The Relationship Between Hot/Cold Factor And LED Efficacy

30 Sep

Energy saving is one of the key combinations between the selling point for LEDs and technology. Compared to the traditional light bulbs, LEDs can significantly reduce electricity consumption for illumination and improve the efficiency of lighting systems. Although the advantage is significant, there is a negative factor: under the same driving current, the junction temperature increase will lead to the decrease of light output, which both reduces the light output and luminous efficacy at the same time.

To compensate for this phenomenon, designers often employ the method of using low current to drive more LEDs to maintain a reasonable junction temperature. The use of multiple LEDs may consumes extra power and increases system costs. However, LED’s hot/cold factor can reduce the influence and improve the system performance.

What is the cold/hot factor?

This terminology describes junction temperature as the function of light output decreasing, the industry has not defined standards for the cold/hot factor. Lower temperature is always 25 ℃ (room temperature), but higher temperature can be any value within the LED limits. In this paper, we define the hot/cold factor as the light output proportions at 25 ℃ and 100 ℃. Figure 1 shows the relationship between standard luminous flux and heat sink temperature. Heat sink temperature used is equivalent to the LED junction temperature under the condition of a very short pulse test.

Figure 1, the relationship between heat sink temperature and luminous flux

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

The influence of hot/cold factor

At first glance, 16% LED luminous flux reduction may have little influence. However, when considering that a lighting device is composed of many LEDs, this is a serious problem. Comparing a recessed down light with 10 LEDs and a flashlight with 1 LED, the influence of hot/cold factor will emerge.

For an ordinary user, a flashlight reducing 16lm light output does not seriously affect its application. However, 160lm light output reduction will seriously influence the use of a recessed down light, so we need to add one or more LEDs to compensate for the light loss. Thus, the overall power consumption and costs of the recessed down light will increase. ENERGY STAR has very strict requirements for the light efficiency of LED lighting fixtures, and such light output reduction makes it difficult for the LED lighting fixtures to to meet these requirements.

Table 1: The influence of hot/cold factor to 10 LEDs and single LED respectively
Lighting type Numbers of LEDs Hot/cold factor Total light output at 25℃ (lm) Total light output at 100℃ (lm)
Flashlight 1 0.84 100 84
Recessed down light 10 0.84 1,000 840

The improved cold/hot factor

The latest LED technology in the chip extension level, phosphor, mold accessories and other aspects has been developed, hot/cold factor is improved accordingly.

Currently, the hot/cold factor some high-power LEDs in the market is 0.94. This means that when LEDs work at 100 ℃, they will lose 6% of the standard luminous flux. Figure 2 shows the function of light output decreasing under a typical and improved hot/cold factor.

In Figure 2, the improved hot/cold factor makes LED’s brightness improved significantly

Hot/cold factor improvements increase the working temperature range of LEDs, which enables lighting designers to have the opportunity to work at any junction temperature within LED limits.

Performance comparison

In many cases, the product instruction provided by many LED suppliers will give a high light output efficiency. Lighting designer may be premature to conclude that LEDs which have a high light output in the data table will perform better in the real world. But this may be a wrong conclusion, because the all the values in the data table are limited to condition that when the LED junction temperature is 25 ℃. The performance of LEDs in lighting system must be assessed at a higher junction temperature. Once this is done, we can compare according to real conditions and pick out better products.

For instance, we analyze two warm white LEDs(see table 2): LED1 has an improved hot/cold factor, while LED2 has a typical hot/cold factor.

Table 2: Performance of two types of LEDs at the junction temperature of 25℃ and current of 350mA
Light output (lm) at junction temperature of 25℃ Maximum input current (mA) Maximum junction temperature (℃)
LED 1 80 1,000 150
LED 2 84 1,000 150

At 25 ℃ junction temperature and 350mA forward current described in the data table, LED2 performs better than LED1. However, a more realistic comparison would be carried out at a higher junction temperature (see table 3).

Table 3: Performance of two types of LEDs at the current of 350mA and higher junction temperature
Number of LEDs Junction temperature (℃) Total light output (lm)
LED 1 9 106 662
LED 2 10 119 608

As a result of LED1’s high hot/cold factor, the total light output of 9 pieces of LED1 is 50lm higher than that of 10 pieces of LED2. Although at 25 ℃, the rated luminous flux of LED1 is lower than that of LED2, under the driving of 350mA current, its performance is obviously better than LED2’s. Figure 3 (left) shows that under any forward current driving, LED1’s driving current is 100mA higher than that of LED2’s. Figure 3 (right) shows that when drived by the same driving current, LED1’s efficacy is higher than LED2’s.

Figure 3: Performance comparison between 9 pieces of LED1 and 10 pieces of LED2

Therefore, improving the hot/cold factor can significantly improve the performance of LEDs which operate at higher junction temperature, we can get the same light output with fewer LEDs to reduce power consumption and overall system costs. When selecting LEDs for a specific application, it is important to assess the performance of LEDs under real conditions, rather than depending solely on data sheet.



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