Dimming Method Research Articles

Design of a digital controller for an LED driver with a digital dimming

This letter presents a new digital controller (DCL) with a new dimming method for high power-factor LED driver circuits. The core of proposed digital controller is based on a zero-voltage (ZVD) detection and a digital sine generator (DSG). By using the DSG, an effective dimming can be carried out easily. The proposed DCL was simulated and fabricated using the 1μm-650V DMOS process. The results obtained indicate that the inductor current waveform is in phase with the input voltage to gain a high power factor, because the inductor current is synchronized to the zero voltage signal from ZVD as expected. Also, the LED current changes according to the reference voltage, which is controlled by an external switch that adjusts the brightness of the LED.

An Integrated Lighting Unit With Regulated Pulse Current Driving Technique

This paper proposes an integrated lighting unit using the regulated-pulse-current-driving technique for low-power application. On-chip/package integration of a white light-emitting diode (WLED) with a light-emitting diode (LED) driver, also named a lighting unit in this paper, may be ideal for smaller and lighter portable devices. The integrated lighting unit is connected to a pulse current driver fabricated with the complementary metal-oxide-semiconductor (CMOS) technology and a WLED on-chip integration. Such a combination would help reduce the packaging costs and the size of the device as well as increase the luminous density. Two control signals are used in the proposed driver to modulate the pulse current. One signal modulates the amplitude of the pulse current from 100 to 500 mA, and the other modulates the pulse current frequency between the ranges of 350 and 750 kHz. The dimming method and experiments on the prototype of the lighting unit have been built and measured. The active area of the designed chip is 0.64 mm × 0.56 mm in a 0.5-μm CMOS process. The resulting integrated lighting unit works well; therefore, this compact device is suitable for portable applications.

Standard color liquid crystal displays by a dimming technology

Dimming technologies have been used in liquid crystal display (LCD) products, especially for mobile applications, to reduce power consumption. It is widely known that dimming technologies tend to degrade the color accuracy. This article, however, presents that color performance of a dimmed LCD is better than a conventional one. We show a max method, one of dimming methods, converting an input digital data into higher values that have less color shift phenomenon. We propose how to compare the color performance of original and dimmed colors based on a modified standard. The average value of measured Δu′v′ of 25 dimmed colors was 0.0040, whereas that of the original colors was 0.0190. In addition, the average value of measured Δ of 25 dimmed colors was 2.40, whereas that of the original colors was 9.24. The measured results prove that the dimming technology can make LCD better in not only power consumption but also color performance. This article is the world's first report that breaks the usual stereotype on dimming technologies. This work proposes a dimmed LCD as a standard color display. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 480–485, 2014

방송조명용 LED 램프를 위한 플리커 저감 조광방식

LED lamps are appropriate to the light source for use in broadcasting lighting. When the lighting frequency of LED lamp is different from the shutter frequency of camera, however, the flicker appears on the moving image which is taken by the camera under the dimmed light of LED lamp with conventional dimming methods. In this study, a new dimming method for the LED lamps used as the broadcasting lighting is proposed to minimize the flicker. A lighting period in the proposed dimming method is divided into the several sub-periods which have weight of the bits. Also, the proposed dimming method has characteristics that are optimizing the turn-on timings of two LED lamps to reduce the turn-off periods of the lamps and not to overlap the turn-on periods of the lamps. In the experiments, the incidence of the flicker is taken by the camera under several lighting conditions of the conventional dimming methods and the proposed dimming method. In addition, the brightness values of the frames are obtained in the moving image, and incidence of the flicker is evaluated and compared with conventional methods. From the results, the incidence of the flicker in the proposed method is more improved than the conventional methods.

A cost‐effective PWM dimming method for LED lighting applications

AbstractThis paper presents a novel cost‐effective pulse‐width‐modulation (PWM) dimming method for light‐emitting diode (LED) lighting applications. High efficiency, high power factor, circuit simplicity, and low cost can be achieved by a Flyback power factor correction converter with the studied adaptive PWM dimming method. The operation principles and design considerations of the proposed LED dimming method are analyzed and discussed. A 45 W laboratory prototype has been built and tested to verify the feasibility of the proposed scheme. Copyright © 2013 John Wiley & Sons, Ltd.

Influence of Voltage and Frequency Dimming on Power Losses in HF Electronic Ballasts for Compact Fluorescent Lamps

Compact fluorescent lamps (CFLs) incorporating electronic ballasts are widely used in lighting. In many cases, the ability to dim the lamp is a requirement. Dimming can be achieved by varying the switching frequency of the inverter or by changing the voltage supplied to the inverter. The effect of dimming by both approaches on the power losses in the inverter is studied in this work. The lamp and associated inverter has been modeled in Pspice, using a behavioral model for the CFL. Predicted losses are in good agreement with experimental data obtained from calorimetry. After verification, the model was then used to determine the distribution of losses within the inverter, enabling a comparison of the effects of the two dimming methods to be made.

A Study on the Working Performance of Dimming Methods for Single- and Multichip Power LEDs

Light emitting diodes (LEDs) have many positive features, such as long life, low power consumption, and high luminous efficacy. Consequently, they are very attractive for use in many applications. Also, the use of LEDs ensures energy savings. Power LEDs have increasingly been used for general lighting via multichip module technology. The output of power LEDs should be controlled to obtain life and illuminance values in catalog. In this study, the effects of different dimming methods are investigated for single-and multichip LEDs. To achieve this aim, pulse width modulation (PWM) and current variation (CV) dimming methods are applied to single and multichip power LEDs during ten stages of dimming. A current-regulated DC source is used for current variation (CV) dimming, and a driver designed by the researcher is used for pulse width modulation (PWM) dimming. Moreover, all of the applications are made for active and passive cooling. Finally, the results are compared.

Progressive dimming method for mercury-free flat fluorescent lamp

A progressive dimming circuit and method for 32 inch mercury-free flat fluorescent lamp (FFL) were presented based on a newly developed FFL, which showed a large operating margin, high luminance and high luminous efficacy in the case of unipolar driving. The proposed dimming circuit is simple in structure and the dimming method is easy to fulfill. The prototype of 32 inch FFL was built and verified. The results show that a progressive dimming can be implemented by pulse density modulation (PDM) method or frequency modulation method and the power consumption can be reduced since the light output of FFL can be controlled progressively. A dimming range about 100% to 6% can be achieved by using the proposed method.

A novel ZVS double switch flyback inverter and pulse controlled dimming methods for flat DBD lamp

To reduce the voltage stress on the switching device and improve the converter efficiency, a zero-voltage switching (ZVS) double switch flyback inverter is proposed. Moreover, two kinds of voltage pulse density modulation dimming methods with the double switch flyback inverter for the driving of flat dielectric barrier discharge (DBD) lamp were presented to improve the dimming performance of DBD lamp. The luminance of the DBD lamp was adjusted by controlling an adopted auxiliary switch. The dimming circuits were built and demonstrated. The luminance and luminous efficacy with variation of input power and dimming ratios were provided. The experiment results show that a ZVS was achieved with the proposed double switch flyback inverter and the flicker caused by the uncontrolled voltage oscillation during the turn-off time of dimming pulse could be reduced. Therefore, a wider dimming range about 100% to 2% could be achieved with the proposed method and the power consumption was reduced.

Cooperative local dimming for accurate backlight brightness matching

A cooperative local dimming (CLD) method that effectively controls the brightness of the backlight unit is proposed. This CLD method takes into account the brightness of neighbouring light sources to achieve a backlight brightness that should be minimally higher than the brightness required by the pixels on the screen. The proposed CLD method matches pixel brightness more accurately and saves more energy than conventional local dimming methods do, thereby improving the quality of images and energy efficiency.

A Novel Adaptive Dimming LED Backlight System With Current Compensated X-Y Channel Drivers for LCD TVs

In this paper, a novel two-dimensional adaptive dimming and current compensation technique with X-Y channels for light-emitting diode (LED) backlight system in LCD TVs is proposed . The proposed LED backlight has matrix structured LED modules with row and column switches to control the brightness of individual division block. It shows local dimming effects such as reduced power consumption and high dynamic contrast ratio even with much less number of LED drivers than that of the conventional two-dimensional local dimming method. Therefore, low cost and compact design of LED drivers can be achieved. This paper also contains a new adaptive dimming algorithm and image compensation technique for the proposed LED backlight system. Moreover, luminance difference among blocks caused by different I-V characteristics of LED and the luminance variation caused by temperature change are compensated by current sensing system. The proposed dimming technique is verified by simulation and experimental results based on a RGB-LED backlight of a 32-inch LCD TV.

LED backlight driving circuits and dimming method

In this paper, light‐emitting‐diode (LED) backlight driving circuits and dimming method for medium‐sized and large liquid crystal displays (LCDs) are proposed. The double loop control method, the intelligent‐phase‐shifted PWM dimming method, the fast‐switching current regulator, and the current matching techniques are proposed to improve not only the current regulation characteristics and the power efficiency but also the current matching characteristics and the transient response of the LED current. The brightness of the backlight using the proposed local dimming method was determined from the histogram of the local block to reduce the power consumption of the backlight without image distortion. The measured maximum power efficiency of the LED backlight driving circuit for medium‐sized LCDs was 90%, and the simulation results showed an 88% maximum power efficiency of the LED backlight driving circuit for large LCDs. The maximum backlight power‐saving ratio of the proposed dimming method was 41.7% in the simulation with a high‐contrast image. The experiment and simulation results showed that the performance of LEDs as LCD backlight units (BLUs) improved with the proposed circuits and method.

A color compensation algorithm to avoid color distortion in active dimming liquid crystal displays

In LCD applications, various low power techniques, such as low voltage driving LEDs, high efficiency DC-DC converter circuitries, active backlight control schemes and the likes, have been applied to a backlight, which occupies the huge portion of total power consumption. Especially, it is a well-known fact that active dimming methods have accomplished considerable power reduction by as much as 20% up to 50% compared to the constant backlight method. However, the former technique may involve several artifacts such as color distortion, clipping, and so on. In this paper, color distortion in an active dimming LCD system caused by the mismatch between gamma values of pixel compensation algorithm and RGB sub-pixels, is analyzed and lowered by the proposed color compensation algorithm. This color compensation algorithm achieves the dramatic reduction on the color distortion without any loss of power saving performances. The maximum color difference ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sup> has been measured for each test image with the previous pixel compensation approaches (method-1 and method-2), and the average value of ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sup> has been obtained over 24 KODAK images. The resultant average value of a color compensated active dimming algorithm has been reduced to 0.413 and 0.478, compared to 24.595 and 9.527 of the conventional active dimming algorithms for method-1 and method-2, respectively.

Backlight power reduction using efficient image compensation for mobile devices

Since backlight dimming widely used for power reduction of mobile devices degrades the image quality in brightness and contrast, image enhancement is exploited to compensate the degradation. In this paper, we propose a novel backlight power reduction scheme using an efficient image compensation algorithm that scales discrete cosine transform (DCT) coefficients of image in the compressed domain. To reduce the computational burden of the conventional backlight dimming method where decompression is performed prior to image compensation in the spatial domain, we introduce a computationally efficient image compensation algorithm that processes only dominant DCT coefficients of images using a mapping function in the DCT domain. Experimental results indicate that our proposed method provides higher visual quality with reduced complexity as compared to conventional image compensation schemes.

An optical feedback system for local dimming backlight with RGB LEDs

One of the major issues plaguing RGB LED backlight is color shift. This phenomenon is due to the characteristic changes of each Red, Green, and Blue LEDs as the time and temperature variation. To compensate this variation, many researchers are proposed color control method for the stable color management. However, their method is not suitable for the local dimming backlight because of the light interference between blocks. Therefore, we proposed an optical feedback system for the stable color control regardless of its dimming method. The test results show that our proposed method is very suitable for RGB LED backlights.

Design of an RGB LED Backlight Circuit for Liquid Crystal Display Panels

This paper presents a red, green, and blue (RGB) light-emitting diode (LED) backlight circuit for liquid crystal display (LCD) panels. Some studies have been conducted to devise dimming methods that regulate the RGB LED currents. In this paper, design considerations for the RGB LED backlight are presented. A laboratory prototype for 20-in panels is implemented and tested. The color gamut of an LCD panel with the designed RGB LED backlight is measured and discussed. The experimental results of the laboratory prototype are also shown to verify feasibility of the findings.

Electro-optic characteristics and areal selective dimming method for a new highly efficient mercury-free flat fluorescent lamp (MFFL)

A highly efficient mercury-free flat fluorescent lamp (MFFL) with dielectric barrier Xe gas discharge was developed for a LCD-TV backlight source. The unit cell of the lamp has a simple structure with two main electrodes running parallel to each other and an auxiliary electrode. The adoption of the auxiliary electrode resulted in a wide, stable operating voltage margin, high luminance and efficiency. The 4 inch diagonal size lamp showed a luminous efficacy of 44 lm W−1 at a luminance of 3400 cd m−2 with Ne–Xe(18%) gas mixture. We demonstrated that the 4 inch unit cell can be used to construct a 5 × 8 multi-structured lamp of 32 inch diagonal size for application in a large-sized LCD backlight source by a simple repeat of the unit cell. Despite the increase in size, the 32 inch lamp showed the same discharge voltage and margin of the 4 inch unit cell. Using the proposed MFFL with the auxiliary electrode as the data electrode and the subfield method, we developed a driving scheme for 2-bit areal selective dimming control of an M × N multi-cell lamp which can be operated using only one inverter.

A Novel Discrete Dimming Ballast for Linear Fluorescent Lamps

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A novel discrete dimming ballast for linear fluorescent lamps is proposed in this paper. A proposed dimming control circuit is combined with a ballast module for multiple lamps to realize control of three discrete lighting levels. Compared with conventional step dimming or <emphasis emphasistype="smcaps">on</emphasis>– <emphasis emphasistype="smcaps">off</emphasis> control methods, the proposed discrete dimming method has the following advantages: 1) digital signal is generated by the dimming control circuit to control the lamps' turn- <emphasis emphasistype="smcaps">on</emphasis> and -<emphasis emphasistype="smcaps">off</emphasis>, which makes the system more reliable and integrated; 2) the proposed discrete dimming system replaces relays, which are necessary in conventional lamp <emphasis emphasistype="smcaps">on</emphasis>–<emphasis emphasistype="smcaps">off</emphasis> control, and therefore decreases the system cost; 3) the proposed dimming ballast can be installed by keeping the original wiring system. This makes the upgrading of a lighting system more effective and efficient; 4) the dimming control circuit also provides a good isolation for operating the low-voltage wall switches by hand safely. Both theoretical, simulation, and experimental results are in good agreement. </para>

Dimming Control Characteristics of Electrodeless Fluorescent Lamps

The study described in this paper was undertaken to identify the dimming control characteristics of the electrodeless fluorescent lamp. First, to accomplish this, a model of the electrodeless fluorescent lamp was made. Two kinds of dimming control methods, the variable switching frequency control method and the variable dc-link voltage control method, were then applied to the resulting model of the lamp, after which the results of the application of the two methods on the lamp model were compared. The hypotheses that were initially formulated were confirmed by the results of the experiment that was conducted on the 100-W ENDURA-type electrodeless fluorescent lamp. The results show that the lamp's dimming control of the switching frequency control is nonlinear and that of the variable dc-link voltage is linear over a wide range, and the theoretical value obtained by the proposed model well matches that of the experimental value.

A Novel Two-Dimensional Adaptive Dimming Technique of X-Y Channel Drivers for LED Backlight System in LCD TVs

In this paper, a novel two-dimensional (2-D) adaptive dimming technique with X-Y channels for light-emitting diode (LED) backlight system in LCD TVs is proposed. The proposed LED backlight has matrix-structured LED modules with row and column switches to control the brightness of individual division block. It shows local dimming effects such as reduced power consumption and high dynamic contrast ratio even with much less number of LED drivers than that of the conventional 2-D local dimming method. Therefore, low cost and compact design of LED drivers can be achieved. This paper also contains a new adaptive dimming algorithm and image manipulation technique for the proposed LED backlight system. The proposed dimming technique is verified by simulation and experimental results based on a RGB-LED backlight of a 32-in LCD TV.