In the conventional white LED package structure, the phosphor is directly coated on the chip. When working, the heat released by the chip is directly loaded on the phosphor, which causes the temperature rise of the phosphor, so that the conversion efficiency of the phosphor at a high temperature is lowered. Introducing a layer of low thermal conductivity thermal isolation between the phosphor and the chip can effectively prevent the heat of the chip from being directly loaded onto the phosphor, reducing the temperature of the phosphor layer, so that the white LED can be kept under high current injection. High lumen efficiency.
In addition to the heat released by the chip, when the coated phosphor is excited by blue light, the conversion efficiency of the phosphor has not reached 100%, and the phosphor particles themselves have a small amount of heat release due to the existence of other losses such as scattering. It is easy to form local heat accumulation. For this reason, when the conversion efficiency of the phosphor material is low, it is necessary to provide a heat dissipation channel for the phosphor to prevent localized heat generation of the phosphor particles. The thermal interaction between the chip and the phosphor in the two structures is compared by two experiments in the conventional phosphor coating method and the thermal isolation packaging method.
1. LED chip heats the phosphor
In order to evaluate the effect of LED chips on the thermal performance of phosphors, we have produced two sets of white LED package structures, one using traditional phosphor coating and the other using thermally isolated phosphor coating. Figure 1 A cross-sectional sample drawing of the thermally isolated package structure.
Figure 1 Cross-section of a conventional white LED (a) Phosphor thermal isolation package structure (b), h = 1mm [14-16]
The phosphor thermal isolation package structure is realized by means of phosphor coating. Phosphor coating technology is a new type of phosphor coating method, which is designed according to the requirements of light emission. The structure of the phosphor film layer is completed in a special mold. After peeling, the phosphor film is removed. The layer is transferred over the LED chip, and a layer of low thermal conductivity silicone layer is also present between the LED chip and the phosphor film layer. To show the difference in thermal performance between the two package structures, we compared the temperature profiles of the surface of the two package structures. Figure 2 shows the radial distribution of the measured IR camera surface of the two package structures at 200, 350, and 500 mA DC drive. At 200 mA drive current, the thermally isolated package structure is 1.6°C cooler than the traditional package center temperature. At 350mA and 500mA injection current, the temperature difference of the phosphor layer reached 8.5 ° C and 16.8 ° C, respectively, and the maximum surface temperature of the conventional structure phosphor has reached 130.2 ° C at 500 mA injection current. In addition, the temperature of the entire phosphor surface layer of the thermal isolation package structure is uniform, and the phosphor center temperature is higher in the conventional structure, especially at a large current.
We use finite element simulation to analyze the effect of parameter changes in the package structure on the performance of white LEDs. The results show that the temperature of the phosphor layer can be regulated by the package structure design and the thermal conductivity adjustment of the package material. Figure 3 is a longitudinal distribution of temperature in the LED thermal isolation package structure, the temperature of the phosphor layer being greatly reduced by the introduction of the thermally isolated silicone layer.
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