Thermal Testing And Simulation Of High Power LED

With the increasing luminous efficacy of Light emitting diodes(LEDs) and the successful fabrication of high power LED(HP-LED) chips,white HP-LED based solid-state lighting becomes feasible.Due to the outstanding benefits of LEDs compared with the traditional light sources,as well as the huge promising future markets,governments and industries around the world have put huge investments into the R&D of LEDs light sources.However, HP-LED has also faced two bottlenecks—relatively low luminous efficiency and high heat generation.This thesis mainly focuses on the research of the thermal issues of HP-LED, including thermal testing and simulation of the single-chip and multi-chips HP-LEDs.The structure of the thesis was organized as follows:Firstly,it briefly introduces the history and lighting mechanism of LEDs,as well as their characteristics and applications.The influences of junction temperature on properties of LEDs are also described in detail.And it points out the necessity of the research of thermal issues of HP-LED.Secondly,it introduces the basic principles of heat transfer,as well as thermal parameters and thermal models of LEDs.Various kinds of thermal measurement methods of LEDs are reviewed.Thirdly,physical mechanism of the forward voltage as a function of the junction temperature in light-emitting diodes(LEDs) was analyzed in detail.Experimental investigation was carried out systematically on the relationship between the forward voltage and the junction temperature in AlGaInP and InGaN high-power LEDs over wide current range(0.1～200mA) and temperature range(60～350k).It is discovered that at a constant current,their relationship can be divided into two segments,i.e.a linear inverse relationship at high temperature regime,where the voltage-temperature coefficient is closely relevant with forward current;while at low temperature regime,forward voltage increases drastically as the junction temperature decreases.A model is proposed to well explain the experimental results. Fourthly,cooling transient temperature curves of the single-chip HP LEDs were measured by the electritical method.Cumulative and differential structure functions were extracted from these curves to analyse thermal resistances and thermal capacitances of all regimes of LED by numerical computation method,1μs of sampling resolution of transient data and high repetition assure the accuracy and reliability of experimental result.Subsequently,thermal conduction capabilities of three different metal core printed circuit boards(MCPCBs) with the single-chip HP LEDs were compared by this method,from which it is discovered bergquist's MCPCB has the best thermal conduction capability,ANT's MCPCB takes second place,and the common MCPCB is the worst.The single-chip and three-chips HP-LEDs with the same package structure were also tested and discussed,and it is found that the experimental results are in good agreement with the thermal model theory.And it is discovered that the thermal resistance between the slug and MCPCB of three-chips HP-LEDs has the most obvious influence on the junction temperature of multi-chips LEDs,which should be carefully considered during heat management.Lastly,according to the actual packaging structure,thermal finite element models of the single-chip and multi-chips HP-LEDs are established,which are verified through experimental results of actual samples.The effect of different thermal conductivity of substances to thermal management of the single-chip HP-LED is discussed.And the effect of the different distance between two chips to thermal management of the multi-chips LED is discussed as well.The testing and simulation of thermal characteristics of HP-LEDs are two important aspects,especially in the process of their commercialization.As the packaging has a great impact on the cooling performance of LED devices,this research has an important practical value for HP-LED packaging.