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. In other words, temperature will directly affect its performance and parameters. As an electrical-optical conversion device, its most important indicator is how many watts of electrical power is input and how much luminous flux (lumens) is output. We call it luminous efficiency (luminous efficiency). This indicator represents the most important quality indicator of LED. The user chooses the LED with the smallest wattage according to the luminous flux (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 a part of the light effect. We do not consider the light transmittance of the photomask (assumed to be 100%, in fact it is usually around 90%), but mainly consider the light efficiency of the LED light source itself. This kind of light effect is also divided into cold light effect and thermal light effect. Probably most people will not pay attention to the difference between the two light effects, so we will first discuss the cold light effect and thermal light effect.

　　1. What is cold light effect and what is thermal light effect

　　Simply put, 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 stable. Specifically, we usually use an integrating sphere to measure the luminous efficiency of the LED light source, then the luminous effect measured immediately after the power is turned on is the cold light effect (assuming that the room temperature at that time is 25 degrees), this "immediately" is about 5 Within seconds. The thermal light effect is to wait at least half an hour to 3 quarters later to reach thermal equilibrium (or thermal stability). Now we usually say that the light effects are cold light effects, or that most of the light efficiency indicators we mark on the outer casing or packaging are cold light effects.

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

　　In any case, I suggest that in the future, the thermal light effect should be used as the light efficiency index, and it should be marked on the packaging box.

　　2. What is the difference between cold light effect and thermal light effect

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

　　The initial value is measured immediately after the power is turned on, and the final value is the measured value after half an hour of waiting.

　　In fact, the stable value has not been fully reached in half an hour, and it may take three quarters or one hour to truly stabilize.

　　The following conclusions can be drawn from the above table:

　　(1) The initial value and final value of the luminous efficiency of all brands of LED lamps are different.

　　(2) The minimum difference is 0.97 and 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 luminous efficiency is mainly due to the increase in LED junction temperature.

　　(5) The increase 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 a thermally stable value. But the size of the increase is related to the quality of the radiator. The better the radiator, the lower the temperature rise. Therefore, in order to reduce the difference between thermal light effect and cold light effect, a good heat sink should be used as much as possible.

　　3. The hot and cold light effect of the LED light engine

　　Why do we need to study the hot and cold light effects of LED light engines? This is because light engines have their special features. The so-called light engine actually puts the LED power supply on the aluminum substrate where the LED light source is located. In this way, the electrical energy consumed by the LED power supply is also released on the aluminum substrate, which increases the heat that the aluminum substrate must dissipate. It will increase the junction temperature of the LED after thermal stability. Let’s give an example to illustrate that the most common type of electrolytic capacitor light engine on the market at present, its power efficiency is usually only about 85%, which means that 15% of the input power will be converted into heat and added to On an aluminum substrate. Assuming that the total input power is 10 watts, the remaining 8.5W is added to the LED, and assuming that the luminous efficiency of the LED itself is 40%, that is, only 3.4W of power is converted into useful light, and 5.1W of power is converted into heat. Originally, the aluminum substrate only needs to dissipate this 5.1W of heat, and now 15% of the power supply is added, which is 1.5W of heat. A total of 6.6W of heat needs to be dissipated, which increases the heat by 29.4%, which is close to 30%. The thermal resistance of the aluminum substrate does not change. The increase in power means the temperature rises, and the temperature of the aluminum substrate increases, which means that the ambient temperature of the LED on the same aluminum substrate increases, and the junction temperature also increases. , The result is reduced light efficiency.

　　The 60W electrolytic capacitor light engine of Epheline Optoelectronics, its power dropped from 62.53W to 60.354W within 30 minutes (closer to the rated value of 60W), so its light efficiency was 115.68lm/W from the beginning, It dropped to 107.48lm/W, which was only a decrease of 7.1%, which was almost the same as that of general lamps. This is equivalent to the normal luminous efficiency drop caused by the junction temperature of the LED rising from the room temperature of 25 degrees to the junction temperature of the designed rated value of about 85 degrees. This is because Epheline's constant current source efficiency is as high as 99%. Therefore, integrating such a high-efficiency constant current source on the aluminum substrate will not increase the junction temperature of the LED after thermal stability. Therefore, the difference between cold light effect and thermal light effect is still within the normal range.

　　By the way, the difference between the two light engines we are comparing here is by no means caused by the difference in the light efficiency of the LED itself. The LED used in a company’s non-electrolytic capacitor light engine is a very large chip 3535. The area is 1225mil2, and the LED used in the light engine with electrolytic capacitor is 3518, its area is only 630mil2, the difference is as high as 595mil2, and the former is larger. The absolute value of Epheline's thermal and light efficiency is as high as 107.48lm/W, which is 25% higher than the company's 86lm/W.

　　5. The difference between various non-electrolytic capacitor light engines

　　Although we only measured the difference between hot and cold light effects of a certain company and a certain product. Some people may suggest that this is a point-to-point, not comprehensive opinion. If this is true from the accuracy of the data, but if you use the approximate results of the data, these results are very representative. In fact, the reason is very simple. The main reason for the difference between cold and hot luminous efficiency is that the low-efficiency constant current source is integrated into the aluminum substrate together with the light source, which causes the junction temperature of the LED to be more than that of the junction without a constant current source. The temperature must be high, so its heat and light efficiency must be low. For the switching power supply, its efficiency may reach 95%, but due to the large number of switching power supply components and large volume, it is impossible to integrate it on the aluminum substrate. So all currently integrated on the aluminum substrate can only be linear power supplies. Of course, there are many types of linear power supplies. Nowadays, the most widely used is an electrolytic capacitor light engine with segmented lighting, and an electrolytic or non-electrolytic capacitor light engine that directly uses constant current diodes, but no matter it is Which kind of linear power supply, its efficiency curve is exactly the same, all decrease linearly with the increase of the mains voltage, and it is about 85% at 220Vac (see the figure below).

Efficiency of all linear constant current sources

　　Although Epheline’s constant current source is also a linear constant current source, it uses adaptive intelligent adjustment technology (Epheline’s power chip is called AICS, which is the abbreviation of "Adaptive Intelligent Current Source"). Makes its efficiency as high as 99% (see figure below).

Effile constant current source efficiency (the blue line is the efficiency of the constant current source itself, the red line is the total efficiency including the rectifier)

　　In order to remove the influence of the rectifier, Ephele’s light engine usually puts the rectifier and all protective components outside the aluminum substrate, and there is only a constant current source with 99% efficiency on the aluminum substrate. So its hot and cold light effect can be kept the same as without constant current source.