Introduction to Active Thermal Management of LED Drivers

LED light sources are the general trend of the lighting industry, but the overall luminous efficiency has not yet met market requirements. Among them, heat dissipation defects are the first and most difficult issue to be solved. Currently, the industry has developed active thermal management using LED drivers, which uses built-in thermal energy return functions to improve the development obstacles of LED service life and heat dissipation.

In 2007, the U.S. Department of Energy's Energy Information Administration (EIA) published a report that specifically mentioned that lighting equipment in the United States (in commercial and residential areas) consumes a total of about 526 billion kilowatt-hours of electricity. If we refer to the 2008 EIA report, we can find that a nuclear power plant in the United States can generate 12.4 billion kilowatt-hours of electricity. Therefore, through simple calculations, it can be seen that the use of lighting equipment in the United States alone consumes the electricity of 42 nuclear power plants. As the population gradually grows, the lighting industry must find new types of lighting sources to improve lighting efficiency and reduce energy consumption.

In the lighting industry, light-emitting diodes (LEDs) have always been expected to replace traditional incandescent lamps, fluorescent lamps, and various gas discharge light sources. The main reason is that LEDs can reduce power consumption and protect the soil from harmful chemicals. However, even though the luminous efficiency of LEDs has long surpassed high-efficiency light sources such as xenon lamps (HID), the internal circuit composition and overall brightness performance are still not ideal.

The luminous efficiency of LEDs is limited by the way light is generated, and has nothing to do with the materials used to make them. The way light is generated inside LEDs is usually non-radiative, so most of the light is reabsorbed and generates heat. White light LEDs based on phosphors will reduce the luminous efficiency of LEDs due to the heat generated by the Stokes Shift effect. However, there are several innovative solutions that can improve the problem of radiative recombination rate and photon absorption to increase the brightness and luminous efficiency of LEDs.

Improving brightness by implanting photonic crystals

Luminus Devices has pioneered a solution that uses implanted photonic crystals to provide waveguides for photons trapped in the substrate. Photonic crystals are naturally occurring substances that can be seen in materials such as opals. Their compact structural characteristics can provide a path for light radiation and can produce a higher density of light field intensity. This feature, along with other features such as quantum wells and optical resonant cavities, can increase the brightness of LEDs beyond imagination. For applications that require high brightness, such as projectors, liquid crystal display (LCD) backlight light sources, and other special applications, these special components are already quite popular, and we hope to use them in general lighting equipment in the future.

Every lighting device has the problem of light source life. Common situations include the tungsten filament of an incandescent bulb suddenly burning out, or the light source material gradually aging and dimming due to long-term use, and LEDs also age due to the junction temperature of the components. All lighting devices provide stable brightness, and after a certain number of years of use, the brightness will inevitably gradually fall below the effective brightness. Different lighting application devices have service life time limits. For example, when the brightness of a street lamp device has decayed to 50% of its original brightness, it represents the end of its service life, but the brightness of a light source device used in a doctor's endoscope may not have decayed that much.

Managing brightness loss through thermal management/LED driver IC

The end of LED life means that its lighting brightness has fallen below the usable level, and the length of LED life is highly correlated with temperature (Figure 1). If the curve in Figure 1 is used as a design reference, to design a general-purpose lighting device that can still have 50% of the initial brightness after a service life of at least 50,000 hours, the junction temperature must be controlled below 100°C. Without active cooling, it will be an extremely difficult task. In addition, since LEDs cannot use radiation to dissipate heat, this will make the heat dissipation problem even worse. Traditional incandescent bulbs can disperse a large amount of infrared heat energy through radiation, while LEDs must consider how to solve the heat dissipation problem. In addition, during use, if the junction temperature instantly exceeds the temperature range limited by the manufacturer, it will cause severe and permanent damage to the life of the light-emitting component.

Figure 1 Temperature effect of LED lighting

If LEDs must be degraded to handle severe junction temperatures, two important things must be considered when designing lighting systems or core components: thermal management and understanding of LED drivers. These two factors must be considered not only in general lighting applications, but also in other applications. The simplest solution is to use passive thermal management, with sufficient airflow through the heat sink to carry away the waste heat. Nuventix has developed an active cooling device that does not use any fan components, but uses high-speed air jets to generate airflow. Its operating principle is like a cyclist holding the back of a large truck with one hand and riding fast as the truck moves forward. The cooler designed by Nuventix uses a similar principle to provide a highly efficient cooling method. This technology has been used in standard lighting equipment such as MR16, PAR20, and PAR25.

Another important issue in the solution is the active thermal management of LED drivers. Traditionally, LED drivers have a fixed current source, which is mainly used to monitor and control the amount of current flowing into one or more groups of LEDs. Generally speaking, this is enough to drive the LED and maintain the same brightness during the change of input voltage. However, with the improvement of brightness and architecture, LEDs can provide higher brightness light sources and gradually evolve into core components of complete light source engines with thermal control requirements.

To prevent damage to components or shorten their lifespan, thermal management is necessary, and there are a number of ways to address this problem. Figure 2 shows two different foldback curves that limit the current flow of the LED as the temperature rises. When the junction temperature remains within the default temperature range, the main control loop is used for current regulation; but when the junction temperature exceeds the default range, another set of control loops is used to limit the current. The junction temperature foldback point and the attenuation rate of the drive current can be changed according to different applications and the LED manufacturer's recommendations.

Figure 2 Example of thermal energy reversal curve

The LED driver LM3424 launched by National Semiconductor (NS) has a built-in thermal foldback function, which allows designers to select the required set point and system gain value (by adjusting the slope of the curve). This allows LEDs to be used in general lighting and other special applications (such as emergency notice boards) to better understand the phenomenon of LED damage due to age or high temperature.

Thermal management improves reliability and significantly extends LED life

Although LED technology continues to evolve, it has also spawned many new problems. As LED luminous intensity and efficiency continue to improve, more lighting applications will gradually shift from traditional light sources to new types of LED light sources, and more new applications will be generated. When new cooling solutions and driving technologies are introduced, thermal management will become easier and can extend the service life of LEDs, making LEDs potentially unnecessary to replace, which is a difficult goal to achieve with the incandescent bulbs currently used.