Driving current versus high power white LED phosphor conversion efficiency

Introduction

As a new type of light source, LED has the advantages of small size, long life, high reliability, fast response, high saturation, low radiant heat, low power consumption, easy modulation and integration, etc. [1~3], widely used. Medical, display, electronic communication, signal indication, car headlights, decoration and white light lighting [4 ~ 7]. There are a variety of methods for making white LEDs [8-11]. Considering the technical and cost factors, blue LED chips and YAG phosphors are the main methods for making white LEDs, and they have been commercialized [12]. The principle of illumination is that the blue LED chip radiates the combined blue light, a part of the excited YAG:Ce3+ phosphor emits yellow light, and the other part transmits and excites the yellow light to produce white light [13,14]. As a new type of illumination source, the conversion efficiency of phosphors used in LEDs has been highly concerned. At present, the research on phosphor conversion efficiency mainly focuses on the preparation process of phosphor, doping modification, smearing technology, temperature quenching characteristics and matching of emission peak wavelength and absorption wavelength [15], but regarding driving current versus fluorescence There are few studies on the influence of powder conversion efficiency, especially the intrinsic reason and influence law of the influence of current change on the conversion efficiency of LED phosphors. The design of the driving current used by LEDs cannot be well guided and helped. Therefore, it is important to study the effect of current on the conversion efficiency of phosphors.

In this paper, four kinds of 1W power white LEDs for illumination were prepared. The internal mechanism of phosphor conversion efficiency with drive current was analyzed. The variation of phosphor conversion efficiency with current was studied.

2 experiment

First, the GaN-based blue chip samples of different structures produced by the four companies were tested for variable current. The current range was 100-900 mA and the test temperature was 25 °C. The instrument used was a three-color LED photoelectric color heat test system from Zhejiang University to test the electroluminescence (EL) spectrum of LED samples.

Next, after applying the same YAG:Ce3+ phosphor to the four chips, four 1 W power white LED samples were prepared using the same transparent silica gel package. The four kinds of samples have the same package conditions except for the chip. The sample number obtained by using the Taiwan chip is T, the sample number obtained by the US chip is A, and the samples made by the domestic two chips are numbered B and C, respectively. The variable current test of 100 to 900 mA was performed on the four samples, and the variation of each parameter of the LED sample with the current was analyzed.

Finally, the same phosphors were excited by blue light chips with peak wavelengths of 450, 452 and 454 nm, respectively. The photoluminescence (PL) spectra of the phosphors excited at different wavelengths were tested, and the PL spectra of the phosphors at different temperatures were measured. . The excitation light source uses a blue LED having an emission wavelength of 460 nm.

3 Results and discussion

It is found that the luminous flux of four kinds of LED samples is close to linear change at a small current. When the current increases, the luminous flux increases, and the luminous efficiency decreases with increasing current. Because the performance of Blu-ray chips is different, the law of variation of luminous flux and light efficiency is also different. Figure 1 shows the luminous flux of the four samples at different driving currents, and Figure 2 shows the luminous efficacy of the four samples at different driving currents.

Not only does LED luminous flux and light efficiency change greatly with current changes, but other optical characteristics of LEDs also change greatly. Figure 3 shows the spectral curves of the four samples at different driving currents. It can be seen that the spectral curve changes regularly with the increase of the driving current. The most important one is that the peak wavelength appears blue-shifted, while the dominant wavelength appears red-shifted.

The change in peak wavelength has a great impact on LED performance, especially for LED light conversion efficiency.

Table 1 shows the peak wavelengths of four and four samples at different driving currents. As the driving current increases, the spectral shape of the blue chip changes to some extent. The peak wavelength of the blue band spectrum shifts to the short wavelength direction, that is, a blue shift occurs.

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