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Thermal Characteristics of High Power Leds
High power LEDs are commonly known as power-saving lights that come with power ratings in the range of 500mW to 10 Watts. They are based on technology meant to increase their luminescent efficiency and therefore can be used almost everywhere from car headlights to high-power lamps. They can be characterized on the basis of a few parameters, such as forward voltage, wavelength, luminous intensity and type.
The wavelength is measured in nanometers and is different for all three basic colors – red, blue and green. By setting these colors to different wavelengths we can get a whole range of exciting colors, also called rainbow colors. This is one of the electro-optical characteristics of an LED. Another important variation is the peak wavelength which suffers the lowest loss due to heat.
Every LED has two kinds of voltages – forward and reverse. A light emitting diode can be made to act like a variable resistor or a capacitor by interchanging reverse and forward voltages. Usually the reverse inclusion is the one with the higher voltage – up to 5V.
Some other vital parameters of high power LEDs are axial intensity and luminous flux. Luminous flux is also known as luminous power and it gives the measurement of the perceived power of light. The power is different at different wavelengths and luminous flux is calculated by adding up power ratings at all wavelengths. This is not the actual power measurement; instead it is the measure of the ‘useful’ power of an LED. Power dissipation is the amount of power consumed by the LED. It is also referred to as the energy lost in the form of heat. Another important characteristic is the emitted color. This color is what we see when the LED is switched on. It can be red, yellow, green, blue or white.
One of the most important concerns regarding the design of high power LEDs is the amount of heat produced due to increased efficiency. High temperatures are not suitable for proper LED operation. They serve to decrease its life and make it less efficient. Thermal management in these LEDs is therefore a major concern in order to make them more productive and longer lasting. There is a critical temperature defined for every LED beyond which it is permanently destroyed. The temperature of an LED keeps on changing with the passage of time and therefore it’s important to maintain a chart to record different temperature ratings against time.
There are three ways by which heat can be discharged: conduction, convection and radiation. Traditional incandescent tungsten light bulbs radiate energy in the form of heat. LEDs, on the other hand, conduct heat from one part to another which takes a long time to be released to the atmosphere. The heat sink should be made by using a material which has high thermal conductivity. Aluminum and copper are two of the most widely used heat sink materials.
Radiation takes less time than conduction and therefore tungsten bulbs do not heat up very quickly. When designing high power LEDs, proper heat conduction is a main issue to be addressed.
The temperature of an LED is directly related to its color. White light is produced by combining base colors in their highest values and therefore the temperature is also the highest in case of white light. Some mechanism is required to keep the color constant when temperature changes due to conduction. A color sensor can be used for this purpose.
Wavelength, forward voltage and luminosity are also affected by the junction temperature of the diode and therefore they also have to be optimal in order to make the LED perform well.
Positive feedback or “thermal runaway” is another problem related to high temperature of the LED. This is a condition in which increased temperature causes the above-mentioned parameters to change unexpectedly resulting in a further increase in temperature. This situation can be critical to the operation of an LED and has to be prevented. It happens when the LED is directed to consume more power in order to produce the desired color. As a result, more heat is produced which raises the temperature and hence the effect.
Thermal runaway is more pronounced when increased junction temperature decreases the luminous intensity and as a result the color sensor reports there is not enough light. This negative cycle goes on and on until the critical temperature is reached and the LED is destroyed. Thermal sensors can be used to correct this problem which constantly measure the junction temperature and make the system shut down if there is an alarming condition.
However, usually this mechanism is not advisable in these LEDs because it makes them useless and obliterates their purpose. Other methods like using natural graphite to spread heat are more practical and useful. This material conducts a large portion of heat dissipated by the LED and as a result releases it to the atmosphere more quickly.
In conclusion, the most important thing to consider in a high power LED design is the selection of appropriate materials that are not affected by the unexpected rise in temperature. This is to ensure that the LED works at all times without shutting down because of the failure of one of its components. Efficient thermal management can increase the critical temperature of an LED and as a result its reliability and longevity is also enhanced. These design decisions on the part of manufacturer help the customer save money and make the most out of these powerful and efficient high power LEDs.
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