Relationship between hot/cold factor and LED efficacy

Compared to traditional bulbs, LEDs can significantly reduce lighting power 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 cold/hot factor

This term describes the function of junction temperature as a function of light output degradation, and the industry has not defined a standard for 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.

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 cold/hot factor

At first glance, a 16% reduction in LED luminous flux may not seem like a big deal. However, when you consider that a lighting fixture is made up of multiple LEDs, the problem becomes more serious. Compare a 10-LED recessed downlight to a 1-LED flashlight, and the impact 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 efficiency 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, phosphor, mold 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 works at 100°C, it will lose 6% of its standard luminous flux.

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 product specifications that give very high light output rates. Lighting designers may prematurely conclude that LEDs with 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, better products can be picked out according to real-world comparisons.