Optical Research And Thermal Analysis Of High-power Led Backlight

With the improvement of LED's lighting efficiency, compare to the conventional light resource, due to its advantage of low power consumption, environmental protection, small volume, long lifetime and so on, it is widely used in back lighting of LCD screen. This paper researchs a kind of high power, high bright LED back light resource, firstly we use ray-tracing software to model and simulate the optical system, and then based on optical design ,analysis of the thermal state, we testify whether the back light module is working in the rated temperature.By these means, we simplify the system design process and shorten the time of the design.Firstly, the luminous principles of LED devices and electrode structure of the chip were presented; the principle of white light LED was introduced, and the current international mainstream of white packaging technology was also introduced with the situation of evolution of LED chip packaging structure, and then analyzed the impact of junction temperature to LED devices.Secondly we introduce the basic measure of basic test, the basic elements of optical modeling and the optical tracing theory and steps of ray-tracing software tracepro, establish the simplified optical model of LED, research the illumination distribution of 1W LED, and then derive the formula for illumination uniform distribution. According to the design, in the case of the thickness of light mixing cavity is 20mm, when the distance between the LEDs is 20mm, the illumination of backlight is the most uniform, and it is testified by the simulation. So extend to the LED backlight ray, illumination homogeneity can got reach to 95%. Under the same illumination, replacing the 1W LED with 3W LED, the spacing of LED is 28mm, which enable the uniformity reach to 85%.Then we discussed the heat produced in LED, its character of thermal and thermal management. Meanwhile, LED is brought forward and its thermal problem is introduced. Then the methods that are used to analyze thermal problem are discussed, especially the ANSYS FE software. We established a thermal resistance model for our object, and get the results of inner thermal resistance is 4.2999K/W, the outer is 72.2367K/W and the total is 80.691K/W. Heat distribution of the package at steady condition is showed by simulating. We get the result of thermal resistance, heat flow density vector and temperature grads. Inner resistance is 5.875K/W, which is closed to the result of the theoretic analyzed. Thermal stress distribution in the LED is also simulated, which is in the allowable range. We also analyzed the relationship between the junction temperature with LED heat power and the convection coefficient. The LEDs array was also simulated by ANSYS. Due to the small space of the arrays,there are thermal coupling in LEDs,which cause the LED chip temperature rise. The effects of different LED array density on device operation were calculated. Then we replace 1W LED with 3W, the result shows that the temperature is under the rated range.At the end of this paper, an experiment to test the temperature is carried out and the results are compared with that of simulation. We conclude an indicative method to deal with the thermal in LED: get a character temperature by a simple method, then combined it with the relations that concluded by simulating, at last, junction temperature and thermal resistance are gained.