Analysis of the progress of LED thermal resistance structure measurement and analysis technology

   　1 Overview:

　　LED solid light sources have the advantages of high efficiency, long life, flexible application, and no pollution, and are currently widely used in the field of lighting . However, most of the electrical energy consumed by LEDs is converted into heat energy, which significantly increases the temperature of the chip. Temperature has an important impact on LED performance, including changes in color temperature, reduced efficiency, reduced life and reliability, etc. Therefore, improving LED thermal management performance has become a key technical link that needs to be urgently solved in the design of high-power LED structures.

　　Commonly used LED thermal management analysis techniques include simulation using thermal design software and measurement using thermal resistance analysis equipment. The former is usually used for thermal management design of LEDs, while the latter focuses on thermal resistance measurement and analysis of actual samples to verify the actual effect of the design and product quality, and to improve the manufacturing process or guide secondary design.

　　2. Basic principles of thermal resistance

　　The heat dissipation of LED is carried out in three ways: heat conduction, convection, and heat radiation. In the LED, heat conduction is the main heat dissipation path, and its heat conduction performance depends on the thermal impedance of the medium. Thermal impedance is determined by thermal resistance and heat capacity. The definition of thermal resistance is:  . In the formula, ΔT is the temperature difference, Rth is the thermal resistance, and P is the thermal power.

　　As shown in Figure 1, heat flow corresponds to current, potential corresponds to temperature, thermal resistance corresponds to resistance, and heat capacity corresponds to capacitance. For any heat conducting medium element, it can be simplified to an RC parallel circuit:

RC parallel circuit model

　　When heat flows through the dielectric unit, a temperature difference will be formed at both ends. Similar to a circuit, heat will initially accumulate in the thermal capacitance, and the temperature difference between the two ends will gradually increase until thermal equilibrium is reached. The thermal resistance at this time is usually called "steady-state thermal resistance". Before the device reaches thermal equilibrium, the junction temperature of the device changes continuously due to the combined influence of the thermal capacitance and thermal resistance, and the corresponding thermal resistance also changes with time. This thermal resistance is called "transient thermal resistance". The measurement of transient thermal resistance is the basis for the measurement of thermal resistance structures.