Han-Kuei Fu

Investigation of dynamic color deviation mechanisms of high power light-emitting diode

Abstract Environmental concerns have led to the popularity of solid stating lighting, in which a high quality white light source depends on the stable property of light emitting diode. This study examines a white-light high-power light-emitting diode composed of a blue chip and yellow phosphor. A white-light light-emitting diode can be divided into four parts—a blue chip, yellow phosphor, transparent silicone, and reflector. In a transient experiment, the wavelength shift of the blue chip markedly affects the conversion efficiency of yellow phosphor, causing white-light deviation, especially in the sharp variation region of absorption of yellow phosphor. A series of short-term experiments was conducted to identify the mechanisms of color deviation between yellow phosphor and transparent silicone. The robustness of commercial phosphor and silicone was much stronger than expected. In addition to a yellowed reflector and blue chip degradation, several combinations of degradation mechanisms between yellow phosphor and transparent silicone. In a long-term experiment, damaged silicon confines blue light resulting in warm white light. Two suggestions are provided to obtain white-light light-emitting diodes with high color reliability.

Evaluation of temperature distribution of LED module

Abstract For high-power light-emitting diodes, heat management and reliability are important issues. The reliability and long lifetime of solid state lighting are especially essential in the high temperature applications, such as street lighting and automotive lighting. Because of the characteristics of semiconductor, the electrical property of light-emitting diodes is varying with operating temperature. Then, the alternation of electrical property changes the heating power and operating temperature of light-emitting diodes. This is a mutual interaction between electrical property and operating temperature, until they reach the steady state. In this paper, we designed experiments and calculation to optimize the simulation of temperature distribution of light-emitting diode module. With forward voltage measurement and thermal transient testing of light-emitting diodes, we obtained the initial values of simulations. The infrared camera captured the thermal images to verify the simulation results. With this method, we can effectively evaluate the temperature distribution of light-emitting diode module.

Curved remote phosphor structure for phosphor-converted white LEDs

A phosphor-converted light emitting diode with curved remote phosphor layer was obtained in this study. By changing the geometry of the flat remote phosphor layer in a conventional remote phosphor structure, the package extraction efficiency and luminous efficacy of the curved remote phosphor structure was improved by reducing the total internal reflection at the interface of the air and phosphor layer. Ray tracing simulations were performed to analyze the package extraction efficiency of the curved remote phosphor structure. Experimental results showed that the convex remote phosphor structure had high luminous efficacy with high color uniformity. The convex remote phosphor structure would be a suitable structure if the remote phosphor structure is used in lighting applications.

Analysis of Thermal Resistance Characteristics of Power LED Module

Multichip LED arrays are widely used for lighting to provide high luminance. Luminous efficacy, lifetime, and color temperature are highly dependent on the temperature at p-n junction. This paper investigated the effects of distance, number of chips, and driving current on the thermal resistance of LED module. Thermal resistance dramatically increased as the distance between LED chips decreased due to significant thermal spreading impedance for heat dissipation from junction to ambient. The parallel-resistance formula substantially underestimated the junction temperature of the LED modules due to significant thermal crowding effect. Thermal boundary can also rise junction temperature as the distance to the board edge decreased in both the two-chip and four-chip modules. Infrared results showed that chip temperatures were highly consistent with thermal resistance measurements under different driving currents.

The Modeling of Two Phosphors in Conversion White-Light LED

Because of the high color rendering property, the two phosphors that convert into white light in a light-emitting diode are essential in the general lighting applications. Predicting the spectrum after two phosphors mix is challenging because of the mutual interaction between the absorption of long wavelength phosphor and the emission of short wavelength phosphor. To efficiently model the target spectrum, this paper provides a systematic method for simulating the concentration and proportion of two phosphors by using empirical equations. The simulated and experimental spectra overlapped effectively. The results show that the maximum differences are about 0.0046 for CIE 1931 color coordinates (x, y), about 59 K for correlated color temperature, and about five for color rendering index, respectively. The results show that this spectral optimization is an effective approach.

The Study of Thermal Resistance Measurement of Multichip LED

The high-power light-emitting diode (LED) lighting is especially essential in the high-temperature applications such as the street and automotive lighting. However, >70% of electrical power is converted into heat due to the low electrical-optical conversion efficiency. Because of the characteristics of semiconductor, the electrical property of LEDs sensitively varies with the operating temperature. These deviations affect the measurement of light and electricity. In this paper, the steady-state thermal measurement techniques developed by the National Institute of Standards and Technology are used to study the thermal resistances of multichip LEDs. The experimental results show that the measurements of thermal resistances with different connections are constant at the same power dissipation. The simulation results show that the detail of thermal interaction between powered and unpowered chips and confirm the experiment results.

Measurement of LED chips using a large-area silicon photodiode

We propose the output power measurement of bare-wafer/chip light-emitting diodes (LEDs) using a large-area silicon (Si) photodiode with a simple structure and high accuracy relative to the conventional partial flux measurement using an integrating sphere. To obtain the optical characteristics of the LED chips measured using the two methods, three-dimensional ray-trace simulations are used to perform the measurement deviations owing to the chip position offset or tilt angle. The ray-tracing simulation results demonstrate that the deviation of light remaining in the integrating sphere is approximately 65% for the vertical LED chip and 53% for the flip-chip LED chip if the measurement distance in partial flux method is set to be 5-40 mm. By contrast, the deviation of light hitting the photodiode is only 15% for the vertical LED chip and 23% for the flip-chip LED chip if the large-area Si photodiode is used to measure the output power with the same measurement distance. As a result, the large-area Si photodiode method practically reduces the output power measurement deviations of the bare-wafer/chip LED, so that a high-accuracy measurement can be achieved in the mass production of the bare-wafer/chip LED without the complicated integrating sphere structure.

Low-Frequency Noise Analysis of Electrostatic Discharge Tolerance of InGaN Light-Emitting Diodes

In recent years, with an extensive use of InGaN light-emitting diode (LED), how to assess the LED quality and further improve the LED reliability are very important. In this paper, the noise spectrum measurement techniques are used to assess the electrostatic discharge tolerance and quality of InGaN LED devices. Experimental results show that the noise spectrum measurement distinguishing the LED device reliability is more effective than the current-voltage curve measurement. In the evidence, emission microscope, scanning electron microscope, and transmission electron microscopy images show that the noise source and the cause of failure of the LED device are attributed by the poor quality of the SiO2 and Indium Tin Oxide (ITO) interface.

Evaluating the junction temperature of AC LEDs by DC and AC methods

Alternating-current (AC) driven light-emitting diodes (LEDs) have become the trend of solid-state lighting (SSL) products. The junction temperature is an important index of LEDs reliability and efficiency. In other words, with proper thermal management of AC LEDs lighting products, the high performance of SSL products will be achieved. In order to obtain the junction temperature, we study and compare two published evaluating methods differentiating between the measurements of DC and AC in this paper. The first method is in which a low reference current having a pulse width was applied and the corresponding voltage across the device was measured and correlated to the junction temperature (Tj). The second method is using an active heat sink for recovering the root mean square (RMS) current of the first half cycle to estimate the junction temperature. The experimental evidence showed different aspects and variations of evaluating the AC LEDs junction temperature. The variations of evaluating junction temperature were caused by the switch time and phase of different source measurements in the first method and the capture time of the first half cycle in the second method. With proper capture time, the rising junction temperature in the second method might be negligible.

Thermal transient characteristics of flip chip high power light emitting diodes

The die-attached quality and the thermal transient characteristics of high power flip chip light emitting diodes (LEDs) are investigated using thermal transient tester. Various die-attached materials were utilized to compare the difference in their thermal resistances and long term performance. By applying accelerated aging stress, the deterioration rates at the die-attached layers were obtained. Numerical simulation provided further understanding of LED device temperature distribution and also revealed that the thermal variance at the die-attached interface can be recognized within only few milliseconds for the flip chip structure. The effects of bump arrangement and material were analyzed to achieve high temperature uniformity and low thermal resistance for high power LEDs.