LED

Light Emitting Diodes (LED) are now being widely used in homes, offices, textiles and other applications because of their less energy consumption, longer lifetime and good quality lighting when compared with conventional lighting systems such as incandescent and fluorescent lamps. The LED’s used for lighting purpose are power LED’s which are 1-3 W rated devices usually run at 350mA were as low power LED’s are usually used as indicators. The working principle is same for both small and power LED’s. LED is basically a PN junction diode which works on the principle of electroluminescence. That is, when a voltage is applied across the diode, electrons from the conduction band recombines with the holes from the valence band releasing energy in the form of photons. The recombination can be direct or indirect recombination. The colour of the emitted light depends on the energy band gap of the semiconductor.

Power LED's

Even though the working principle is same, the construction of LED differs to a great extent depending on its type. LED structure plays an important role in emitting light from LED surface and therefore the LED’s has to be structured such that the photons generated should be allowed to escape from the device without being reabsorbed. LED’s used for lighting purpose are usually surface mount LED’s or SMD LED’s and is fixed using Surface Mount Technology (SMT). The LED lights such as bulbs, tubes, down lights, street lights, lanterns etc. available in the market are of either cool white or warm white in colour. Warm white is having a colour temperature of about 2700-3000K and that of cool white ranges from 4000-5000K. The selection of colour is based on personal preference and application.

In case of lights, in order to denote efficiency, the term efficacy is used. It’s the ratio of lumen output produced per watts. For example, in case of a 5W bulb if the lumen output is 500, then its efficacy is 100.  Now LED lights with efficacy more than 100 is available in the market.

LED Characteristics

LED is a constant voltage load with low Equivalent Series Resistance (ESR). As LED is a device that emits light when electrically biased and is similar to standard diodes, most of the electrical characteristics of LED matches with that of diodes. As shown in the V-I characteristics, any further increase in voltage after reaching the threshold voltage of LED, will lead to rapid increase in current through them. Because of this behavior it is better to drive an LED with constant current rather than constant voltage. From the luminous flux versus forward current characteristics, it’s clear that as current increases luminous flux also increases. Thus the current through LED is a key parameter that determines the intensity, colour and forward voltage of light emitted from the LED.

Need for LED Driver

LED works on DC power and therefore it requires a device that can convert incoming AC power to DC voltage and regulate the current flowing through the LED during its operation. From V-I characteristics it’s clear that slight variations in supply voltage can cause very large change in current.  If the current exceeds manufacturer recommendations, LED’s become brighter but their light output degrades due to the junction temperature rise and this may reduce the useful life of LED. Since LED’s are very fast devices, if there is any variations in forward voltage, even for very short time, it will cause big variation in current which may damage the LED. Inorder to avoid these problems and to protect LED from line voltage fluctuations, LED’s must be driven from a current source. Without an appropriate driver circuit LED’s may become too hot and unstable, leading to poor performance or failure of LED lights. Thus like the use of ballast for fluorescent lamp, driver circuit is used for LED lights to provide correct power supply to function and perform at their best.

Mainly LED drivers consist of rectifier and a DC to DC converter circuit. Driver IC’s were also available in the market. The power factor, harmonic distortion etc. of an LED light depends on the LED driver used.  LED’s with power factor greater than 0.98 are available in the market whereas the power factor for CFL lights are usually below 0.7. Since converter circuits such as buck converter, boost converter etc. were used it is possible to control the light output by using switches like MOSFET in the converter circuit by giving appropriate gate signals. The cost of LED lights mainly depends upon the power factor, light output, quality and price of LED’s, driver IC’s, heat sinks etc.

Temperature dependence of LED lights

The major problem facing by the LED light manufacturing industries is the thermal issues related with LED. As heat is generated at the junction within the LED due to the inefficiency of the semiconductor processes that produce light, the junction temperature of LED rises. The LED properties, including luminous flux, forward voltage, color characteristics, useful life and reliability dependents on junction temperature. Useful operating life is one of most important factor based on which LED light source is selected. LED lighting is claimed to have very long life compared to traditional light sources. The temperature of the junction is the most critical factor as it affects the reliability and performance of the LED luminaire, in terms of functional and useful lifetime of the luminaire. The manufacturers measure the performance of LED by testing them at junction temperature of 25 degree celsius which is the ideal laboratory condition. But in practice LED’s are working at much higher temperature. This also affects the light output of LED. Various studies show that the luminous efficiency of LED's typically decreases by approximately 0.2 to 1 % per degree celsius rise in temperature. As junction temperature increases the light output gets reduced and LED colour shift occurs.

Thermal path of LED

In order to ensure long life and dependability of LED lighting products, LED light manufacturers must deal with heat management issues at the system and component level. Heat must be moved away from the LED die in order to maintain expected light output, color and life. The amount of heat that can be removed from the LED junction depends upon the design of the thermal path and ambient temperature from the die to the surroundings. The above picture shows the thermal path for LED. Use of heat sink for LED lights helps to reduce the heat at junction of LED’s. Heat sinks first conduct heat away from the LED junction and then it will convect and radiate heat to the ambient air. Proper design of heat sink helps to keep the temperature away from the LED’s junction. Usually Aluminium heat sink is used as it has high thermal conductivity (120-240W/m K). 

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