperature data to be interpolated between temperatures and extrapolated into the future to predict output over long periods of time. The key points of the standard are:
- Assuming exponential decrease in light output (LOP)
- LOP can be extrapolated 6 times in time
- Interpolation between temperatures based on "activation energy"
In mathematical terms, the reduction in light output can be expressed as:
where Ea is the activation energy, k is Boltzmann's constant, T is the Kelvin temperature, and C is a constant. The TM-21 standard only allows interpolation of temperature data, so for two temperatures T1 and T2 we can calculate the activation factor Ea/k as follows:
Once the activation factor is known, the decay rate and the resulting time dependence can be calculated directly from the Arrehenius expression for the intermediate temperature. Figure 8 shows a graph of real LOP data and extrapolated and interpolated values based on these calculations (Reference: Mark Richman LEDs Magazine).
The important role of drive current in lumen maintenance
The driving current of an LED plays an important role in determining lumen maintenance. As discussed at the beginning of this article, recombination in semiconductors leads to increased defects and reduced radiative efficiency. Therefore, it stands to reason that drive current must play an important role in lumen decay. Unfortunately, there are currently no standards that recommend testing in this situation, and it is up to the LED manufacturer to choose the current value used in the test. Fortunately, manufacturers made smart choices and have plenty of data.
Figure 7 vividly illustrates the importance of drive current, especially at high temperatures. In this chart, TM-21 is used to extrapolate LED lumen maintenance data when the semiconductor junction temperature is approximately 127oC. It can be clearly seen that as the current increases from 0.35 A to 1 Amp, there is a very large increase in the degradation rate. It can also be seen from this chart that the L70 value (light output reduced to 70% of the original value) is reduced from more than 91,000 hours at 0.35 amps to 22,900 hours at 1 amp, a significant reduction in effective service life. (Strictly speaking, the LM-21 only allows 6x extrapolation beyond time-dependent data; I've obviously gone above and beyond to illustrate this)
The Importance of LEDSense Technology
Given that operation at high current levels can result in significant reductions in lumen maintenance, it is important to provide a method in the driver electronics to protect the operation of LED lighting products. Thermal reentry provides such a safety net. Hot folding is important because:
-It achieves brightness and long service life regardless of the conditions
-It provides OEMs with a "worry-free" solution
-It virtually eliminates the consequences of poor installation on site
TerraLUX's thermal foldback implementation is called LEDSense? , using a microprocessor controlled constant current driver. Figure 5 shows a block diagram of the basic features of the LEDSense circuit. Various buck or boost topologies are used depending on the input voltage range and the length of the LED string that determines the required output voltage. The LED's temperature is measured by a thermistor located in the device's thermal path to determine the LED's operating temperature. The microprocessor measures this value via an A/D converter and compares it with the known operating characteristics of the LED via a built-in algorithm. The processor then sets the driver's current through its D/A converter. With a properly designed and operated luminaire-light engine combination, the current (and temperature) is maintained at a predetermined safe level. If the temperature exceeds a preset threshold, an algorithm in the processor gradually reduces the current set point through a firmware algorithm to ensure the longevity of the LED. The microprocessor is also used to analyze the power waveform to tailor driver operation to the specific dimmer-transformer combination used, providing additional benefits to OEM manufacturers.
The resulting performance characteristics are shown in Figure 6. This graph compares drive current versus LED temperature for two examples of LED light engines, one (blue square) without LEDSense technology and one (red circle) with LEDSense technology. In this example, the externally controlled temperature is the temperature on the back of the light engine heat sink, but we plot the bulk temperature of the LED; the blue dashed line is intended to illustrate the temperature excursion caused by component thermal resistance and drive power level.
Also shown is the calculated L70 time for the LED based on the TM-21 standard (although again I extend this beyond the recommended time under low current conditions). The LED temperature is not controlled, but when operating under current, its value is significantly shortened to only 13,900 hours. In contrast, the LEDSense feature has reduced the current to a point that effectively "offsets" the negative impact of existing high temperature conditions on longevity. Although users will notice a noticeable darkening in this case, less obvious overheating conditions may be difficult to detect due to eye insensitivity at high brightness. Nonetheless, the LEDSense algorithm keeps LEDs safe even in areas with only slight overheating.
The industry has developed comprehensive standards to test LEDs and describe their degradation over time. So far, however, these standards only allow predictions based on operating temperature. We have demonstrated that the combination of drive current and operating temperature has an extremely important impact on the LED lifetime defined by L70. Thermal foldback provides a useful way to ensure control is within the safe operating range of an LED; TerraLUX’s LEDSense? The technology has the added advantage of quantitative control algorithms to ensure LED longevity and light output.
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