Relationship between heat / cold factor and LED efficiency

Compared to traditional light bulbs, LED can significantly reduce the electricity consumption of lighting, lighting and improve system efficiency. Although the advantage is significant, but there is a disadvantage: under the same driving current, the junction temperature increase will lead to a decline in light output, this change caused by the decline in light output and efficiency. Energy saving is one of the key selling points of LED.

To compensate for this, designers often use a low current drive more LED method to maintain a reasonable junction temperature. The use of multiple LED has the potential to consume additional power and increase system costs. However, the thermal / cold factor like lED can reduce the impact and improve the performance of the system.

What is the cold / thermal factor

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

At 25 DEG C, the standard luminous flux is about 1, at a temperature of 100, so that the cold / heat factor is 0.84., which means that when the heat sink temperature is at, the LED will lose 16% of the luminous flux.

Effect of cold / thermal factor

At first glance, 16% of the LED luminous flux reduction may not affect. However, the problem is serious when considering a lighting device made up of multiple LED. Is composed of 10 LED block with a LED lamp consisting of flashlight, heat / cold impact factor emerged.

For an ordinary user, the light output of the flashlight 16lm is not a serious impact on its application. However, the light output reduction of 160lm is huge impact on embedded downlight, therefore need to add one or more LED to compensate the loss of light. In this way, embedded downlight overall power consumption and cost will increase. The energy star has strict requirements on the luminous efficiency of LED lamps, so it is difficult to meet these requirements.

Improved cooling / heating factor

The latest LED technology has been developed in the aspects of epitaxial level, phosphor, die attachment and so on.

At present, some of the high power LED on the market has a thermal / cold factor of 0.94. which means that the LED will lose 6% of the standard luminous flux when operating at a temperature of 100.

The improvement of thermal / cooling factor increases the operating temperature range of LED, so that the lighting designer has the opportunity to work at any junction temperature in the LED limit.

performance comparison

In many cases, a lot of LED suppliers will provide a very high light output rate. Lighting designers may be premature to conclude that LED with higher light output in the data sheet will perform better in the real world. But this may be a false conclusion, since all values in the data table are limited to 25 LED junction temperature. The performance of LED in the lighting system must be evaluated at a higher junction temperature. Once you do this, you can pick out a better product in terms of real conditions.