LED Lighting and Effectively Managing Heat Production|
Article- May 2013 By Larson Electronics.com
Larson Electronics Blue and White LED Light Bar
By now most of us are becoming quite familiar with LEDs and how advanced their design is in comparison to traditional forms of illumination such as incandescent and fluorescent bulbs. The lack of a wire filament, the passing of electrical current through semi-conducting materials to force the radiation of photons, and the extremely efficient process of electroluminescence utilized by the LED versus electrical resistance upon which incandescent lamps are based, while providing wide ranging improvements in efficiency and durability, also presents some problems which are also unique to the LED.
Unlike traditional incandescent lamps which are designed to produce light by using the electrical resistance of a wire filament to radiate light energy and thus run extremely hot, LED lights on the other hand are quite sensitive to increases in temperature. Like traditional lamps, the lifespan of an LED is measured in hours, however, during the course of operation, the light output of an LED exhibits a gradual decline. As a result of this gradual decline, there comes a point when the light output of the LED drops to a level which makes it considered no longer effective. So, the operational life of an LED is generally measured by the number of hours it takes for output to drop to 50% of initial levels.
One of the biggest contributors to decline in LED output and overall operational life is heat. Although LEDs are considered cool radiators and produce far less heat than incandescent lamps of comparable output, they still produce significant increases in temperature. In order to maintain LED output and longevity, it is thus necessary to provide a means for effectively dissipating the heat produced to avoid accelerated degradation of the emitter. Additionally, the luminous output of an LED is limited by the amount of heat produced. As heat increases, lumen output decreases, which results in reduced efficiency as well as shortened operational life. This is a serious problem in today’s high performance LEDs, as manufacturers seek to increase luminous efficiency as well as output, which in turn generally means the use of higher current and thus increased heat generation.
The most common method of controlling the heat produced by LEDs is through the use of passive radiators or “heat sinks” which act to increase the surface area of the LED, thus increase the amount of area exposed to the ambient atmosphere thereby more effectively dissipating heat. Although generally effective, this in turn presents additional problems as one of the large benefits of LEDs is their compact size. The need for an artificial radiator effectively increases the overall size of the LED assembly, oftentimes in the case of high power LED assemblies, to the extent that any improvements from reduced fixture size are lost. Additionally, this creates problems with LEDs designed to act as direct replacements for incandescent or fluorescent fixtures as it then becomes difficult to keep the size and overall design of the LED assembly compatible with existing fixtures.
Another newer and effective means of managing heat production in LEDs involves the use of active heat management systems. LED fixtures designed for general illumination use require that the current fed into the LED assemblies be regulated at a precise voltage for optimal performance. The circuitry that does this is known as “drivers”, and they take incoming voltages and reduce or step them up as needed before they reach the LEDs. Almost any LED connected to a power source need some sort of “buffer” between it and the power source to provide protection against uneven voltages and spikes that could reduce LED life, and in the most basic forms this is just a resistor or two place in line with the power leads. Larger assemblies intended for use in general illumination applications however require more complex controllers that can maintain and manage the higher variances and increased demands produced by the stronger currents needed to power these assemblies. It is these more complex drivers that have provided an added method for controlling heat that can allow the use of smaller heat sinks while also allowing the use of a wide range of voltages.
LED drivers can be tailored to act as more than just a voltage regulator. These circuits can be designed and programmed to also provide different LED operating modes such as hi/low output, dimming, flashing or strobing and signaling capabilities. For heat control though, the real benefits come from the incorporation of Pulse Width Modulation technology into LED drive controllers which allows all of these potential control options. PWM in essence controls the duty cycle, or off/on cycle of the current being applied, and . Basically, PWM allows current levels to be lowered or raised according to the duration of the on/off cycle. This provides some interesting possibilities for LED lighting, as not only can the duration of the on/off cycle be managed to produce a specific voltage level, the effective rate at which this takes place can also be controlled. In a simplified sense, this means that rather than run the LED at a constant on state, which will produce a specific increase in heat related to voltage level, the LED is actually turned off an on at an extremely fast rate, effectively reducing the amount of time the LED is actually switched on.
By altering the duty cycle of the current applied to the LED, the controller can effectively keep temperatures at a specific level, which as can be imagined, offers increased LED longevity and performance. Additionally, this also allows the LED to be run at full power without an matching increase in temperature, thus allowing the LED to reach its full output potential without detrimental effects. Although heat is still produced, this ability to actively manage its production while maintaining LED output also means that the need for a heat sink is reduced. Instead of large heat sinks that significantly increase the overall size profile of the LED assembly, smaller more compact sinks can be used without any loss in performance.
As LED lighting technology continues to mature, the innovations adding to its suitability as a replacement for traditional forms lighting are certain to grow as well. Although heat management is currently one of the biggest stumbling blocks to LED development, resulting in increased costs and limits to the amount of light output an LED can achieve, improvements such as PWM technology are already proving effective at meeting the challenges of heat management.