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Factors that affect LED efficiency, ways to adjust efficiency, and reduce system cost analysis

Updated:2017-08-23 Click:328times

As the LED has a variety of advantages, has now become a semiconductor light source for general lighting applications an important part. For some time, LED efficiency was significantly higher than the traditional lighting technology. But how do you define the efficiency of a solid state lighting (SSL) system? In ordinary everyday language, when it comes to the efficiency of light sources, we usually refer to their luminous efficiency in units of per watt lumen (lm / W). Another measure is radiated power. Need to take into account the impact of effectiveness and efficiency, discuss the decision-making process that product developers must follow, as this affects the efficiency of the entire system.

The computational function measures include determining the luminous flux of the light source (lm) and the electrical input power (W). On the contrary, the corresponding amount of photometric radiation does not take into account the sensitivity of the human eye, but purely efficient measurement. The electrical input power (W) divided by the optical radiation output power (W) yields the percent efficiency. The efficiency of LEDs depends on many different factors. We will cover some of these in detail in this article.

Color temperature affects LED efficiency

The choice of color temperature has an important effect on the efficiency of the lighting and can be used as a method of efficient planning lighting solutions within the framework of existing lighting regulations. High color temperature LEDs (such as 5000K) are generally more efficient than color temperature (eg, 3000K). The graph in Figure 1 shows the spectra (or spectral power distribution - SPD) of different CCT values when the LED is in color rendering index (CRI) Ra> 80. The SPD curve is based on the human sensitivity curve Vλ.

In order to make the LED produce white light, usually use the blue LED chip. Some of these lights are converted into light of longer wavelengths (green, yellow and red) by converters or phosphors, adding all these colors together and then producing white light. However, the conversion process will be depleted, the wavelength of light conversion increases, the greater the loss, because the higher energy level (blue) light and lower energy level (red) between the energy difference is converted into heat. To minimize the loss, it is necessary to accurately calculate the absorption and emission wavelengths of the converter.

However, the simplification of the situation is sufficient to explain the basic principles. For example, for a warm color of 3000K, you need to convert a lot of red light. However, this requirement leads to greater losses, compared with the 4000K, reducing the luminous efficiency. For the high color temperature of 5000K, Blu-ray only need to be converted into green light, do not need to convert to red, which is why more than 4000K luminous efficiency to increase. The effect is shown in Figure 2.

Effect of color on LED efficiency

As described above, the appropriate converter is selected. After conversion, the composition of the chromatogram has a decisive influence on the efficiency of the LED. The converter portfolio was developed for different CRIs, specifically optimized for CRI and efficiency. The difference between CRIs 70, 80 and 90 can be clearly seen when the red system is displayed. In order to reproduce these colors as truly as possible, a very high proportion of long wave light is required; in other words, the red end of the spectrum.

Figure 3 shows the SPD at 4000K LEDs at different CRI values. The high aspect of the CRI 90 version can be clearly seen. As mentioned above, generating such a high proportion will involve high losses. In addition, most of the red energy produced is significantly beyond the sensitivity curve of the human eye Vλ, which leads to a further reduction in luminous efficiency. The effect of different color temperatures on LED luminous efficiency is within ± 5%, and the effect of CRI at different values is usually within ± 15% (Figure 4).

However, the choice of LED color temperature and CRI will be limited by specifications, standards and specific requirements for specific applications. Conventional lighting technology can also see similar effects.

LED efficiency adjustable

Compared to conventional light sources, LED optoelectronic semiconductors provide additional dimensions, and the lamp manufacturer can adjust and set efficiency or luminous efficiency, i.e., current density, on a specific application basis.

LEDs are usually grouped according to the brightness and color of a particular operating current. For a particular grouping condition, it is possible to adjust the efficiency that is suitable for application and expectation to achieve the luminous efficacy level by changing the current density.

For example, if a 130 lm / W luminous efficiency LED is used at a specified packet current, the operating current can be reduced to 40%, resulting in an increase in luminous efficiency of 20% to 156 lm / W. If the operating current is increased to 140% Luminous efficiency will be reduced by 10% to 117 lm / W. Table 1 summarizes the variable current density.

The efficiency curve also depends on other parameters, such as the operating temperature or the maximum operating conditions that need to be met. At these two points, the LED's absolute luminous flux will naturally change. We will look at this effect carefully in the next section.

Use more efficient LEDs to reduce system cost

In the development process, LED becomes brighter, so the efficiency is getting higher and higher. However, some applications do not necessarily require higher efficiency. So, why is there a bigger demand for brighter LED? There is no doubt that one reason is that more efficient LEDs can significantly reduce costs at the solid-state lighting system level.

Look at an example. To make the following comparison, suppose you want to create a product with 100% efficiency and luminous flux. Here you can choose one of two LEDs: 100% LED 1 brightness, 110% more effective LED 2. Figure 5 and Figure 6 graphically depict two LEDs at system-level operating conditions.

Use system A with LED 1 as a reference. When the normal operating current is 100%, the relative efficiency of the system is 100% and the relative luminous flux is 100%. The relative number of LEDs required for the system is also 100%.

If the LED 2 is used, the system B in Fig. 5 can be implemented. In this system, the LEDs are operated at the same current and the same number of LEDs are used. As a result, the efficiency and brightness of the system B is increased by 10%. Brighter may be the selling point of the fixture, or may provide options for the lamp manufacturer to change the operating parameters.

If there is no need to improve efficiency at the system level, perhaps because the system has reached the threshold of energy efficiency levels, the efficiency can be reduced from 110% to 100% by increasing the current density. This means that with the addition of the original 10%, the new system C will be brighter, due to the same efficiency as System A, the brightness level is 42% higher than the system A.

But if the application may not need a larger luminous flux output. Table 2 summarizes this scenario and presents a System D option. Because system C is much brighter than system A, it is possible to choose to reduce the number of LEDs. System D uses 70% of the number of LEDs in system A, thereby significantly reducing the cost of the system.

This example can be very easily applied to systems with a large number of LEDs. If the flux package can be reduced and a packaged LED with a smaller chip can be used, it can also be used for a single LED system. The level of savings in each case depends on a variety of other parameters and may vary depending on the operating point.

The examples given here show that the color temperature and CRI have a significant effect on the luminous efficiency of the LEDs. Specifically, the higher the color temperature, the higher the luminous efficiency; the higher the CRI, the lower the luminous efficiency. In this respect, the key difference between the LED and the conventional light source is that the efficiency of the LED can be adjusted by the operating current. As can save LED cost, so in the choice of LED system should be fully considered

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