Maintenance of stable light emission in high power LEDs
Introduction
The recent improvements on light-emitting diodes (LEDs) technology directly promote the noticeable growth of many and diverse products, such as the mobile headlamp [1] and the backlight module for the LCD panels [2]. Additionally, unlike mercury-containing fluorescent lamps, LEDs do not carry the pollutant substance inside, so LEDs possess higher potential than fluorescent lamps to be the main light source for general lighting [3]. However, there are still some critical issues unsolved for the practical applications. As well known, the most important one among these issues is the light-emission characteristics of LEDs sensitive to the diverse driving conditions, especially the injected electric current and the junction temperature in operation. When the electric current injected into LEDs, some part of the electric energy is transformed into the optical energy and the rest is converted into the heat energy. Accordingly, the more the heat energy accumulated in the junction, the higher the junction temperature is. Hence, the internal efficiency of LEDs will be dropped down and the light-emission properties will be changed, such as the degradation on optical power and the shift on chromaticity [3], [4], [5]. Therefore, the subject of how to maintain the light-emission characteristics of LEDs as time goes in the practical operation is much more important and very urgent at present [6].
In previously work, several literatures had proposed several methods for the light-emission characteristics of LEDs in real time through the feedback control mechanism [7], [8], [9]. For example, in US Patent No. 7,132,805, a control mechanism composed of a temperature sensor and a current waveform sensor was proposed for monitoring working characteristics of LEDs, such as the operating temperature and the driving current. For the purpose of maintaining the light power of LEDs, an input current was adjusted in real time according to feedback signals from these working sensors. In addition, a dynamics of color mixing LED device composed of color sensors and temperature sensors for monitoring chromaticity and operating temperature of LEDs in real time was presented by US Patent No. 7,119,500B2. Similarly, for the purpose of maintaining the chromaticity of LEDs, the operating temperature of LEDs was adjusted in real time according to feedback signals from sensors. However, these aforementioned schemes for modulating light-emission characteristics of LEDs must work restrainedly with external sensors, such as color sensors and temperature sensors, which not only complicate structures of the whole LED module, but also significantly increase production costs.
In this work, the dependency of the emission light from high power LEDs upon the driving electric current and the junction temperature will be explored in details. One integrated measurement system is proposed for the study in simultaneously obtaining all the thermal-optic-electric characteristics of LEDs throughout the measuring. Based on the basic feedback control methodology, one simple maintaining procedure is applied for the stable light emission in high power LEDs. It shows the robustness of the maintaining procedure from the environment change with the least heat dissipation in the operation of the high power LEDs. The results imply that all the thermal, the optic, and the electric properties of the high power LEDs should be taken into consideration in the same time rather than separately when maintaining their operation.
Section snippets
Integrated measurement system
As the p–n junction locating deep inside the LED, it is almost impossible to detect the junction temperature directly by any temperature sensors. Recently, there are several research have reported on the measurement of the junction temperature for LEDs, including the forward voltage method [10], the peak wavelength shift method [10], the high energy slop method [8], the nematic liquid crystal method [11], and the radiation energy method [12]. Among these methods, the forward voltage method is
Results
In order to differentiate the performance of LEDs between with the control scheme and without the control scheme, the whole experimental process is arranged to be of three stages: with control scheme (W-1, the baseline), without the feedback control scheme (W/O, the contrast), and with control scheme applied again (W-2, the verification). In Fig. 3, it presents the results of the control scheme. With the comparison among the results of stages W-1, W/O, and W-2, it proves that the control scheme
Conclusion
In conclusion, an effective feedback scheme for controlling light-emission characteristics of LEDs is developed and implemented. It does not need any optic sensors as those in the conventional ways. The experiments prove the effectiveness of the feedback control scheme. Furthermore, it is worthy noted that this control scheme exhibits a great reliability on controlling light-emission properties of LEDs. With the effective feedback control scheme, the operation of LEDs will be much more robust
Acknowledgements
This wok was partly sponsored by the National Science Council, Taiwan, under the Grant Nos. of NSC 100–3113-E-008–001 and NSC 101–2911-I-008–501, and also partly sponsored by National Central University, Taiwan, under the Grant No. of 100G903–2.
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