How efficient is the LED? How to improve?

  1. High efficiency

LED is a current driver, which requires current limiting when the voltage source provides current. In most applications, it is driven by a constant current source. No matter how the power supply voltage changes, the current source can adjust the current through the LED lamp.

Linear LED drive
Linear LED drive

LEDs are no longer just used as red or purple indicators in electronic devices. With the progress of technology, it has become a practical lighting source. The biggest benefit of LED is its durability and luminous efficiency. If used properly, a high-power LED can work continuously for 100000 hours without light attenuation. The typical efficiency of power LED is 50-80m / W, which is several times that of incandescent lamp and is equivalent to that of fluorescent lamp. Because LED is a solid-state device, it can effectively resist the impact and vibration that damage the filament or glass cover.

In order to effectively improve the stability of LED, a constant current source is needed to drive it. Most LEDs have a specified current value to maintain their normal life and make their brightness reach the maximum. An LED can be driven by a linear voltage controlled current source. However, high-power LED is not suitable for this method. Switching power supply (SMPS) can provide a more effective solution to drive LEDs.

Under the given current drive, a forward voltage difference will be generated at both ends of the LED. The power supply voltage and the forward voltage characteristics of LED determine the design requirements of switching power supply. Under the given current drive, multiple LEDs can be connected in series to increase the forward voltage drop.

  1. Directivity of LED lamp

The light output of LED is directional and only emits at a small angle in a certain direction. In order to create a 360 ° light output effect like an incandescent lamp, its chip needs to be assembled in different directions.

  1. Light diffusion of LED lamp

The strong light generated by the LED is too bright compared with the surrounding ambient light, so that it will feel uncomfortable or painful when looking directly at the eyes. Therefore, it is necessary to diffuse the light emitted by the LED. Any material that can diffuse or scatter can be used in LEDs to produce softer lighting effects. According to the practical application, these materials can be coated glass or structured glass, or transparent or opaque plastic.

Light diffusion of LED lamp
Light diffusion of LED lamp
  1. Color rendering index

The human eye’s perception of the color of the observed object is closely related to the light source. The same object may show different color effects under the conditions of sunlight and low-pressure sodium lamp. The color rendering index (CRI) of the light source is defined within 100. It describes the fidelity of the light source to the color of the observed object in the process of observing the object, and reflects the ability of the light source to reproduce the real color of the observed object. For light sources, high color rendering index is as important as high luminous efficiency in order to be well recognized and accepted.

The CRI of incandescent lamp is almost 100, that of fluorescent lamp is 95, and that of LED is only about 80. For low-pressure sodium lamps, it is lower, only about 25. CRI was first defined by CE (International Lighting Committee) in 1974, and scientists have also established other evaluation methods for CRI [10]. In fact, in 2007, CIE proposed not to use it as an indicator of white LED. Nevertheless, in order to evaluate the color rendering of a light source, it will still be compared with a reference light source with a CRI of 100. Note: CRI is used to compare the color rendering of different light sources, but in 2007 (11,12), CIE had reservations about using it to measure the color rendering of white LED, and suggested that it is necessary to establish a new measurement method.

CIE mentioned in its technical report 177:2007: color rendering of white LEDs: “the technical committee concluded that CIE’s CRI color rendering standard is not suitable for evaluating the color rendering arrangement sequence of a series of light sources such as white LEDs.” The simple reason is that the CRI obtained by this method is not consistent with the color rendering observed in the real world, whether it is a white LED based on phosphor luminescence or RCB trichromatic luminescence.

  1. Relevant color temperature

The concept of color temperature comes from blackbody radiation. The color of heated metal will change with the change of temperature. As the metal heats up, its color changes from the initial red to orange, yellow, white and blue white. Color temperature is very intuitive for incandescent lamps, but for light sources such as fluorescent lamps and LED lamps, the concept of (related) color temperature does not mean the temperature of the light source, but is used to compare and refer to the color of the light source. In the field of lighting, the color temperature is expressed by Kelvin temperature K, which represents whether the light source is warm or cold. High color temperature (3600 ~ 5500k) is considered as cold color light, and low color temperature (2700 ~ 3000K) is considered as warm color light, which sounds a bit confusing. Cold light is basically used for display because it has better contrast than warm light. Warm color light is mainly used for indoor lighting.

By definition, the color temperature of light is different from the color of light. The color of light can be defined by chromaticity: in the three-dimensional coordinate system established with 700.0nm red light, 546.1nmm green light and 435.8nm blue light as the unit quantity of three primary colors, if x, y and Z represent the light of three primary colors respectively, the chromaticity of any light source can be expressed by the ratio of three primary colors contained in this light source:

Chroma = XX + YY + zz

This formula is based on the isoenergy principle that white light can be obtained when the three primary colors of red, green and blue are mixed.

CIE first defined the standard chromaticity system in 1931, which established the corresponding relationship between the color and wavelength of light. This system, called CIE color model, can be considered to have evolved from Maxwell triangle.

CIE chromaticity principle is based on the fact that the pyramidal cells on the human retina have three color types of photoreceptor cells, which will respond differently to different wavelengths of light. The response of the three photoreceptors is measured by the unit quantity of the three primary colors of X, y and Z.

  1. White LED lamp

What led brings real revolutionary changes to the lighting industry is the birth of blue and ultraviolet LED. For natural semiconductors, there is no material with a unique band gap that produces white light. In order to produce white light with LED, two main processes need to be realized on inorganic led: wavelength mixing and wavelength conversion. White LED can be made by mixing three colors of light: red, green and blue. The advantage of this technology is that the hue of white light can be controlled by changing the proportion of each color light.

White LED lamp
White LDE lamp beads

In the white LED based on phosphor, its luminescence is a wavelength conversion process caused by the Photoluminescence Characteristics of phosphor. At present, the common white light is to excite the phosphor coated on the surface through the blue light or near ultraviolet LED, and synthesize it with the original blue light or near ultraviolet light to obtain the white light similar to the fluorescent lamp. Phosphor can be directly coated on the surface of LED chip or on the surface of plastic cover.

At present, most commercial LEDs are made of in GA n blue LED coated with yttrium aluminum garnet (YAG) phosphor (YAG: CE) doped based on Paving [13]. In order to make LED replace incandescent lamp and fluorescent lamp as general lighting source, in addition to considering its efficiency and service life, good color rendering is also a very important requirement. In this country, led with high color rendering must have high luminous efficiency at the same time. The early white LED was about the package size of 5mm in diameter and was composed of a blue LED chip coated with phosphor surrounded by epoxy resin.

The main difference between blue LED and white LED based on blue chip is that the latter is coated with phosphor on the inner surface of epoxy resin or on the surface of the chip. The color of LED is completely determined by the selected phosphor. Appropriate phosphor can be selected according to the color requirements of various lighting conditions. However, in order to meet the lighting requirements of white LED lamps, it is necessary to optimize its parameters, including the efficiency of phosphors.

In the past 10 years, the luminous efficiency of in GA n-led solid-state lighting has been improved by more than 10 times: the efficiency of cold white LED has exceeded that of linear fluorescent lamp (> 100M / W), while the efficiency of warm white LED has exceeded that of compact fluorescent lamp (50 ~ 80lm / W). By 2015, the U.S. Department of energy plans to improve the efficiency of warm white encapsulated LEDs to 1381 M / W, which will be a huge technological achievement that will lead to significant changes in the solid-state lighting market. As expected, YAG: CE doped with a small amount of red phosphor improves the CRI of white led to an acceptable range (color rendering index > 80) and improves the light conversion efficiency. The research on phosphors and their compounds is constantly expanding their excitation frequency to near ultraviolet and blue light (380-450 nm), which can produce light emission in the visible range.

  1. Temperature degradation and aging of LED

The reliability of a lighting system depends on its lumen maintenance rate and the stability of its color. With the continuous growth of high brightness white LED market, LEDs with several watts of input power have emerged, which are ideal alternative light sources for traditional lighting systems. However, the self heating effect of the device also increases with the increase of input power.

Compared with incandescent lamp and fluorescent lamp, the heat generated by LED cannot be radiated with the monochromatic light. One of the disadvantages of LED is that it must be effectively managed. Too high temperature will lead to the degradation of P n junction, the degradation of epoxy and the change of color, reduce the transmittance and refractive index of epoxy, affect the light emission of phosphor, and finally reduce the output light power and change the spectral characteristics (color); In other words, it leads to reduced brightness and color changes. The micro analysis of LED shows that the influence of temperature stress on LED packaging devices mainly lies in the carbonization of plastic packaging materials.

In order to maintain the high light output of LED and ensure its long service life, effective heat dissipation is very necessary. Incandescent lamps can convert about 8% of the input electric power into visible light, and other energy is radiated out in the form of heat. Fluorescent lamps can convert about 21% of the input electrical power into visible light and the rest into heat. For LED, 15% – 25% of the input electric power can be converted into visible light, and the rest can be converted into heat to raise the temperature of the device itself, so it must be effectively dissipated.

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