Analysis of Modified Current Controller and its Implementation in Automotive LED

A novel highly dimmable current controller which is a linear one is employed in the application of low power automotive is discussed here. Light Emitting Diode is the one which drives current that is linearly controlled to decrease the intensity of LED to limit the destruction of the LED and improve its reliability. Although many dimming techniques for LED lighting are available, our proposed method outperforms the existing methods in terms of power consumption and the no. of transistors used in the proposed design. This emits 100mA and decrease the LED current which is going linearly based on the theory of dimming control voltage. Toggling of LED is avoided finally in comparison with the existing system. This circuit is developed in 0.18?m process technology and Cadence ADE with Spectre is employed for simulation purpose. The proposed method utilizes a maximum power of 392.85 mW when the supply voltage is 4V and the control voltage is 4V at the temperature of 27oC

Precise Dimming and Color Control of Light-Emitting Diode Systems based on Color Mixing

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Nonlinear Dimming and Correlated Color Temperture Control of Bi-Color White LED Systems

This paper proposes a nonlinear approach of controlling the luminous intensity and correlated color temperature (CCT) of white light-emitting diode (LED) systems with dual color temperatures. This LED system is made up of a warm color LED source (2700 K) and a cool color LED source (5000 K). The luminous intensity of each of these LED sources is individually controlled by pulsewidth modulation. The overall intensity of the LED system is due to the combined emitted flux of both LED sources. Its overall CCT is the mixed average CCT of both LED sources. This proposed method is based on the nonlinear empirical luminous and CCT models of the LEDs, which take into consideration the thermal effect of LEDs on its luminance and CCT properties. With reasonable approximation, the theoretical models are simplified into practical solutions, which are translatable into real-life applications. It is experimentally validated that the proposed approach is considerably more accurate than existing linear approaches that do not consider color variations of LED sources. The idea is applicable to LED systems with multiple color temperatures and is not limited to white LEDs.published_or_final_versio

Digital implementation of the feedforward loop of the asymmetrical half-bridge converter for LED lighting applications

The Asymmetrical Half Bridge converter (AHBC) has proven to be a promising candidate for LED lighting applications. It provides high efficiency, galvanic isolation and, at the same time, its output filter can be very small and, therefore, easily implemented without electrolytic capacitor. On the other hand, its main drawback is its poor attainable bandwidth. In any ac-dc LED lighting application, the input voltage of the AHBC is provided by a Power Factor Corrector (PFC) converter which has to be also implemented without electrolytic capacitor in order to assure the long lifetime of the whole LED driver. As a consequence, its output voltage (input voltage of the AHBC) is affected by a low-frequency ripple. Due to the poor bandwidth of the AHBC, this voltage ripple will be transferred to the converter output voltage, leading to flickering. A possible solution is using a feedforward loop for cancelling the effect of this low-frequency ripple without affecting stability. Due to the complex and non-linear transfer function of the AHBC, any analog feedforward loop has to be tuned for a given operating point, leading to a poor performance (i.e., high flickering, high ripple) when the AHBC moves away from that point. Dimming, which is a very frequent requirement in many LED drivers, implies large variations of the output voltage and, consequently, moving away from the aforementioned operating point. In this paper, a digital feedforward loop is proposed in order to solve this problem. The digital implementation allows the feedforward loop to perfectly cancel the ripple under any condition (e.g., output voltage variation due to dimming). Besides, despite its complex transfer function, this digital feedforward loop has been designed and optimized for its implementation in small-size microcontrollers. Experimental results with a 40-W prototype prove the usefulness of the proposed feedforward loop and the validity of the equations used in the optimized desig

Characterizing Light Output Variations from Solid State Lighting Due to High Frequency Electromagnetic Interference

Consumer electronic devices employing active power electronic switching have been increasingly used in the last decade. With the rise in number of these devices, the emission of harmonic currents by these devices has changed both in magnitude and character. The effects of harmonic frequencies up to 2000 Hz on various electrical and electronic devices has been the subject of considerable scrutiny over the past decade. However, newer consumer devices employ switched mode power electronic circuits that switch in the multiple kilohertz range. The emission from these devices, along with power line communication, are sources of high frequency currents in the range of 2 to 150 kHz. As a result, there has been an appreciable rise in the amount of conducted emission in the frequency range 2 to 150 kHz. One of the important outcomes of rising emission in this frequency range is that there have been reported cases of interference with various consumer electronic devices. Among the devices in which interference has been reported are the new generation of solid state LED lamps which have become popular in the last 3-5 years. Considerable research has been done in the past about the effects of light flicker and the modulation of light output from incandescent lamps, on human beings. However, the utilization of power electronic converters changes this paradigm considerably. Unlike incandescent bulbs, where low frequency modulation of input voltage resulted in visible flicker, observations and reports have shown that LED lamps may be susceptible to flicker from frequencies above the 2 kHz mark. As a result, old methods of predicting flicker and studying it may no longer be applicable. This thesis attempts to shorten this gap in knowledge by exploring the topic of LED flicker due to high frequency distortion, and the factors that affect it. This was achieved by exposing LED lamps of various sizes and from various manufacturers, to realistic voltage distortion signals, recorded in the power system. Signals with high-frequency distortion superimposed on to the fundamental, were used. The test set-up used, allowed for the testing of light equipment with various types and levels of distortion at different points on wave. For the first time, experimental results showed that not only does high frequency voltage distortion cause changes in average value of light output and the modulation of light output, but that this change depends upon the point-on-wave at which the high frequency distortion appears. The mathematical tool of cross-correlation was proposed to quantify the effect of point-on-wave of high frequency distortion on light output. The utilization of this tool showed that LED lamps are susceptible to distortion appearing near the peak or near the zero crossing of the input voltage. In order to understand the dependence of LED flicker on the topology of the LED driver, five LED driver development boards available commercially were also subjected to the above mentioned high frequency voltage distortion. The results showed that light flicker from LED lamps is not necessarily a by-product of LED driver topology. The utilization of discontinuous conduction mode of operation and an isolation transformer in the LED driver is not sufficient to disconnect the LED load from input voltage variations. LED drivers of the same topology can behave completely different, likely due to the control methodology employed by each manufacturer. Finally, a simulation model of a popular LED driver solution: a flyback DC-DC converter with primary side regulation was developed to verify the experimental results and perform root cause analysis for the observed phenomena. Changes in control methodology and circuit design were suggested to overcome this flicker problem and evidence of the degradation of circuit components due to excess heat generated by high frequency distortion was shown

Optimizing energy-efficient buck converter as LED current source

Energy-efficiency demands get tighter and tighter and traditional lighting devices don’t satisfy the demands. LED-lighting efficiency is high, therefore they are a good choice for replacing traditional lighting devices. There are a vast variety of LED-modules on the market, which are setting high demands for an LED current source, especially when high quality dimming is demanded. There is the intention to design a high quality LED current source in this thesis. LED-modules need current flow through them, which is produced by a constant current step-down switched-mode converter. A switched-mode converter has a high efficiency and a small size. Buck converter output current is the same as inductor current. When inductor current is desired then output current is too. Buck converter control can be implemented in many ways, all of them based on precise inductor current feedback. A control system controls the main switch ON and OFF with a very high frequency and inductor current average value depending on the inductance of the inductor, the switching frequency and the duty ratio. Although a buck converter efficiency is high, it produces heat, which has to be minimized. This heat shortens the lifetime of the components and increases the total power loss. However, switched-mode converters have a much better efficiency than linear regulators. Since the operating frequency is high, the converter produces electromagnetic interference to its input, output and to the air. These interferences have to be minimized by a good design. An LED-modules light output is proportional to the current through it. Dimming can be implemented by reducing the average current flow through it. There are three dimming methods: blocking LED current flow rapidly with a specific duty ratio, reducing the current value linearly, or combining both earlier dimming methods. All dimming methods have pros and cons. Dimming may have the following problems: visible steps between light levels, flickering, stroboscopic effect, audible noise and possible color change. On the basis of the study, a prototype device was designed in which efficiency and power losses was simulated and measured. The prototype device is hybrid dimmable and its output is optimized for human eye sensitivity. Especially the hybrid dimming results are encouraging, because the dimming is stepless for the human eye. In addition the main circuit is chosen so that the control circuit can be implemented by using low voltage levels, and then a microcontroller can be used

Using an LED as a sensor and visible light communication device in a smart illumination system

The need for more efficient illumination systems has led to the proliferation of Solid-State Lighting (SSL) systems, which offer optimized power consumption. SSL systems are comprised of LED devices which are intrinsically fast devices and permit very fast light modulation. This, along with the congestion of the radio frequency spectrum has paved the path for the emergence of Visible Light Communication (VLC) systems. VLC uses free space to convey information by using light modulation. Notwithstanding, as VLC systems proliferate and cost competitiveness ensues, there are two important aspects to be considered. State-of-the-art VLC implementations use power demanding PAs, and thus it is important to investigate if regular, existent Switched-Mode Power Supply (SMPS) circuits can be adapted for VLC use. A 28 W buck regulator was implemented using a off-the-shelf LED Driver integrated circuit, using both series and parallel dimming techniques. Results show that optical clock frequencies up to 500 kHz are achievable without any major modification besides adequate component sizing. The use of an LED as a sensor was investigated, in a short-range, low-data-rate perspective. Results show successful communication in an LED-to-LED configuration, with enhanced range when using LED strings as sensors. Besides, LEDs present spectral selective sensitivity, which makes them good contenders for a multi-colour LED-to-LED system, such as in the use of RGB displays and lamps. Ultimately, the present work shows evidence that LEDs can be used as a dual-purpose device, enabling not only illumination, but also bi-directional data communication

조명 장치에서 플리커를 낮은 위험 수준으로 줄이기 위해 두 개의 평행한 플로팅 벅 구조를 사용한 교류-직류 엘이디 구동 회로

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2016. 8. 정덕균.This dissertation presents an ac-dc LED driver that consists of two parallel floating buck converters. To buffer the twice-line-frequency energy, one floating buck converter conveys energy to a storage capacitor, simultaneously performing the power factor correction (PFC). The other floating buck converter regulates the LED current to maintain a constant brightness in the LEDs for reducing the light flicker to low-risk levels. The proposed architecture reduces the voltage stress and the size of the storage capacitor, enabling the use of a film capacitor instead of an electrolytic capacitor. Considering the power factor and the flicker standards, a de-sign procedure to achieve a high power factor, while minimizing the storage capac-itance and the LED current ripple, is presented. A prototype of the proposed LED driver has been implemented with an on-chip controller IC fabricated in a 0.35 μm CMOS process and its functionality and performance have been verified experi-mentally. It demonstrates a power factor of 0.94 and a peak power efficiency of 85.4% with an LED current ripple of 6.5%, while delivering 15 W to the LEDs.CHAPTER 1 INTRODUCTION 1 1.1 MOTIVATIONS 1 1.2 FLICKER METRICS AND STANDARDS 5 1.3 PRIOR WORKS 9 1.4 THESIS OBJECTIVES AND ORGANIZATION 15 CHAPTER 2 BACKGROUND ON LED DRIVER 17 2.1 POWER CONVERTER TOPOLOGIES 17 2.1.1 LINEAR REGULATOR 17 2.1.2 SWITCHED-CAPACITOR CONVERTER 18 2.1.3 INDUCTOR-BASED CONVERTERS 19 2.2 BASICS FOR LED DRIVERS 31 2.2.1 LED CONFIGURATIONS 31 2.2.2 CURRENT SENSING TECHNIQUES IN LED DRIVERS 32 2.3 PFC TECHNIQUES IN LED DRIVERS 35 2.3.1 POWER FACTOR 35 2.3.2 PASSIVE PFC CIRCUIT 35 2.3.3 ACTIVE PFC CIRCUIT 36 2.4 DIMMING TECHNIQUES 38 CHAPTER 3 DESIGN OF AN AC-DC LED DRIVER WITH A TWO PARALLEL FLOATING BUCK TOPOLOGY 40 3.1 PROPOSED SYSTEM ARCHITECTURE AND OPERATION PRINCIPLE 40 3.1.1 OVERALL ARCHITECTURE 40 3.1.2 OPERATION PRINCIPLE 42 3.1.3 DISCUSSION ON DIMMING 50 3.2 DESIGN OF THE PROPOSED TOPOLOGY 52 3.2.1 RELATIONSHIP BETWEEN THE INPUT CURRENT WAVEFORM AND THE POWER FACTOR 52 3.2.2 DESIGN CONSIDERATIONS FOR DECIDING THE STORAGE CAPACITOR VOLTAGE 54 3.2.3 ANALYSIS OF THE PROPOSED LED DRIVER WITH LINE VOLTAGE VARIATIONS 57 3.2.4 DESIGN OF THE FLOATING BUCK CONVERTER FOR PFC AND ENERGY BUFFERING 59 3.2.5 DESIGN OF THE FLOATING BUCK CONVERTER FOR LED CURRENT REGULATION 63 3.3 CIRCUIT IMPLEMENTATION 67 3.3.1 CONTROLLER CIRCUIT ARCHITECTURE 67 3.3.2 LED CURRENT REGULATION LOOP DESIGN 68 3.3.3 BUILDING BLOCKS 70 CHAPTER 4 EXPERIMENTAL RESULTS 79 4.1 EXPERIMENTAL SETUP 79 4.2 MEASUREMENT RESULTS 84 CHAPTER 5 CONCLUSION 93 BIBLIOGRAPHY 94 초록 101Docto

Integrated circuits for efficient power delivery using pulse-width-modulation

Circuits and architectures for efficient power delivery have become crucial in emerging smart systems. Switching power amplifiers (PA) are very attractive for such applications, because they exhibit better efficiency compared to linear PA designs, due to saturated operation. Switching PAs also allow for utilization of deep submicron CMOS technologies, due to which these designs can be easily integrated with digital circuits, and can benefit from process scaling, in performance as well as in area. Pulse-width-modulation (PWM) is commonly used with switching PAs. A PWM signal typically employs a high-frequency switching pulse waveform as a carrier signal, wherein the pulse-width or duty-cycle of each pulse is modulated by a given low-frequency input signal. The carrier frequency can vary from several kHz to GHz, and is typically determined by the target application. In this thesis, efficient power-delivery circuits that use PWM with switching class-D stages are presented. Advanced circuit techniques, as well as architectures for PWM are proposed to enhance efficiency and circumvent the limitations of conventional architectures. A digitally-intensive transmitter using RF-PWM with a class-D PA is described in the first part of the thesis. The use of carrier switching for alleviating the dynamic range limitation that can be observed in classical RF-PWM implementations is introduced. The approach employs the full carrier frequency for half of the amplitude range, and the second harmonic of half of the carrier frequency, for the remainder of the amplitude range. This concept not only allows the transmitter to drive modulated signals with large peak-to-average power ratio (PAPR), but also improves the back-off efficiency due to reduced switching losses in the half carrier-frequency mode. A glitch-free phase selector is proposed that removes the deleterious glitches that can occur at the input data transitions. The phase-selector also prevents D flip-flop setup-and-hold time violations. The transmitter has been implemented in a 130-nm CMOS process. The measured peak output power and power-added-efficiency (PAE) are 25.6 dBm and 34%, respectively. While driving 802.11g 20-MHz 64-QAM OFDM signals, the average measured output power is 18.3 dBm and the PAE is 16%, with an EVM of -25.5 dB. The second part of the thesis describes a high-speed driver that provides a PWM output using a class-D PA. A PLL-based architecture is employed which eliminates the requirement for a precise ramp or triangular signal generator, and a high-speed comparator, which are typically used for PWM generation. Multi-level signaling is proposed to enhance back-off as well as peak efficiency, which is critical for signals with high PAPR. A differential, folded PWM scheme is introduced to achieve highly linear operation. 3-level operation is achieved without the requirement for additional supply source or sink paths, while 5-level operation is achieved with additional supply source and sink paths, compared to 2-level operation. The PWM driver has been implemented in a 130-nm CMOS process and can operate with a switching frequency of 40-to-170 MHz. For 2/3/5-level PA operation, with a 500 kHz sinusoidal input and 60 MHz switching frequency, the measured THD is -61/-62/-53 dB and corresponding efficiency is 71/83/86% with 175/200/220 mW output power level, respectively. Performance has also been verified for 2/3-level PA operation with a high PAPR signal with 500 kHz bandwidth. While intended as a general purpose amplifier, the approach is well-suited for applications such as power-line communications (PLC). The final part of the thesis introduces an efficient buck/buck-boost reconfigurable LED driver that supports PWM and PFM operation. The driver is based on peak current control. Rectified sin as well as sin² functions are employed in the reference signal to improve the power factor (PF) and total harmonic distortion (THD) of the buck and buck-boost converters. The design ensures that the peak of the inductor current maintains a constant level that is invariant for different AC line voltages. The operating mode of the design can be changed between PWM and PFM. The LED driver has been implemented in a 130-nm CMOS process. PF and THD are improved when the proposed reference is employed, and peak PF and lowest THD are 0.995/0.983/0.996 and 7.8/6.2/3.5% for the buck (PWM), buck (PFM), buck-boost (PFM) cases, respectively. The corresponding peak efficiency for the three cases is 88/92/91%, respectively.Electrical and Computer Engineerin