LED Strings Research Articles

Flicker-Free Resonant LED Driver With High Power Factor and Passive Current Balancing

A flicker-free resonant LED driver with high power factor (PF) and passive current balancing is proposed and analysed in this paper. The proposed LED driver are composed a buck power factor correction (PFC) unit and a resonant dc-dc unit by sharing an active switch. Based on this topology, the low output current ripple can be achieved via small voltage ripple on bulk capacitor and fast voltage-mode control loop. By the charge balance for resonant capacitor, the output current of each LED string can be balanced. In a half line cycle, owing to the small voltage ripple on bulk capacitor of pre-stage output, the duty cycle of active switch is almost constant. Therefore, when the inductor current of buck PFC unit operates in discontinuous conduction mode, high PF can be realized. Moreover, as the resonance is existed between resonant capacitor and leakage inductor of transformer, the diodes in resonant dc-dc unit can achieve zero-current turn-off. Thus, the efficiency can be improved. Finally, an 82W LED driver prototype with three-output is built to verify the studied results.

LLC LED Driver with Current-Sharing Capacitor Having Low Voltage Stress

In this paper, an LLC light-emitting diode (LLC LED) driver based on the current-sharing capacitor is presented. In the proposed LED driver, the LLC resonant converter is used to step down the high input voltage, to provide galvanic isolation, to offer a constant current for LEDs. Moreover, the current-sharing capacitor connected to the central-tapped point of the secondary-side winding is used to balance the currents in two LED strings. By doing so, the voltage stress on this capacitor is quite low. Above all, the equivalent forward voltages of the two LED strings are generally influenced by the temperature and the LED current, and this does not affect the current-sharing performance, as will be demonstrated by experiment on the difference in number of LEDs between the two LED strings. In addition, only the current in one LED string is sensed and controlled by negative feedback control, while the current in the other LED string is determined by the current-sharing capacitor. Moreover, this makes the current control so easy. Afterwards, the basic operating principles and analyses are given, particularly for how to derive the effective resistive load from the LED string. Eventually, some experimental results are provided to validate the effectiveness of the proposed LED driver.

Low Flicker Dimmable Multichannel LED Driver With Matrix-Style DPWM and Precise Current Matching

A low flicker dimmable four-channel LED driver with precise current balancing is proposed in this paper. It is able to achieve highly linear dimming and low lighting flicker at the same time by utilizing a new matrix-style digital pulsewidth modulation (DPWM) along with LED string rotation. Only one LED string needs to be modulated to reduce the total LED current variation and switching loss. To avoid color shift and reduce the effect of subharmonic flicker, a fourth-order common centroid layout is applied to match the current-controlling MOSFETs for precise current balancing. Low-voltage and high-temperature protection circuits as well as a temperature-compensated relaxation oscillator are all integrated to complete the driver design and reduce the overall cost for practical applications. The chip is designed in a TSMC 0.25-μm 1P3M 60-V high-voltage CMOS process with a chip size of 1.76 × 1.37 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The current mismatch among LED strings is measured to be only ±0.28% along with four times reduction in lighting flicker.

High Voltage, Low Current High-Power Multichannel LEDs LLC Driver by Stacking Single-Ended Rectifiers with Balancing Capacitors

In this paper, a new LLC converter for series-connected, high-voltage LEDs is proposed. The proposed LLC converter consists of two stacked, single-ended rectifiers and one balancing capacitor, to compensate for the current deviation of two individual LED strings. The proposed LLC LED driver can use a diode with low voltage stress, even if the secondary LED is connected in series to have a high driving voltage. In addition, even if several series-connected LEDs are changed into two-stacked structures, the balancing capacitor can compensate for the current deviation of the two separated LEDs, as well as the difference in leakage inductance of the two stacked single-ended rectifiers. The balancing capacitor can be made equal to the voltage tolerance of the stacked, single-ended rectifier diodes. The proposed circuit can be easily extended to a series channel LED driver circuit, without increasing the voltage stress. To verify the characteristics and operation of the proposed LLC LED driver, a 260W high-power LED driver is implemented.

Self-Configurable Current-Mirror Technique for Parallel RGB Light-Emitting Diodes (LEDs) Strings

Traditional current-mirror circuits require buck converter to deal with one fixed current load. This paper deals with improved self-adjustable current-mirror methods that can address different LED loads under different conditions with the help of one buck converter. The operating principle revolves around a dynamic and self-configurable combinational circuit of transistor and op-amp based current balancing circuit, along with their op-amp based dimming circuits. The proposed circuit guarantees uniformity in the outputs of the circuit. This scheme of current-balancing circuits omitted the need for separate power supply to control the load currents through different kinds of LEDs, i.e. RGB LEDs. The proposed methods are identical and modular, which can be scaled to any number of parallel current sources. The principle methodology has been successfully tested in Simulink environment to verify the current balancing of parallel LED strings.

Universal control algorithm for automatic current regulated LED driver

In the present work, a universal control algorithm is proposed which automatically regulates the output current of an LED driver as per the connected load requirements. The unique features of the proposed control algorithm is its operability in a wide output power range and remaining unaffected by the unequal current sharing among the LED strings arising due to mismatch among same wattage LED chips. The proposed control algorithm is tested by modelling and simulating an LED driver in MATLAB Simulink to operate LED modules having series-parallel combination of 3 W LED chips in the range of 18 W-72 W. The results obtained from simulation show that the proposed control algorithm successfully regulates the output current of the LED driver according to load requirements. The electrical performance of the simulated LED driver on universal AC supply complies with IEC 61000-3-2:2014 class C regulations and shows significant improvement over previously proposed LED drivers.

Implementation of a Dimmable LED Driver with Extendable Parallel Structure and Capacitive Current Sharing

A dimmable LED driver along with an extendable series structure and interleaved capacitive current sharing is presented herein, the LED connection of which is changed from the traditional series structure to the proposed parallel structure. The number of LED strings can be extended. As the number of LED strings is increased, the output voltage of this LED driver and the voltage stress on the main switch are ideally not influenced. Moreover, only one current sensor is needed to achieve current control and dimming. In this paper, the basic operating principle of the proposed LED driver is described and analyzed. Finally, the effectiveness of this LED driver is demonstrated by experiment based on the field-programmable gate array (FPGA).

Experimental verification of a linear AC LED driver with a light flicker compensating circuit and high power factor operation by modulating the current regulator

AbstractThis article proposes a linear PFC regulator with a light flicker compensating circuit for LED lighting applications. The proposed circuit is small in size because it includes no reactors or electrolytic capacitors. The light flicker compensating circuit, which is connected in parallel to the LED string, consists of four ceramic capacitors and diodes. These capacitors are operated in series in the charge state and in parallel in the discharge state by using a diode network circuit. Therefore, an external controller and large capacitors are not required. The proposed circuit is experimentally validated using a 9 W prototype. From the experimental results, it is found that the power fluctuation of the LED string is less than the JIS and JLMA limitations, and the power factor is 0.95. In addition, the power loss is analyzed at an efficiency of 89.4% for the prototype circuit. Also, by modulating the current regulator, sinusoidal input current can be achieved. The second proposed circuit is experimentally validated using a 7.5 W prototype, and the power factor and input current distortion are found to be 0.999 and 2.1%, respectively.

Flicker-Free Single Switch Multi-String LED Driver With High Power Factor and Current Balancing

A flicker-free single switch multi-string LED driver with high power factor and current balancing is proposed and analyzed. It combines a pre-stage boost PFC converter and multiple second-stage buck dc-dc converters by a single active switch and multiple dc-link capacitors. With the proposed circuit configuration of multiple dc-link capacitors, the output current of each LED string can be balanced. Therefore, by controlling the output current of one LED string, the output currents of other LED strings can be controlled via current balancing of dc-link capacitors. The effect of the ripple of dc-link voltage on each output can be eliminated by fast voltage mode control loop. Therefore, flicker-free low ripple current for each LED string is achieved. With small voltage ripple across dc-link capacitors, the turn on time of active switch is almost constant in a half line cycle. Therefore, power factor correction can be achieved with inductor current of pre-stage operating in discontinuous conduction mode (DCM). Finally, prototypes of dual-string LED driver with 90-135 Vac input and 175-265 Vac input for 47 W output power are built to verify the studied results.

Some Remarks on Design of LED Lamps and their AC Direct Drivers

Many examples of capacitorless and inductorless drivers used to power LED lamps directly from the AC mains use complicated circuitry in the form of a dedicated integrated circuit which is designed for a particular multistring lamp in order to obtain the value of the power factor very close to 1 and high efficiency of the whole unit. However, the authors of such descriptions do not provide any details concerning the optimum choice of the number of strings in a LED lamp and the number of LEDs in each string. The aim of this work is to show that acceptable values of both electrical parameters mentioned above can be obtained using a LED lamp with just one or two strings. Some formulae and results of their numerical analysis for the 230 V 50 Hz mains are presented, thanks to which designers of LED lamps and their AC direct drivers may easily estimate the optimum conduction voltage of a LED string or the number of single LEDs in a string.

Thermoelectric Conversion System With Dimmable LED Lighting

In this paper, a step-up converter is presented, which possesses a high voltage gain, a continuous input current, and galvanic isolation. This converter is applied to a thermoelectric conversion system along with the maximum power point tracking (MPPT) based on three-point-weighting method, so as to extract energy from heat via the thermoelectric module effectively. For LED dimming to be considered, a switched LED module is used to adjust the number of LEDs in LED strings, so as to achieve a dimmable function. Moreover, a field-programmable gate array (FPGA) chip, named EP1C3T100C8N made by Altera Co., is used as a control kernel, so as to realize not only MPPT but also LED dimming. Eventually, several thermoelectric modules, manufactured by Kryotherm Co., are used to verify the effectiveness of the proposed thermoelectric conversion system.

一种LED灯串故障自检测电路的设计与实现

一种LED灯串故障自检测电路的设计与实现

Integrated Stage Dual Buck -Boost Fly Back Converter LED Driver

This paper presents a novel integrated stage converter configuration being applied as a LED driver. In order to prove the betterment of the topology, its performance is compared in terms of power factor, output voltage and settling time with three other randomly selected topologies: A single switch AC-DC LED driver based on boost fly back power factor correction converter with a lossless snubber, Integrated stage double buck- boost converter LED driver, Integrated stage LED driver based on buck-boost and class E resonant converter. As a consequence of relevant simulations of all the chosen topologies with resistance as the load, it is understood that Integrated buck boost fly back converter based LED Driver is beneficial as a compromise between all circuits. Hence the same is translated into an experimental prototype which is capable of a yield rated 45 V, 84.6 W and 1.88 A for a LED string load.

Independent Control of Multicolor-Multistring LED Lighting Systems With Fully Switched-Capacitor-Controlled $LCC$ Resonant Network

Current imbalance is a major problem in multistring LED lighting systems. Significant research efforts have been directed to solve the problem by either an active or passive approach. However, in view of some system requirements or other emerging lighting applications, independent control of different LED strings is getting more important to obtain different desirable brightness and/or color spectrums. The existing active and passive approaches have their limitations to perform current balancing and independent control simultaneously, thus posing another great challenge for the design of LED drivers. In this paper, a fully switched-capacitor-controlled $LCC$ resonant converter is designed to address this challenge. With the replacement of all fixed capacitors in the resonant tank of the $LCC$ converter with switch-controlled capacitors (SCCs), the regulation of the common ac bus voltage, current balancing, and individual current control of LED strings can be readily achieved with a single LED driver by varying the effective capacitances of the SCCs under constant switching frequency. In this way, the circuit complexity can be greatly reduced while the circuit performances can be significantly improved. An experimental prototype of driving LED growth light for indoor farming applications is designed and constructed to verify the idea.

Series-Based AC LED Driver with Efficiency Improved

A dimmable AC LED driver circuit with high power factor (PF) and low total harmonic distortion (THD) is proposed, and such a circuit takes series driving based on the basic unit mainly built up by the base of the bipolar junction transistor (BJT) connected with the drain of the metal oxide-semiconductor field-effect transistor (MOSFET). Accordingly, the isolated base driver for the BJT is not required. In addition, the efficiency is further improved by adding one switchable LED block constructed by several LED strings along with individual switches. As the switch in this block is turned on, the corresponding LED string is reverse-biased, whereas as the switch in this block is turned off, the corresponding LED string is forward-biased. By doing so, the redundant input voltage across the BJT current source can be decreased, thereby reducing unexpected power dissipation and improving overall efficiency. Eventually, a prototype circuit is built up and controlled by the field programmable gate array (FPGA). By means of experimental results, the effectiveness of the proposed topology can be verified.

An Auto-Zero-Voltage-Switching Quasi-Resonant LED Driver With GaN FETs and Fully Integrated LED Shunt Protectors

An auto-zero-voltage-switching (ZVS) quasi-resonant LED driver with gallium nitride (GaN) FETs for general lighting applications is presented. The proposed LED driver switches at high frequency to minimize the inductors to microhenry range. ZVS can be automatically achieved with the proposed controller to eliminate switching loss. The GaN FETs enable high-frequency operation and improve the power efficiency. A fully integrated LED shunt protector is proposed to bypass the failed LEDs in series-connected LED strings. The overall lifetime of the LED strings can be improved, and the maintenance cost can be reduced. The characteristics of the ZVS quasi-resonant LED driver with small inductors and the conditions for ZVS are also discussed in detail. The LED driver is fabricated with a 0.35- $\mu \text{m}$ 120-V high-voltage process. It can provide up to 25-W power to the LED with $2 \times 3.3\,\,\mu \text{H}$ inductors and achieves 91.4% peak efficiency and a 0.973 peak power factor from 60-Hz 100- to 120- $\text{V}_{\mathrm {ac}}$ input.

Nonisolated Two-Phase Interleaved LED Driver With Capacitive Current Sharing

In this paper, a nonisolated two-phase interleaved LED driver with capacitive current sharing is presented. In the proposed LED driver, the current sharing capacitor can be used to balance the input inductor currents and the LED currents. Furthermore, only one grounded LED string current is sensed and controlled. Moreover, under the interleaved control, the input current ripple can be reduced. Finally, the operating principles, analyses, and experimental results are provided to verify the effectiveness of the proposed LED driver.

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%.

Design of LED constant current controller using multi-channel current-sharing topology

In this study, a multi-channel current sharing topology controller is designed for LED lighting application. This controller can regulate three channels of LEDs. Each LED string may have a different forward voltage and current setting. A constant buck regulator is also integrated inside this controller to regulate the total current, which is fed into three LED output channels. The circuit also contains a feedback loop in order to precisely control the current passing through each LED string and is immune to transient conditions. The start-up procedure is designed to ensure both acceptable overshooting of output current and that the turn on time is as short as possible. In the final simulation, the maximum output current overshooting and ripple under transient condition do not exceed 20% of the nominal value. The output current buildup time is less than 25ms for all three channels. The design is laid out using 350nm technology in a 1.5mm×1.5mm area. The chip does not contain a driver for power MOSFET.

Dimmable AC LED Driver Based on Series Drive

In this paper, a dimmable ac LED driving circuit with high power factor and low total harmonic distortion is presented, which takes series driving via the basic unit mainly constructed by the base of the BJT connected with the drain of the MOSFET. By doing so, the problem in floating driving can be removed, i.e., no additional isolated gate drivers are needed, and furthermore, if any one of LED strings opens, then turning on the corresponding switch can still make this LED system work. Finally, a prototype circuit is established and controlled by the field programmable gate array. Via experimental results, the effectiveness of the proposed topology can be demonstrated.