Yongzhi He

Angular CCT Uniformity of Phosphor Converted White LEDs: Effects of Phosphor Materials and Packaging Structures

In this work, we studied the effect of different packaging parameters on the angular homogeneity of correlated color temperature (CCT) in phosphor-in-cup light-emitting diode (LED) packages, including phosphor particle concentration, phosphor-layer surface curvature, and lens surface curvature, using a Monte Carlo ray tracing simulation tool. An optimized packaging geometry with convex phosphor layer was achieved with good CCT uniformity and highest lumen efficiency. Moreover, an approach was developed for the prediction of color uniformity and lumen efficiency when varying the surface curvature of either phosphor layer or clear lens, since the inconsistency of phosphor layer or lens cap happened commonly to the fabricated phosphor conversion white LEDs (pc-WLEDs).

One-Component, Low-Temperature, and Fast Cure Epoxy Encapsulant With High Refractive Index for LED Applications

The cationic polymerization of diglycidyl ether of bisphenol-A (DGEBA) using a latent cationic thermal initiator, ammonium antimony hexafluoride, and the effect of addition of cycloaliphatic epoxy resin on the polymerization were studied with DSC, TGA, and hardness measurement. It was found that the cationically polymerized DGEBA at 0.5 phr initiator level gives the best compromise of chemical reactivity and optical properties, and therefore is suitable for light emitting diodes (LED) applications as an optically transparent encapsulant. This encapsulant, on one hand, is room temperature stable for at least six months, yet cures fast at low temperature, on the other hand. Moreover, the encapsulant has a high refractive index of 1.6 resulting from the high density of aromatic structure in the cross-linked network. Packaged with this encapsulant, the LED device is shown to exhibit an increased light output as demonstrated by both simulation and experimental measurement. The introduced system provides a new one-component encapsulant that is optically transparent, cures fast at low temperature with no sacrifice of room temperature stability, and has high refractive index at the same time.

Influence of Die Attach Layer on Thermal Performance of High Power Light Emitting Diodes

In this paper, the influence of the die attach adhesive (DAA) layer on the thermal performance of high power light emitting diodes was first investigated by using finite element analysis, and some key results were verified by the experimental data. Effective thermal management of the studied light emitting diode package can be achieved by selecting a DAA material with a proper thermal conductivity and by manipulating the geometry parameters of the DAA layer, such as the DAA area, and the bond-line thickness. The significance of DAA thermal conductivity to heat dissipation was further demonstrated by an analysis of the bottleneck to heat transfer.

Thermal management of high power LEDs: Impact of die attach materials

Thermal management plays an important role in high power light emitting diodes (LEDs). The physical properties, such as light output, dominate wavelength and life span of LEDs are affected adversely by the junction temperature. As the result, heat flow path in LEDs becomes more and more important with increasing input power. The packaging materials including the die attach materials can decide heat flow rate from junction to board. However, die attach materials have far lower thermal conductivities compared to other packaging materials. The thermal conductivity of die attach materials thus can be reflected by the thermal resistance from junction to board. We developed an in situ measurement method after LEDs packaged incorporated with the measurement of junction temperature, light output and input power to exactly determine thermal resistance from junction to board. The results provide a reference in the selection and development of die attach materials for high power LEDs.

Packaging of Phosphor Based High Power White LEDs: Effects of Phosphor Concentration and Packaging Configuration

Two types of packaged white light emitting diodes in which one has a flat-top (FT) emitting surface and the other is aflat-top-with-lens (FTWL) type are fabricated by using the same leadframe and investigated on their optical properties, such as optical power, luminous efficiency, correlated color temperature (CCT), chromaticity coordinate, and color-rendering index (CRI), as a function of phosphor concentration in silicone encapsulant. It is found out that the optical power, CRI, and CCT decrease steadily as the phosphor ratio increases, while the luminous efficiency increases up to a level and then drops after a certain value of the phosphor ratio for both types of packages. Due to the totally internal reflection (TIR) at the encapsulant-air interface, the FT package shows a 10∼11% power (in mW) reduction compared with the FTWL package at the same phosphor concentration. However, it is demonstrated that the FT package provides a more efficient way of utilizing phosphor than the FTWL package based on the same targeted chromaticity coordinates due to the TIR effect inside, resulting in a reduced phosphor usage with a lumen output only about 3% lower than that of the FTWL package.

Design optimization of wedge-shaped lensed fibers for fiber-laser coupling: Fresnel reflection and non-Gaussian mode effects

Two approaches, i.e., a diffraction model considering Fresnel reflection and a paraxial beam propagation method are utilized to optimize the design of wedge-shaped lensed fibers for coupling between laser diodes of highly asymmetrical beam and single-mode fibers. It is demonstrated for the first time that, in contrast to the prevailing view, the non-Gaussian fiber-mode shape modification by the wedged fiber tip greatly reduces the coupling efficiency when the wedge angle is relatively large, while the Fresnel reflection is found to play an insignificant role. The fundamental cause of the non-Gaussian shape effect is fully explored and, consequently, a novel design is proposed to suppress this effect and increase coupling efficiency. Moreover, the novel design is relatively simple, and it does not complicate the fiber tip fabrication process.

Packaging and AC powering of LED array

High power LEDs for lighting application can be implemented with dies. Multiple LEDs are usually connected in series to sustain high power supply voltage like AC 110V due to low LED forward voltage. AC-DC converter is often required for LED arrays working under an AC power supply. But the AC-DC converter brings power consumption overhead and degrades the overall system efficiency as much as 15%. In this work, a high power ceramic COB (Chip On Board) LED array packaging technology capable of working under AC 110V is developed. A total of 40 LEDs are used in the ceramic COB LEDs array to allow it work directly under AC 110V power supply. A special powering method is designed and dedicated to the COB LED array. The special powering method ensures both high light output and high LEDs array reliability especially under supply voltage variations. The measured COB LEDs current vs. AC supply voltage variation is shown in Fig.1 An extremely high driving efficiency (>98%) at the max power and a high power factor are achieved due to the elimination of the AC-DC converter. The electrical efficiency over LEDs forward current is plotted in Fig.2. The structure and top view of COB LED packaging is shown in Fig.3 and Fig.4. The COB package dimension is 68mm by 28mm. With 150mA LEDs forward current, the measured total power is 20W. The max LEDs forward current for this design is 700mA and the max power is 95W. The cross section of the PCB design is shown in Fig.5. Since AC-DC converter reliability is becoming the bottleneck of the high power LED lighting system, the presented ceramic COB LEDs array package with the special powering circuit reliability is enhanced due to the elimination of AC-DC converter. The system reliability is performed and shown in Fig.6. There is no performance degradation of both the ceramic COB LED and control circuit for 1000 hours. Long term reliability test of the system is still under testing and will be presented.

Improved full-vectorial beam propagation method with high accuracy for arbitrary optical waveguides

An improved finite-difference (FD) full-vectorial beam propagation method is introduced for arbitrary optical waveguides with a dramatic improvement in accuracy. This method is developed based on the generalized Douglas scheme and a novel FD formula for the cross-coupling terms which, expressed in explicit form, is independent of specific types of waveguides. The present method is demonstrated for a strongly guiding rib waveguide.

Investigation of LED Light Output Performance Characteristics Under Different Alternating Current Regulation Modes

LED performance characteristics under dc regulation modes have been studied extensively. But limited literatures are dedicated to studies of LED performance characteristics under different ac regulation modes. With direct ac powering LEDs for lighting application being available and more attractive in efficiency due to the elimination of ac-dc converter, investigation of the influence of different ac current regulation modes on LED performance characteristics becomes important and essential. This paper presents an experimental investigation of different current regulation modes on LED light output with different LED connection configurations. It is found certain current regulation mode under which a higher light output with the same absolute average current can be obtained. The result is due to facts that the absolute average current is the measurement of electrons and holes recombination rate inside the LED, and second-order nonlinear effect and thermal issue lead to the different light output for different ac regulation modes.

Materials Challenges and Solutions for the Packaging of High Power LEDs

As a future lighting source, high power LED lighting is significant in that it provides decades of lifetime under normal operation, requiring a fraction of the power demanded for traditional lighting solutions. For LED lighting to be a viable lighting source, there are many technical challenges to be resolved. Among them the light extraction efficiency, the chip overheating, and the light output degradation are the key issues, which turn out to be all related to the packaging materials. This talk outlines those challenges and reports some of the recent progresses in resolving the key problems