【作者】 王静；

【导师】 蔡忆昔；

【作者基本信息】 江苏大学 ， 动力机械及工程， 2014， 博士

【摘要】 随着汽车工业的不断发展以及节能减排、发展低碳经济政策的推行,人们对绿色汽车照明光源的需求与日俱增,使得LED在汽车照明领域的应用越来越广泛,汽车灯具的LED化成为必然的发展趋势。与传统汽车前照灯光源相比,LED具有使用寿命长、体积小、耗能低、可靠耐用、响应快、设计灵活等优点。尽管如此,LED要在汽车前照灯中取代传统光源并非易事,需解决配光性能、驱动电源和散热性能等关键问题。研究中,基于大功率白光LED的光学特性,实现了一种满足标准配光要求的汽车近光灯光学系统；基于LED汽车前照灯的电学特性,对该近光系统对应的驱动电源工作性能展开了研究；以结温和热阻作为控制指标,从被动式散热和主动式散热两个角度揭示了LED前照灯的散热机理。汽车前照灯的光学性能,尤其是近光灯的配光性能是影响汽车行驶安全和照明质量的关键因素。研究中,基于大功率白光LED芯片的光电特性和GB25991-2010标准要求,以非成像光学理论和能量守恒定律为指导,实现了一种由变截面椭球体反射器、挡板和自由曲面透镜组成的投影式LED前照灯近光光学系统；基于Monte-Carlo光学追迹原理,对该近光系统的配光性能和光能利用率进行仿真研究；基于多场耦合理论揭示了LED前照灯光学特性与散热性能的相互作用规律。结果表明：该近光系统的配光性能能够满足标准要求,其光能利用率相对于传统椭圆截面投影式近光系统有较大提高；LED前照灯的光学特性与散热性能彼此影响,相互制约,其工作过程是复杂的多场耦合过程。汽车照明电源的输出电压稳定性不高、工作环境恶劣。因此,研究一种高效、高精度、恒流、亮度可调且可靠性高的驱动系统,是LED前照灯得以成功应用的基础。根据LED前照灯的电学特性,选择电感升压式拓扑结构,实现了一种基于MAX16832A驱动器的大功率白光LED汽车前照灯驱动系统,并对其工作性能进行了研究分析；基于热耦合效应揭示了驱动电源对LED前照灯热传递作用效果的影响机制。研究表明：不同负载和输入电压下,该驱动系统能输出700mA稳流精度较高的恒定电流；相同负载下,驱动系统转换效率随输入电压增加呈现先增加后降低的趋势；负载为2颗LED时,驱动系统的整体转换效率高于1颗LED负载和3颗LED负载,且输入电压为13V左右时,驱动系统转换效率达到峰值94.3%；该驱动电路启动迅速,性能参数稳定,能满足大功率LED汽车前照灯的工作要求；驱动电源对LED前照灯散热性能的影响较为显著,它与LED光源形成的双耦合热源会使LED前照灯热负荷增加,通过折返式热管理可以同时降低驱动电路和LED光源的发热量,进而降低系统的热负荷。LED前照灯工作过程中,大部分输入功率将转化为热能,这些热量若不能及时散去,将导致芯片发光效率下降、使用寿命缩短以及色温漂移。研究了结温和热阻对LED前照灯光学特性和散热机制的影响,根据LED前照灯近场气体流动方式的不同,制定了被动式和主动式两种散热方案,优化了LED前照灯的散热机制。基于多场耦合理论研究了自然对流模式下LED前照灯的散热机理,结合传热强化的场协同理论,对散热系统进行了优化,同时完善了LED前照灯的强化散热理论。研究散热器材质对系统散热性能的影响,探寻了散热装置结构变化对系统散热性能产生影响的物理本质。研究表明：环境温度相同时,发热量越大,材料对系统散热性能的影响越大；改变翅片的拔模角度会影响系统的散热效果,随着翅片拔模角度的增大,散热系统最高温度呈现先下降后上升的趋势；对散热器翅片开缝可以改善气流速度场和温度梯度场的协同性,从而强化散热,开缝宽度过小时不利于气体流动,开缝宽度增大后,气体流动较均匀,流场与温度场的协同角变小；被动式散热方案只适用于LED前照灯在较低温度和较小发热功率的条件下工作,为进一步提高散热性能,需要采用强制散热措施。建立了LED前照灯散热的热阻网络模型,利用无相变换热方式下气体显热的改变、相变换热方式下工质汽化潜热能的变化以及基于帕尔帖效应的热电制冷理论,对LED前照灯的散热性能进行优化。探寻了导热板散热、热管散热和半导体制冷散热方案下LED前照灯的散热机制。当环境温度最高为80℃、LED发热功率最大为25W时,使LED芯片结温低于125℃作为控制目标。研究了不同散热方案下LED前照灯的传热特点,分析了存在的主要热阻；研究不同控制参数下的热传递作用特性,探寻了强化LED前照灯散热的基本途径。结果表明：热管式和半导体制冷式散热系统的热传递作用效果优于导热板式散热系统；采用风扇辅助导热板散热,其主要热阻存在于导热板的前后端,可使LED前照灯在低于65℃的环境温度下正常工作,但不能满足控制目标；热管式散热系统工作性能的影响因素主要有热管充液率、布置方式以及各组成段的长度；优化热管的受热面积与冷却面积之比,有利于热量对外传导,从而改善热传递作用效果,当蒸发段、绝热段和冷凝段长度分别为30mm、40mm和50mm时,试验得到的芯片结温较低；芯片结温随热管充液率的增加呈现先降低后上升的趋势,当充液率为30%时,热传递作用效果较好；由微热沉、U型热管和散热鳍片组成的“三级”散热系统不仅可以减小接触热阻和扩散热阻,还增加了热管的冷凝面积,使回流工质具有更高的过冷度,能够降低补偿室乃至蒸发段的温度,当环境温度为80℃、发热功率为25W时,测得的芯片结温为107.7℃,能够较好地满足控制目标要求；半导体制冷式散热系统的工作响应时间快、光通量衰减小、装置结构紧凑、散热效果显著,相同工况下,采用该方案的芯片结温仅为105.4℃,能够更好地满足控制目标要求,且其制冷量可以通过改变输入电压或电流来调节。更多还原

【Abstract】 LEDs are more widely applied in the field of automotive lighting with the sustainable development of automotive industry, the implementation of energy conservation and environment protection, the development of low carbon economy policy, and the increasing demand of green automotive lighting. It is an inevitable development trend that automotive light sources should be LEDs. LED headlamp is a kind of headlamp with LED light source. LED headlamp has the advantages just like long service life, small dimension, low energy consumption, reliability and durability, fast response, flexible design, etc. compared with traditional automotive headlamps. However, it is not easy for LEDs to replace traditional light sources in automotive headlamp unless it solves several key problems just like photometric characteristic, driver properties and heat dispersion. A new kind of automotive passing beam optical system has been designed which could satisfy the requirement of standard based on the photoelectric properties of high power white LED. Researches have been done to study the driving power performance based on LED headlamp’s electrical properties. It revealed the LED headlamp’s heat dissipation mechanism from two aspects of passive cooling and active cooling with junction temperature and thermal resistance as the control index.The automotive headlamps’ optical performance, especially the passing beam’s light distribution property is a key factor affecting driving safety and lighting quality. A kind of projection LED headlamp low beam system was designed which comprised variable cross-section ellipsoid reflector, flap and free surface lens. And the system was based on the electrical and optical properties of high power white LED light source, requirements of national standard GB25991-2010, the theory of non-imaging optics as well as the energy conservation law. The simulation research was carried out according to the Monte-Carlo optical tracing method. The constraint relationship between LED headlamp’s optical performance and cooling performance was analyzed. The results reveal that the designed LED headlamp low beam system can not only meet the standard requirements but also improve the energy utilization rate. The working process of the LED headlamps is multi field coupling process which contains photoelectric conversion, heat transmission, gas flow and pressure distribution.It is the basis of the successful application of LED headlamps to design a kind of driving circuit with high efficiency, high accuracy, constant current, high reliability and adjustable brightness which can be used in harsh conditions since lead-acid batteries are often used as lighting source in cars and it cannot guarantee the high precision stability of the output voltage. In the research, a constant current driving circuit based on the MAX16832A integrated controller has the functions with open circuit protection, short circuit protection, the anti-protection, over-current protection, and PWM dimming was designed according to LED diving power key design points, vehicle power supply voltage and it has been tested in the designed experimental system. The influence of the driving power on LED headlamp’s heat dissipation performance was analyzed. The test results show that the driving circuit can output high precision constant current steadily under different load and input voltage. The conversion efficiency increases at first and decreases later with the input voltage change under the same load. The conversion efficiency of the driving circuit with two LED chips as load is higher than that with one or three LED chips as loads respectively. The conversion efficiency gets its peak value of94.3%when the input voltage is about13V. Driving power and LED light source form the double coupling heat sources, which makes the LED headlamps heat load increase. Turn-back type thermal management can reduce driving circuit heat and LED light source heat simultaneously.Most of the input power will be converted to heat energy in the working process of LEDs. It will decrease the LED chips’luminous efficiency, shorten the service life, cause color temperature drift and other problems if the heat is not dispersed in time. The influence of junction temperature and heat resistance on LED headlamp’s optical properties and the heat dissipation mechanism have been studied, and solutions of passive and active cooling are offered to optimize the heat dissipation mechanism based on different conditions of the near field gas flow.The heat dissipation mechanism of LED headlamp under natural convection was studied based on multi-field coupling theory, and the cooling system by the field synergy theory of heat transfer strengthen was optimized. The LED headlamps’heat dissipation strengthen theory would be improved. The radiator material has great influence on the cooling performance of the system. Under the same environmental temperature, the bigger the calorific value is, the greater the material impact on cooling performance of the system will be. Changing the draft angle of the fin will also influence the radiator cooling effect. The highest temperature of the cooling system shows the tendency of rising after falling first with the increase of the fin draft angle. It can improve the synergistic effect of the airflow velocity field and temperature gradient field if the radiator fin is slotted, small slit width will go against gas flow. While the gas flow is more uniform, and the synergy angle of the flow field and temperature field become smaller when the slit width increases. In addition, the direction of the radiator fin may also affect the gas flow pattern, thus affecting the cooling effect. Passive cooling scheme is only applicable to the LED headlamps working under the condition that the environment temperature and heating power is lower. It needs to adopt compulsory cooling measures to optimize the cooling performance.The LED headlamps’radiator thermal resistance network diagram was established and its’heat dissipation performance was optimized by the means of gas sensible heat change under none phase-change, working medium’s latent heat of vaporization heat change under phase-change and thermoelectric refrigeration based on post Pal effect theory. The heat dissipation mechanism of LED headlamp has been studied under the schemes of thermal conductive plate cooling, heat pipe radiator and semiconductor refrigeration. The heat dissipation characteristics of different schemes were investigated and the main thermal resistances were founded based on the target of limiting the LED chip junction temperature under125℃when the environment temperature is up to80℃and the largest LED heat power is25W. The results reveal that the cooling performances of heat pipe radiators and semiconductor refrigeration radiator are superior to heat conducting plate radiator. It can ensure that LED headlamps work properly when adopting heat conducting plate radiator with cooling fan below65℃ambient temperature, but it can’t meet the design goals. The main thermal resistance of the system exists in both ends of the heat conducting plate. The influence factors of the working performances of the heat pipe radiator include the length of the heat pipe of each segment, the heat pipe liquid filled ratio and the heat pipe arrangement. It can improve cooling effect of heat pipe radiator if the heat pipes have smaller heated area to receive input heat but larger cooling area for heat dissipation. When the evaporation section length, adiabatic section length and condenser section length are30mm,40mm and50mm respectively, the LED chip will get minimum junction temperature. The junction temperature will rise later after reducing first with the increase of heat pipe liquid filled ratio, and the cooling effect is best when the heat pipe liquid filled ratio is30%. The ’three level’ heat pipe cooling device consisting of micro-heatsink, U-shape heat pipes and cooling fins has better cooling performance than heat pipes in rectangular layout radiator and heat pipes in plane layout radiator because it can not only reduce the contact thermal resistance and diffusion resistance, but also increase the heat pipes condensing area. The backflow working substance has a higher degree of super-cooling, besides, it can reduce the compensation chamber and the evaporation temperature. The LED headlamp equipped with semiconductor refrigeration radiator has fast response time, small luminous flux attenuation, compact structure, remarkable heat dissipation effect and adjustable refrigerating capacity.更多还原