【作者】 于新刚；

【导师】 梁新刚；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2008， 博士

【摘要】 发光二极管(LED)被称为二十一世纪的光源。然而随着发光二极管功率的不断增大,热流密度也不断增大,散热问题已经成为制约其发展和应用的瓶颈,因此必须不断改善LED器件及应用过程的热设计。本文针对GaN基功率型LED,采用数值模拟与实验相结合的方法,从材料、器件、汽车前照灯应用系统三个层面对其进行了深入系统的热分析和研究。III价氮化物是功率型LED芯片制做的主要材料,其位错密度较大。本文在热导率计算的修正Callaway模型中,引入了位错和同位素散射的影响,计算了GaN的热导率。结果表明位错和杂质对热导率的影响都存在临界值。讨论了位错密度和同位素影响的相对大小,当位错密度小于1010 cm-2时,同位素对GaN热导率影响很大,而当位错密度大于这一值时,同位素影响很小,从而澄清了同位素对热导率影响大小的争议。同时在输运性质计算的Monte Carlo方法中引入了位错的影响,发现位错对迁移率的影响也存在一临界值。针对功率型LED芯片,建立了利用准确的芯片热源模型计算芯片温度分布和热应力的方法。研究了基板的热导率和底板换热系数对芯片最高温度和最大应力的影响,指出综合考虑散热和成本,采用热导率为140 Wm-1K-1的硅基板是一个很好的选择。倒装芯片的研究发现,倒装芯片只有合理的设计键合区域才能改善芯片的散热性能。为了方便的对LED热阻进行测试,设计了一种简单易用的器件热阻测试方法——快速切换法,并基于此方法设计了LED热阻测试仪器。利用设计的热阻测试仪对车灯用大功率LED的热阻进行了测试,测试结果与器件标称值符合的较好。针对LED汽车前照灯设计了热测试实验系统。并设计了导热板、热管及直接热沉式的热控制方案。研究发现将LED直接置于热沉上的方案散热效果最优,热管方案次之,导热板方案最差。用6个LEWD1A型LED组成的前照灯的测试表明采用热沉式方案,总功率为40 W,室温时,LED结点温度为70 oC;环境温度升高为80 oC时,结点温度为123 oC,前照灯可以长时间正常工作。更多还原

【Abstract】 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.更多还原