摘要 :
The illumination of an LED may be affected by operating temperature even under constant-current condition. A constant-power driving technique is proposed in the present study for LED luminaire. A linear system dynamics model of LE...
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The illumination of an LED may be affected by operating temperature even under constant-current condition. A constant-power driving technique is proposed in the present study for LED luminaire. A linear system dynamics model of LED luminaire is first derived and used in the design of the feedback control system. The PI controller was designed and tuned taking into account the control accuracy and robust properties with respect to plant uncertainty and variation of operating conditions. The control system was implemented on a microprocessor and used to control a 150W LED luminaire. The test result shows that the feedback system accurately controls the input power of LED luminaire to within 1.3 per cent error. As the ambient temperature changes from 0 to 40℃, the LED illumination varies slightly (-1.7%) for constant-power driving, as compared to that of constant-current driving (-12%) and constant-voltage driving (+50%). The constant-power driving has revealed advantage in stabilizing the illumination of LED under large temperature variation.
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摘要 :
IR LEDs are widely used in the near-infrared (NIR) spectroscopy, especially in handheld devices for the detection and measurement of organic materials, allowing faster and cheaper examinations. The -OH, -NH and -CH functional grou...
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IR LEDs are widely used in the near-infrared (NIR) spectroscopy, especially in handheld devices for the detection and measurement of organic materials, allowing faster and cheaper examinations. The -OH, -NH and -CH functional groups found in organic substances can frequently be detected by spectroscopy through absorbance measurement at the resonance wavelength of valence-bond vibrations. The measured wavelengths are 4-2.5 μm, while signal to noise ratio of photon detectors is low due to thermal noise at room temperature. The lst-3rd harmonic absorption bands are located in the range of the NIR, where smaller signals can be measured effectively in practice. The LEDs have tunable narrow wavelength range therefore they are suitable for such measurements as radiation sources. Further advantages of LEDs compared to incandescent lamps are small dimensions, high efficiency, and low power consumption. In order to tune the emission wavelength of the LED, the composition of the semiconductor light-emitting layer has to be properly set. The change in chemical composition will generally affect the lattice constant too. The crystal defects caused by lattice-mismatch often reduce the efficiency. The emission wavelength is tunable in the quaternary GalnAsP/InP material system meanwhile the lattice-constant remains unchanged. Absorption band of the organic materials is generally broader than the emission spectrum of the LED, therefore a broader emitter is preferred. To achieve this, a number of solutions are known. One of these is to use multiple layers with different composition, where the primary light comes from the active layer at the p-n junction of the diode, then excites the second smaller band gap layer producing longer wavelength secondary light by photoluminescence. The transmitted part of the primary light, together with the secondary light, results in a broader spectrum. This method is not yet implemented in this material system, so my work is focused mainly on this approach.
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