High thermal stability of correlated color temperature using current compensation in hybrid warm white high-voltage LEDs

This study experimentally and numerically examines the correlated color temperature (CCT) stability issue for hybrid warm white high-voltage light-emitting diodes (HV-LEDs) by using a current compensation method. This method could efficiently maintain the CCT stability factor at approximately 1.0 and yield greater color uniformity with ' ' u v Δ values ranging from 0.017 to 0.003 in CIE 1976 chromaticity coordinates. The simulation results show that the red chip intensity drop is the primary cause of CCT instability in the hybrid warm white system when the temperature increases. Therefore, Furthermore, results indicate that the relative lumen drop improves from 21% to 15% by using a current compensation method. ©2013 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (230.2090) Electro-optical devices. References and links 1. E. F. Schubert and J. K. Kim, “Solid-state light sources getting smart,” Science 308(5726), 1274–1278 (2005). 2. S. Nakamura, T. 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Introduction
Light-emitting diodes (LEDs) have been regarded as the next generation of environment lighting sources because of its long lifetime, high efficiency, and energy-saving properties, especially in solid-state lighting (SSL) [1][2][3][4].Using blue chips with yellow phosphor has recently become the most common method for generating white light sources [5,6].Therefore, great efforts have been made to synthesize the novel phosphor with III-Nitride LEDs in the recent years [7][8][9][10][11].As for the part in the InGaN-based LEDs chips, the progress in white LEDs is strongly driven by the advances in high efficiency InGaN-based LEDs emitting in the visible spectral regime [12][13][14][15][16][17][18][19][20][21][22][23][24].These InGaN-based LEDs are used as high-power pump excitation sources in phosphor-based white LEDs.Recent works by using new types of active regions with reduced charge separation [12][13][14][15][16], growths and substrate technologies [17][18][19], nano/microphotonics structures [20,21], and barrier engineering [22][23][24] have led to significant improvement in nitride-based LEDs.Thus, the availability of high efficiency nitride LEDs enables the practical implementation of phosphor-based white LEDs.As a replacement for traditional lamps, high lumen efficacy and suitable color quality are critical standards for white LEDs.The quality of white LEDs is determined by correlated color temperature (CCT) homogeneity and high color rending index (CRI), which can be further improved [25].Several studies have focused on reducing CCT deviation [26,27].Liu et al. proposed that the phosphor location influences CCT performance [28].Furthermore, the patterned remote phosphor structure has also been shown to obtain high chromatic stability [29].Therefore, the control of CCT deviation in white LEDs has become the critical issue to replace the conventional incandescent light for SSL applications.
Conversely, recent research on high-voltage light-emitting diodes (HV-LEDs) has indicated that multiple series-connected micro-diodes in a single large chip obtains high forward voltage with a low driving current, thereby reducing current crowding and efficiency droop [30].Additionally, two types of color temperature primarily exist for SSL: cool white and warm white.Red phosphor is frequently adopted in phosphor composition to achieve lower color temperatures at approximately 3000 K as a warm white lighting source and a higher CRI [31,32].Furthermore, it has recently been proposed that the hybrid warm white HV-LED systems comprising blue chips, phosphor, and red chips achieve greater luminous efficacy and CRI values compared with previous warm white technology.However, the AlGaInP material used in red LEDs is sensitive to temperature variation, thereby resulting in a significant intensity drop compared with other systems [33].Moreover, the dominant drift leakage in AlGaInP is also important to emphasize at the high temperature [34].Therefore, varying thermally intensity drops and wavelength shift behavior between blue and red chips potentially causes CCT instability at different temperatures.
In this study, the characteristics of hybrid warm white HV-LEDs investigated experimentally and numerically.This is the first time the current compensation method has been proposed to obtain uniformity of angular CCT in hybrid white warm HV-LEDs.CCT deviation maintains similar values from 20 to 90 °C using the current compensation method.Furthermore, the lumen drop behavior is improved slightly with the increased temperature.Consequently, the color deviation ' ' u v Δ value is characterized with CIE 1976 chromaticity coordinates.

Experiments and simulation
The hybrid warm white system is composed of four 45 x 45 mil InGaN blue HV-LEDs, two 50 x 25 mil AlGaInP red HV-LEDs, and yellow phosphor powder (YAG:Ce, approximately 15 μm).The domain peak emission wavelength of our blue LEDs and red LEDs are 452 and 617 nm, respectively.The electric circuit for InGaN HV-LEDs and AlGaInP HV-LEDs are independent in the system to demonstrate the current compensation method.For comparison, a constant 20 mA input current on both blue and red chips is used for in the conventional system, whereas the proposed system is treated using the current compensation method with a constant 20 mA current for blue chips and a tunable current (15 to 25 mA) for red chips to control red light intensity and maintain CCT stability.Moreover, the luminous efficiency of HV-LEDs is 100 lm/W, and the CRI is 90 at approximately 3000 K, with a 20 mA driving current.Figure 1 shows the temperature dependence of the HV-LED emission spectra.The peak shift and intensity drop transform especially the red chip.Furthermore, the parameters for peak shift and intensity drop as a function of increased temperature are shown in Table 1.However, these phenomena cause a significant deviation in CCT and consequent instability in white LEDs.This study employs the OptisWorks simulation software based on the Monte-Carlo method to analyze the influence of the thermally dependent intensity drop and wavelength shift of red chips on CCT of the hybrid warm white system [29].In the simulation, the parameters of the refractive index in phosphor, silicone, and the air are set to 1.82, 1.54, and 1.0, respectively.The dimensions of the chips are fixed at 45 x 45 mil for the InGaN blue HV-LED and 50 x 25 mil for the AlGaInP red HV-LED.As for the phosphor layer, the particle sizes and the concentration of phosphor are set as average diameter of 12 μm and 8%, which is uniform distributed in the silicon binder.Moreover, the shape of the phosphor layer is designed as the slightly convex and the thickness of the phosphor layer is about 300 μm in the middle.The phosphor absorbs the blue light from the LED chips and emits the isotropic yellow light and the amount of light being absorbed and scattered by phosphor particles are simulated using Mie Theory [35].The reflectance of the surface on the board and reflector is about 90% in the simulation.The additional procedures are as follows: First, the simulated structure included two red chips and four blue chips in the phosphor mixture, and then the measured spectra at different temperatures were input into the simulation model.Thereafter, the thermal dependent wavelength shift and intensity drop are two calculated values in the simulation.For the wavelength shift, the peak wavelength of the red chip is shifted from 617 to 627 nm (the same as our experimental results in Table 1), as shown in Fig. 2(a).The CCT in the simulation result shows a similar value and indicates that the spectrum shift of the red ship is not the primary reason for CCT instability, as shown in the inset of Fig. 2(a).For the peak wavelength, the red chip spectrum at various light output intensities from 100% to 46% with identical peak wavelengths is entered into the simulation software (Fig. 2(b)).CCT increases at approximately 1000 K with a 47% intensity drop.Therefore, the instability of CCT in hybrid white warm HV-LEDs may be attributable to the red chip intensity drop, rather than the spectrum shift, and become critical when the ambient temperature increases.From the simulation results, the red chip intensity drop in the hybrid warm white LEDs is the primary cause of CCT instability.The wavelength shifts of the LED chips are the minor factors in CCT instability.Thus, the concept of using the current compensation method to increase the red chip intensity is proposed, especially at high ambient temperatures.In this experiment, an integrated sphere is employed for optical characteristics measurement, a thermoelectric cooler for thermal control, and two power sources for applying different currents to the red chip and blue chip separately.The current compensation method in HV-LEDs is shown with a constant 20 mA input current for blue chips and a tunable current for the red chips at a room temperature of 90 °C.To maintain the sample CCT, the input current of the red chip is increased with the ambient temperature.Therefore, CCT stability can be obtained at various temperatures.Additionally, this experiment is designed to simulate a real case in hybrid warm white systems and solve CCT instability.

Results and discussion
The warm white light source is used for indoor lighting, and its CCT stability is crucial.Therefore, the CCT stability factor is defined as the variations in CCT value at different temperatures divided by the CCT at the initial temperature.The CCT stability factor with and without current compensation at various ambient temperatures is shown in Fig. 3.The CCT deviation in hybrid warm white HV-LED with current compensation is improved and maintains a similar value at different temperatures, compared with the device without current compensation.Furthermore, the CCT stability factor maintains a value greater than 0.99, regardless of the temperature using current compensation method, whereas the factor decreases to 0.88 at the ambient temperature of 90 °C in the reference.This result indicates that the red chip intensity drop is the primary factor influencing CCT variation.This finding is consistent with the simulation results.In addition, the compensated current for the red chip at various ambient temperatures is shown in Table 2. To obtain a constant CCT, the compensated current increases with the ambient temperature.This advantage in CCT stability is caused by the supplement of higher red light intensity the current compensation.Moreover, the CRI value would be increased due to the stability of the red light with the current compensation method.The normalized lumen efficacy and luminous efficiency of the two samples with and without current compensation, as a function of ambient temperature, were investigated (Fig. 4(a) and 4(b)).The lumen drop is improved from 21% to 15% by using current compensation.With the increase in ambient temperature, the differences of lumen drop between the two samples become greater.This indicates that the heat generation inside the LEDs increases, thereby reducing lumen efficacy.However, the results show that the current compensation method in hybrid white warm HV-LEDs maintains an identical flux output, confirming the practicality of this method for SSL applications.As for the luminous efficiency, the hybrid warm white LED with current compensation is higher than the conventional one due to the less electrical power from 20°C to 50°C.It obtains the same luminous efficiency at 60°C because of the same electrical power in red chip.With the increase temperature after 70 °C, the luminous efficiency of hybrid warm white LED with current compensation is lower than the conventional one because of the higher driven current in the red LED chips.To compare the color distribution between two samples, the color deviation of a lighting system, ( ' ' u v Δ ) is calculated as follows:

Fig. 1 .
Fig. 1.Temperature-dependent emission spectra of the hybrid warm white HV-LEDs.The inset shows the sample for the hybrid warm white HV-LEDs.

Fig. 2 .
Fig. 2. (a) The simulation spectra with different peak wavelengths of red chip.The inset shows the simulation results on CCT with different peak wavelength of red chip (b) The simulation spectrums with different red chip intensity.The inset shows the simulation result on CCT with different red chip intensity.

Fig. 3 .
Fig. 3.The CCT Stability Factor of with and without current compensation at different ambient temperature

Fig. 4 .
Fig. 4. (a) The relative lumen drop (b) luminous efficiency with and without current compensation at different ambient temperature