High Dimming Ratio Research Articles

PWM Dimming Method for Capacitor-Clamped Current-Sharing Circuit in LED Backlight System

When multichannel light-emitting diode (LED) strings are applied in display backlight application, it’s necessary to keep the brightness identical and adjustable. This paper deals with the capability of pulsewidth modulation dimming control method for capacitor-clamped current sharing circuit. By detecting the rising-edge of dimming pulse and turn on the switches of current sharing circuit in rotation, this method can achieve current sharing at any dimming frequency and dimming ratio. What is more, to improve the dynamic response of LED current and achieve a high dimming ratio, a control method based on the output voltage feedforward is proposed, which can decrease the rising time of the LED forward current significantly. The operational principle and parameter design are analyzed in detail. Based on this analysis, a three-string current sharing circuit driven by Buck converter is fabricated and tasted in the laboratory. Experimental waveforms demonstrate the validity.

Voltage-Controlled GaN HEMT-LED Devices as Fast-Switching and Dimmable Light Emitters

This letter reports a novel voltage-controlled light emitter with fast switching speed and dimming capability using a monolithically integrated GaN HEMT-LED device. The integrated HEMT-LED device, with a transconductance of 113 mS/mm and large light emitting area ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$450\times 470\,\,\mu \text{m}^{2}$ </tex-math></inline-formula> ), exhibits a fast switching speed of 15 MHz. When functioning as an optical transmitter, a data rate of 16.7 Mbit/s was demonstrated. By reducing the duty cycle of the pulsed gate voltage from 100% to 0.1%, the light intensity decreased accordingly, and a linear pulse-width modulation dimming performance with a wide dimming range (0.1%–100%) and high dimming ratio (51 282:1) was achieved.

Multiple String LED Driver With Flexible and High-Performance PWM Dimming Control

The main objectives in driving multiple LED strings include achieving uniform current control and high performance PWM dimming for all strings. This work proposes a new multiple string LED driver to achieve not only current balance, but also flexible and wide range PWM dimming ratio for each string. A compact single-inductor multiple-output topology is adopted in the driver, accompanied by synchronous integrators and variable dimming frequency, to achieve both high efficiency and high performance dimming. By using the proposed variable dimming frequency scheme, high dimming frequency is applied to a string with high dimming ratio, which helps to maintain the deviation of LED string current in an acceptable range, while low dimming frequency is applied to a string with low dimming ratio, which helps to achieve rectangular LED current waveform. Meanwhile, the new time multiplexing control scheme automatically optimizes the LED string's bus voltages, thus minimizes each string's power loss. A three string LED driver prototype is constructed to validate the effectiveness of the peoposed control scheme, where the three strings can have different dimming ratios between 4% and 100%.

A Fast Switching Current Regulator Using Slewing Time Reduction Method for High Dimming Ratio of LED Backlight Drivers

This brief proposes a fast switching current regulator to achieve a high dimming ratio of the light-emitting diode (LED) backlight drivers. The proposed current regulator employs the slewing time reduction method to enhance the dimming ratio by decreasing the rising time of the LED forward current. A six-channel LED backlight driver was fabricated using a 0.35- $\mu\text{m}$ bipolar-CMOS-DMOS process technology which includes an n-type MOSFET with a low voltage of 5 V and a high-side lateral double diffused MOSFET with a breakdown voltage of 60 V. The measured rising time and minimum pulsewidth of the LED forward current are reduced to 4.8 ns and 12 ns, respectively, at a current transition of 60 mA. Consequently, the dimming ratio of the LED backlight driver using the proposed current regulator is calculated to be 160 000 : 1 at a dimming frequency of 500 Hz, which is more than three times higher than that obtained using the previously reported current regulators. Therefore, the proposed current regulator is suitable for high-end displays which require high image quality.

고속 디밍제어를 위한 고출력-LED 드라이버 설계

본 논문에서는 고전압 공정기술을 이용하여 고속 디밍제어가 가능한 고출력-LED 드라이버를 설계하였다. 제안하는 고출력-LED 드라이버는 디밍신호를 통해서 LED에 필요한 전류량을 예측하고, 예측된 전류의 일부분을 인덕터 전류로 피드백시키는 방법을 사용하여서, LED 전류 상승시간이 최소화되도록 설계하였다. 기존 고출력-LED 드라이버의 최소 LED 전류 상승시간은 3㎲로 제한된 반면 제안하는 고출력-LED 드라이버의 최소 LED전류 상승시간은 1/10 정도로 감소되었다. 설계된 LED 드라이버는 0.35㎛ 60V BCDMOS 2-poly 4-metal 공정으로 제작되었으며 측정 결과 입력전압 12V, 9개의 백색 LED, 353mA LED전류, 1KHz 디밍주파수에서 LED전류 상승시간과 전력전달효율은 각각 240㎱, 93.7%로 측정되었다.

Evaluating Process Yield for LED Assembly under Undetected Process Parameter Change

AbstractThe technology of light emitting diode (LED) assembly has become more popular due to the high demand for LED backlight modules, which can provide desirable characteristics such as a wide color gamut, a high dimming ratio, a long lifetime, and high power efficiency. For LED assembly processes, the distance between LEDs is one of the key parameters because the uniformity of an LED backlight module can be affected by it. In order to obtain good optical performance of an LED backlight module, the process yield of the parameter needs to be controlled well. Conventionally, process yields of LED assembly processes are evaluated using typical capability index methods under the essential assumption that the processes are stable. However, some inevitable process parameter changes regarding the distance between LEDs might exist on the shop floor. In order to evaluate the process yield more accurately, this paper presents an accommodated capability index method for process yield evaluation under undetected process parameter changes. For illustration purposes, an application is presented.

Design and implementation of a high dimming ratio LED drive controller

This paper presents a high dimming ratio light emitting diode (LED) drive controller chip with digital mode dimming (DMD). The chip is composed of a boost power converter and a dimming control block. A novel constant on time (COT) control strategy is proposed for boost converter to achieve high dimming ratio. In addition, a fast enough load transient response of the converter power stage ensures its high dimming ratio. The COT control circuit operates mainly based on two current-capacitor timers and a finite state machine (FSM). The LED drive controller chip is designed and fabricated in 1.5 μm bipolar CMOS-DMOS (BCD) process with a die area of 1.31 × 1.43 mm2. Experimental results show that the proposed LED drive system works well. And, as expected, the minimum LED dimming on time of 1.0 μs and the corresponding dimming ratio of 1000:1 at 1 kHz dimming frequency are successfully achieved.

Two liquid crystal on silicon panel projector with efficient light-emitting diode illumination engine

LED-based projectors have numerous advantages compared to traditional projectors. They are more compact, they exhibit a larger color gamut and a longer lifetime, the supply voltage is lower and they can even operate on batteries. LEDs can switch rapidly (possibility to pulse) and they have a high dimming ratio (contrast considerations). However, they have low optical power per étendue, although this is also improving consistently. With an efficient illumination engine design we can build an LED projector with a moderate light output and with superior properties. We present a relatively compact LED projector with two liquid crystal on silicon (LCOS) light valves (LVs). One of these LVs alternately modulates red and blue information, while the other permanently modulates green information to achieve a good color balance. Additionally, we apply some methods to increase the brightness on the screen. Our two-LCOS approach results in a compact, efficient LED projector that produces 171 lm projected D65 flux.

Optical feedback system with integrated color sensor on LCD

TFT LCDs have the largest market share of all digital flat panel displays. A three color RGB LED LCD backlighting system is very attractive considering wide color gamut, tunable white point, high dimming ratio, long lifetime and environmental compatibility. But the high intensity LED has thermal and time-based dependencies. Color and white luminance levels are not stable over a wide range of temperature due to inherent long-term aging characteristics. In order to minimize color point and brightness differences over time, optical feedback control is a key technology for any LED backlighting system. In this paper, we present the feasibility of an optical color sensing feedback system for an LED backlight by integrating the amorphous silicon color sensor onto the LCD panel. To improve the photoconductivity degradation of amorphous silicon, a new LASER immersion treatment has been applied. The integrated color sensor optical feedback controlled LED backlighting system improved the color variation to less than 0.008 Δu′v′ (CIE1976) compared with 0.025 for an open loop system over the temperature range of 42–76 °C.

LED-backlight feedback control system with integrated amorphous-silicon color sensor on an LCD panel

— Thin-film-transistor liquid-crystal displays (TFT-LCDs) have the largest market share of all digital flat-panel displays. An LCD backlighting system employing a three-color red-green-blue light-emitting diode (RGB-LED) array is very attractive, considering its wide color gamut, tunable white point, high dimming ratio, long lifetime, and environmental compatibility. But the high-intensity LED has problems with thermal stability and degradation of brightness over time. Color and white luminance levels are not stable over a wide range of temperature due to inherent long-term aging characteristics. In order to minimize color point and brightness differences over time, optical feedback control is the key technology for any LED-backlight system. In this paper, the feasibility of an optical color-sensing feedback system for an LED backlight by integrating the amorphous-silicon (a-Si) color sensor onto the LCD panel will be presented. To minimize the photoconductivity degradation of a-Si, a new laser exposure treatment has been applied. The integrated color-sensor optical-feedback-controlled LED-backlight system minimized the color variation to less than 0.008 Δu'v' (CIE1976) compared to 0.025 for an open-loop system over the temperature range of 42–76°C.

Development of a dimmable aircraft fluorescent light system

THE DEVELOPMENT program to be described was for the Douglas DC-8 commercial jet airplane and was inaugurated to provide an effective dimmable aircraft fluorescent light system. Prior to this development none was available. The deficiency had made fluorescent lighting for aircraft unattractive, despite its many other advantages. Specifically, the purpose of this program was to design and develop two dimmable systems: one for the cabin which would have a maximum lamp current of 150-ma and an extremely high dimming ratio, and one for the cockpit which would have a maximum lamp current of 200-ma (very high light level) and a moderate dimming ratio.