Relationship between hot/cold factor and LED efficacy

Compared to traditional bulbs, LEDs can significantly reduce lighting electricity consumption and improve the efficiency of lighting systems. Although its advantages are significant, there is a disadvantage: at the same drive current, the increase in junction temperature will cause the light output to decrease. This change causes a decrease in light output and efficiency. Energy saving is one of the key combinations of LED selling points and technology.

To compensate for this phenomenon, designers often resort to driving more LEDs at lower currents to maintain a reasonable junction temperature. The use of multiple LEDs may consume extra power and increase system cost. However, factors such as the hot/cold factor of LEDs can reduce the impact and improve system performance.

What is the hot/cold factor
This term describes the function of junction temperature as a function of light output degradation, and the industry does not have a standard for defining this factor. The lower temperature is always 25°C (room temperature), but the upper temperature can be any value within the LED's limits. In this article, we define the hot/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

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

Effect of the hot/cold factor
At first glance, a 16% reduction in LED luminous flux may not seem like much of a problem. However, when you consider that a lighting fixture is made up of multiple LEDs, the problem becomes more serious. Compare a 10-LED downlight to a single-LED flashlight, and the effect of the hot/cold factor becomes apparent.

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

Improved cold/hot factor
The latest LED technology has made progress in epitaxy level, phosphors, die attachment, etc., and the hot/cold factor has been improved accordingly.

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

Figure 2: Improved hot/cold factor results in significant improvement in LED brightness

Improvements in the hot/cold factor increase the operating temperature range of LEDs, giving lighting designers the opportunity to operate at any junction temperature within the limits of the LED.

Performance Comparison
In many cases, many LED suppliers provide datasheets with very high light output rates. Lighting designers may prematurely conclude that an LED with a higher light output in the datasheet will perform better in the real world. But this may be a wrong conclusion, because all values ​​in the datasheet are limited to when the LED junction temperature is 25°C. The performance of LEDs in lighting systems must be evaluated at higher junction temperatures. Once this is done, the better product can be picked out according to real-world comparisons.

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

At the datasheet-specified junction temperature of 25°C and forward current of 350mA, LED2 performs better than LED1. However, a more realistic comparison would be at a higher junction temperature (see Table 3).

Due to the high hot/cold factor of LED1, the total light output of 9 LED1s is 50lm higher than that of 10 LED2s. Although LED1 has a lower rated luminous flux than LED2 at 25°C, it outperforms LED2 when driven at 350mA. Figure 3 (left) shows that LED1 outperforms LED2 by 100mA at any forward current. Figure 3 (right) shows that LED1 is more efficient than LED2 when driven at the same current.

Figure 3 Performance comparison of 9 LED1 and 10 LED2

Therefore, improving the hot/cold factor can significantly improve the performance of LEDs operating at higher junction temperatures, allowing fewer LEDs to achieve the same light output, reducing power consumption and overall system cost. When selecting an LED for a specific application, it is important to evaluate the LED's performance under real conditions rather than relying solely on the data sheet.