Study On GaN-Based HB LED And Thermal Management For LED Automotive Headlamp

Due to its advantage over bulb and fluorescent lamp, light emitting diode (LED) was reported as light source for the 21th century. However, with the increase of power and heat flux, high die temperature has become the main bottleneck for its further application. Consequently, thermal design should be indispensable in design and application of LED. To improve the performance of LED, GaN-based high-brightness (HB) LED was experimentally and numerically studied from the angles of material, device and application in LED automotive headlamp.First, thermal conductivity and electron transport of III-Nitride were calculated. III-Nitride featuring high dislocation density is the leading material for HB LED chips. Regarding this characteristic, in modified Callaway's model, effects of dislocation and isotope were all put into account to investigate the thermal conductivity of GaN. The results showed that effects of dislocation and impurity on thermal conductivity largely depend on its concentration. To address the ambiguity of the impact of isotope on thermal conductivity, relative importance of isotope and dislocation was investigated. It was found that there are critical values of impurity concentration and dislocation density below which the isotope effect on conductivity has to be considered and beyond which impurity and dislocation have significant impact. Dislocation scattering was also added into the Monte Carlo method to study electron transport. Numerical results of InN showed that electron mobility in InN with structural defects has a critical dislocation density below which dislocations have no effect on the mobility and beyond which the increased dislocation density results in an order-of-magnitude decrease in the electron mobility in InN.Based on accurate heat source, a model to calculate the temperature and thermal stress filed was put forward. Effects of substrate thermal conductivity and heat transfer coefficient on device performance were investigated. Numerical result showed that performance of device can be improved significantly by changing prevailing sapphire substrate with other high conductivity substrate, especially Si substrate. Flip-chip was also studied with this model. It was found that flip-chip may deteriorate the heat dissipation unless bonding area was deliberately arranged.To evaluate the thermal performance of device, a convenient and practical method was established to measure thermal resistance of LED. Based on this model, thermal resistance test instrument was presented. Thermal resistance of an exclusive LED for automotive headlamp was measured using this instrument. Experimental result showed that tested value of thermal resistance was similar to its nominal value.Experimental system for LED automotive thermal test was designed. With this system, three thermal management schemes including aluminum plate, heat pipe and heat sink were tested. From the thermal point of view, heat sink was the best one, heat pipe the second, aluminum plate the third. A headlamp comprising 6 HB LED was also tested. It was found that at room temperature, with a headlamp power of 40 W, junction temperature of LED was about 70 oC by placing LED directly on heatsink. While the ambient temperature was set to 80 oC, junction temperature was 123 oC. Automotive headlamp can work very well.