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In this research, a series resonant LED driver circuit based on the differential-mode transformer for current equalization is proposed. In this circuit, the series resonant converter adopts controlled frequency modulation to change the energy transferred to the output while realizing zero voltage switching (ZVS) turn-on of the half-bridge switch. To deal with the problem of unequal LED currents caused by two different forward conduction voltages of the output LED strings, a differential-mode transformer is employed to balance the currents between the two LED strings, and the relationship between the magnetizing inductance and the percentage of current sharing error are derived to facilitate the design. Furthermore, only the current of one LED string needs to be sensed to achieve current equalization, while the current of the other LED string is automatically determined by the differential-mode transformer.

To match the key features of light-emitting diode (LED) lighting source and further save power, LED lighting driver also requires long life, while maintaining high efficiency, high power factor, pulse-width modulation dimming and low cost. However, a typical LED lighting driver has the following drawbacks: (i) utilise bulky electrolytic capacitor as storage capacitor with short lifetime; (ii) employ a low-frequency diode bridge as the rectifier cell; and (iii) engage multiple stages cascade structure for multiple LED strings. To overcome the aforementioned shortages, this study proposed a bridgeless electrolytic capacitor-less AC/DC converter for offline LED lighting application. In the proposed converter, the conventional diode rectified bridge is replaced by Totem-pole bridgeless configuration for reducing the number of semiconductors in the line-current path. Meanwhile, the valley-fill circuit is introduced to further reduce the capacitor size. As comparison to its counterpart, the proposed circuit requires only one quarter of the capacitor energy when considering the energy amount (CV2) as the capacitor sizing criterion. Furthermore, the isolation type of the studied circuit is compatible with Twin-Bus configuration for achieving higher overall system efficiency. Finally, the experimental results, taken from a laboratory prototype rated at 50 W, are presented to verify the effectiveness of the proposed converter.

A self-adaptive load technology for multiple-string LED drivers is presented. Directly operating from an AC voltage, the proposed technology realises soft switching with a self-adaptive load resistor, so that the LED avoids the current glitch during current switching. Thus, the quality of lighting, the lifetime and the performance of the LED are improved. An AC LED driver using the proposed technology was fabricated in a 0.25 µm BCDMOS process and tested with a five-string LED. The measured results show that the power factor is 0.998 (0.997) with 5.1% (7.2%) total harmonic distortion under 19 W 110 V/AC (220 V/AC) condition.

To obtain sufficient luminance and achieve higher reliability, light-emitting diodes (LEDs) are usually arranged in parallel strings in many high-power applications. Since current imbalance should be avoided among the parallel LED strings, a multi-channel constant current LED driver based on high-frequency AC bus is proposed in this study. The proposed driver is composed of one resonant inverter followed by passive resonant rectifiers according to the number of LED strings. The amplitude of the high-frequency AC bus voltage is regulated by the resonant inverter and each LED string is individually powered with constant current by its corresponding passive resonant rectifier. The proposed driver is characterised by low cost and high efficiency. Finally, a prototype is built to verify the performance of the proposed driver.

The LT3756 controller, a versatile high power LED driver that has all the features required for large (and small) strings of high power LEDs, is described.

In this work, a multi-independent-output, multi-string, high-efficiency, boost-converter-based white LED (WLED) driver architecture is proposed. It utilizes a single inductor main switch with a common maximum duty cycle controller (MDCC) in the feedback loop. A simple pulse skipping controller (PSC) is utilized in each high-side switch of the multiple independent outputs. Despite the presence of multiple independent outputs, a single over-voltage protection (OVP) circuit is used at the output to protect the circuit from any voltage above 27 V. An open circuit in any of the strings is addressed, in addition to the LED’s short-circuit conditions. Excellent current matching between strings is achieved, despite the low ON-resistance (Rdson) of transistors used in the 40 nm process. Most circuits are designed in digital CMOS logic to overcome the extreme process variations in the 40 nm node. Compared to a single output parallel strings topology, a 50% improvement in efficiency is achieved relative to extremely unbalanced strings. Three strings are used in this proposal, but more strings can be supported with the same topology. Each string is driven by a 25 mA current sink. An input voltage of 3.2–4.2 V and an output voltage up to 27 V are supported.

There are many possible LED lighting applications where separate regulation of the LED current (luminous flux) of individual LED strings would be desirable—specialized variable correlated color temperature lights for ambient lighting, decorative lighting, surgical lights, horticultural lights, etc. Separate regulation of the current or light flux of individual LED strings is associated with a known problem: the necessity of using a controllable LED driver for each string, which increases the total component count, overall system complexity and costs. One of the possible solutions—a current source mode single-inductor multiple-output LED driver—was discussed in previous different papers. However, the practical implementation of this solution was not discussed in detail. This article aims to correct this omission.

The wide application of light emitting diodes (LEDs) in lighting systems has necessitated the inclusion of spectral tunability by using multi-color LED chips. Since the lighting requirement depends on the specific application, it is very important to have flexibility in terms of the driving conditions. While many applications use single or rather white color, some recent applications require multi-spectral lighting systems especially for agricultural or human-medical treatment applications. These systems are underexplored and pose specific challenges. In this paper, a mixture of red, green, blue, white (RGB-W) LED chips was used to develop a compact light engine specifically for agricultural applications. A computational study was performed to understand the optical distribution. Later, attention was turned into development of prototype light engines followed by experimental validation for both the thermal and optical characteristics. Each LED string was driven separately at different current levels enabling an option for obtaining an infinite number of colors for numerous applications. Each LED string on the developed light engine was driven at 300 mA, 500 mA, 700 mA, and 900 mA current levels, and the optical and thermal parameters were recorded simultaneously. A set of computational models and an experimental study were performed to understand the optical and thermal characteristics simultaneously.

Thanks to a very high luminous efficacy of LED lamps (over 160 lm/W) they are the most preferred light sources in lighting applications today. The useful lifetime of LED modules exceeds 50,000 hours. Chromatic parameters of lamps making use of SSL (Solid State Lighting) have already equalled classic solutions, although they were noticeably worse not so long ago. High values of the Colour Rendering Index (CRI) and ease of control over the luminous flux cause that lamps with LEDs have become very attractive solutions. Today, the most important problem concerns LED drivers supplied from the 230 VAC mains. The lifetime of switched-mode converters, including electrolytic capacitors, is considerably shorter than that of LEDs. This paper discusses the features of alternative drivers for LED modules which are supplied directly from the 230 VAC mains and do not contain any electrolytic capacitors. In particular, power factor and efficiency of lamps with one or two LED strings are analysed and some hints concerning optimal design of such lamps are given. A unique feature of this work is a detailed analysis of harmonics contents in the supply current of such drivers, proving their conformity with the relevant standard. Finally, some problems associated with flicker resulting from the considered type of supply are mentioned.

This work investigates a novel pulsating DC high-voltage linear driving scheme for GaN-based Light-emitting diode (GaN LED) general lighting to save costs and alleviate flicker. The superiority and practicality of this scheme in three-phase AC power grids were demonstrated for the first time. Compared to applications for single-phase AC grids, linear driving of GaN LEDs for three-phase AC grids can provide superior performance for general lighting. The DC component of the three-phase AC rectified voltage reaches 90.7%, which effectively alleviates the flicker problem. In this paper, we balanced GaN LED power and driving efficiency by optimizing the GaN LED distribution of the linear multi-string GaN LED driving scheme while taking the effects of grid voltage fluctuations into account. In addition, we constructed a double-string GaN LED lighting system as a modular prototype with scalability. The experimental results exhibit high driving efficiency (~94% @380 V line voltage), high power factor (~0.952), flicker-free, and high reliability at a very low cost (~$0.005/W).