Implementation And Heat Dissipation Mechanism Research Of Led Headlamp Passing Beam System

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.