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High-power Leds
To begin let me explain what high-power LEDs means. A high-power LED should be more than 1 Watt in order to be called like that. Usually these devices use 350 mA and have luminous efficacy of 115 lm/W. They produce light and they could be used almost everywhere. Their high efficiency comes from the technology on which they are based.
Now let’s take a look at their characteristics. First we will go through the electro-optical characteristics. The first one is called peak wavelength. This is by definition the wavelength that suffers the lowest loss. Usually it is provided in nanometers.
Next there are voltage characteristics – forward and reverse voltage. Every LED has two ways of inclusion – the forward one in which the necessary voltage is lower and the reverse one usually with more than 5 V.
The axial intensity comes to show the light intensity on the axial line. Luminous flux is another part of the specification table of the high-power LEDs and it is the measure of the perceived power of light. After that comes the viewing angle to provide data on what angle the light is directed on the objects or generally on what angle you will be able to see the light produced from the LEDs. Emitted colour is the colour of the light. It could be orange/red, yellow, green, blue or white. The power dissipation is another important criterion and for example it could be 120 mW. It shows the power consumption of the LED.
One problem for these LEDs could be the temperature and all specifications are usually guaranteed at 25 degrees centigrade. Of course the operation/storage temperature could usually vary from -40 to +85 degrees centigrade but temperatures close to these limits are not well-advised. There is usually a chart that shows how the temperature of the LED is changed in time and the critical temperature which could destroy the LED and of course the time (usually in seconds) which the LED could endure the temperature before it is permanently damaged.
The thermal problems of the high-power LEDs are maybe the most important aspect of their technology and this is why we will discuss them on a large scale.
Unlike other light sources like the incandescent tungsten light bulbs, high-power LEDs do not radiate heat. Instead of that, they conduct heat to their thermal slug. That means that heat goes a longer way to reach the atmosphere.
In the tungsten bulb the heat trajectory is short. It starts with the thermal resistance from the filament to the glass and it ends with the resistance from the glass to the atmosphere. In high-power LEDs it is not that short because of other parts that stand on the way like the slug, the board and the heat sink. However it is very crucial for us to understand the difference of the heat delivering – in high-power LEDs we talk about conduction, not radiation.
The temperature of the LED is directly linked to its colour and if a precise colour is needed the optical design should be optimized. Different colours are made by combining base colours and for white colour - all colours are used. That means that if a white colour is used, the temperature will be higher than the temperature in all other cases. Usually a colour sensor is used to insure the proper colour during the temperature changes and the aging of the LED.
Almost every aspect of the high-power LEDs is linked to the junction temperature of the LED. These aspects are the dominant wavelength, luminosity and the forward voltage and they are the most important criteria of the high-power LED.
One significant thermal problem with high-power LEDs is called “thermal runaway”. It happens because when the colour sensor reports that the luminous intensity is not high enough the processor drives the LED harder and by doing this it uses more power and when more power is used more heat is generated. More heat means higher junction temperature and higher junction temperature again decrease the luminous intensity. And again the colour sensor reports low light emission and the process circles all over again until the temperature of the LED reaches critical state and the LED overheats and permanently fails.
Of course “thermal runaway” usually happens only with colour sensor feedback systems which do not monitor or manage heat. One simple protection could be a thermal sensor constantly measuring the junction temperature and when it reaches a temperature that is too high the system is shut down. It will go on again when the temperature is considered safe but usually this is not an option because the high-power LED needs to work all the time.
More reliable options are needed and one good option is to use heat spreading material like for example natural graphite. When it is applied to the heating body this material will vastly conduct the heat through its capacity and if it is installed properly it will quickly conduct the heat to the atmosphere leaving the endangered body safe.
The brute force method in which the LED shut down is proofed to be useless in most systems. The thermal management should be the most important aspect of the high-power LEDs technology only seconds away from the colour mixing firmware.
First thing to do when designing thermal management plan is to ensure that in any case you will be in the admissible borders of the temperature. That means that none of the materials used in the high-power LEDs construction could fail during the exploitation because of the heat.
Every high-power LEDs designer should take the thermal problem seriously because smart thermal management will increase the temperature range in which the LED could operate and the thermal monitoring will maintain the accuracy of the final product. Systems with programmable mixed-system controllers offer many advantages over conventional circuits. Usually these advantages will save money to the end user and they will ensure proper colour correction which makes them reliable and desired by the market itself.
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