Relationship between heat/cold factor and LED performance

Energy conservation is one of the key combinations of LED selling points and technology. Compared to conventional light bulbs, LEDs can significantly reduce the power consumption of lighting and improve the performance of lighting systems. Although its advantages are significant, there is a downside: at the same drive current, an increase in junction temperature results in a drop in light output, which causes a drop in light output and efficiency.

To compensate for this, designers often use a low current to drive more LEDs to maintain a reasonable junction temperature. The use of multiple LEDs can consume additional power and increase system cost. However, heat/cool factors like lED can reduce the impact and improve system performance.

What is the cold/heat factor?

This term describes junction temperature as a function of reduced light output, and the industry has not defined criteria for this factor. The lower temperature is always 25 ° C (room temperature), but the higher temperature can be any value within the LED limits. In this paper, we define the heat/cold factor as the ratio of light output at 25 ° C and 100 ° C. Figure 1 shows the relationship between standard luminous flux and heat sink temperature. The heat sink temperature is equivalent to the LED junction temperature under very short pulse test conditions.

Figure 1 Relationship between heat sink temperature and luminous flux

The standard luminous flux is 1 at 25 ° C and 0.84 at 100 ° C, so the cold/heat factor is 0.84. This means that the LED will lose 16% of the light when the heat sink temperature is 100 °C.

Cold/heat factor effect

At first glance, the impact of 16% reduction in LED light flux may not be significant. However, this problem is serious when considering that one lighting device is composed of a plurality of LEDs. The effect of the heat/cold factor is revealed by the combination of 10 LEDs and a flashlight.

For an average user, the reduced light output of the flashlight 16lm does not have a serious impact on its application. However, the reduction in light output of 160 lm has a huge impact on the downlight, so one or more LEDs need to be added to compensate for light loss. In this way, the overall power consumption and cost of the downlight will increase. ENERGY STAR has strict requirements on the luminous efficiency of LED lamps, and such a reduction in light output makes it difficult to meet these requirements.

Improved cold/heat factor

The latest LED technology has progressed in epitaxial levels, phosphors, mold attachments, etc., and the heat/cooling factor has been improved accordingly.

Currently, some high power LEDs on the market have a heat/cool factor of 0.94. This means that when the LED is operated at 100 ° C, it will lose 6% of the standard light. Figure 2 shows the light output reduction function for a typical and improved heat/cold factor.

Figure 2 improved heat/cold factor for a significant improvement in LED brightness

The improvement in the heat/cold factor increases the operating temperature range of the LED, giving the lighting designer the opportunity to work at any junction temperature within the LED limits.

Performance comparison

In many cases, many LED suppliers offer high light output rates in the product specifications. Lighting designers may prematurely conclude that LEDs with higher light output in the data sheet will perform better in the real world. But this may be a wrong conclusion, because all the values ​​in the data table are limited to the LED junction temperature of 25 ° C. The performance of LEDs in a lighting system must be evaluated at higher junction temperatures. Once this is done, you can pick a better product based on the comparison of real conditions.

As an example, two warm white LEDs are analyzed (see Table 2): LED1 has an improved heat/cold factor and LED2 has a typical heat/cool factor.

The performance of LED2 is better than that of LED1 at the junction temperature of 25 °C and the forward current of 350 mA described in the data sheet. However, a more realistic comparison will be made at a higher junction temperature (see Table 3).

Due to the high heat/cooling factor of LED1, the total light output of the nine LEDs is 50 lm higher than the 10 LED2. Although the rated light of LED1 is lower than that of LED2 at 25 °C, its performance exceeds that of LED2 under the current of 350 mA. Figure 3 (left) shows that LED1 is 100mA higher than LED2 driven by any forward current. Figure 3 (right) shows that LED1 is more efficient than LED2 under the same current drive.

Figure 3 Comparison of the performance of 9 LED1 and 10 LED2

Therefore, increasing the heat/cooling factor can significantly improve the performance of LEDs operating at higher junction temperatures, allowing the same light output to be achieved with fewer LEDs to reduce power consumption and overall system cost. When choosing LEDs for a particular application, it's important to evaluate the LED performance under real-world conditions, rather than relying solely on data sheets.

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