Cold and heat effect analysis of LED light engine

We all know that LED is a semiconductor light-emitting device, and semiconductor is a heat-sensitive device, which means that it is very sensitive to temperature. Or temperature will directly affect its performance and parameters. As an electro-optical conversion device, its most important indicator is how many watts of electric power is input, and how much luminous flux (lumen) is output. We call it luminous efficiency (luminous efficiency), which represents the most important quality indicator of LED. The user also selects the LED with the smallest wattage according to the luminous flux (the number of lumens) he needs.

However, although the LED device itself has a light effect, it does not mean that the user can directly enjoy the light effect, because assembling the LED device into a lamp will lose some of the light effect. We do not consider the light transmittance of the reticle (assuming 100%, in fact, usually around 90%), but mainly consider the light efficiency of the LED light source itself. We also have two kinds of light effects: cold light effect and hot light effect. Perhaps most people will not pay attention to the difference between the two light effects, so we have to discuss the cold light effect and the thermal light effect here first.

1. What is cold light effect and what is hot light effect?

Simply speaking, the cold light effect is the light effect measured when the LED light source is at room temperature (25 ° C), and the thermal light effect is the light effect measured after the LED light source is thermally stabilized. Specifically, we usually use the integrating sphere to measure the light efficiency of the LED light source. Then the light effect measured immediately after the power is turned on is the cold light effect (assuming that the room temperature is 25 degrees at that time), this "immediately" is about 5 Within seconds. The thermal efficacy is at least half an hour to three quarters of an hour to reach thermal equilibrium (or thermal stability). Nowadays, the light effect we usually say is cold light effect, or most of the light efficiency indicators that we usually indicate on the outer casing or outer packaging are cold light effect.

For the vast majority of users, what they care about is actually hot light. Because LED lighting will be used for more than half an hour, unfortunately, the user only knows the cold light effect of this light fixture and does not know the thermal efficacy of this light. In a sense, this is an unintentional trick. Consumer behavior.

In any case, I suggest that in the future, the thermal efficacy should be used as a light effect indicator, marked on the box.

2. What is the difference between cold light effect and hot light effect?

Let's take a look at how much difference between cold light effect and hot light effect. In order to understand the difference between the hot and cold light effects of LED lamps of various well-known brands, we went to the United States and Japan to purchase several well-known brands of LED PAR lamps for actual testing. The results are shown in the following table:

The starting value is measured immediately after the power is turned on, and the final value refers to the test value after waiting for half an hour.

For example, the output lumen of Philips' 432195 bulb has been reduced from 1860 lm to 1680 lm to 92% after half an hour. The specific lumen reduction process is shown in the figure below.

In fact, it has not yet reached the stable value for half an hour, and it may take three quarters or an hour to be truly stable.

From the above table, the following conclusions can be drawn:

1. The luminous efficacy start and end values ​​of all brands of LED lamps are different.

2. The minimum difference is 0.97, the maximum is 0.86

3. The difference in input power is actually independent of the thermal characteristics of the LED.

4. The difference in light efficiency is mainly due to the increase in junction temperature of the LED.

5. The rise in junction temperature is unavoidable, because the efficiency of the LED itself is only about 40%, and the others are converted into heat, so the heat gradually increases after power-on, and finally rises from room temperature to heat stability. However, the magnitude of the rise is related to the quality of the heat sink. The better the heat sink, the lower the temperature rise. Therefore, in order to reduce the difference between the thermal efficacy and the cold light effect, a good heat sink should be used as much as possible.