Xiaohui Qu

Resonance-Assisted Buck Converter for Offline Driving of Power LED Replacement Lamps

LEDs are potential successors of incandescent lamps with high luminous efficacy and long lifetime. To improve the overall luminair efficacy and lifetime, the power efficiency and lifetime of LED ballasts become important factors. Efficiency gain in transformerless power converters appears attractive for applications without isolation. Driving solid-state LED bulbs in an existing lighting fixture such as PAR30 style housing from universal mains necessitates a high-voltage step-down ratio in order to produce an output voltage of about 10-20 V, which is very common in LED lighting applications. Traditional nonisolated step-down pulse width modulation buck converters may suffer from poor efficiency due to the long diode freewheeling time at small duty cycles. In this paper, we propose a resonance-assisted buck converter to achieve a high-voltage step-down ratio and high converter efficiency, whilst maintaining durability and compatibility with existing incandescent dimmers. The performance of the proposed LED driver is verified experimentally..

Noncascading Structure for Electronic Ballast Design for Multiple LED Lamps With Independent Brightness Control

LED light sources, which are more compact, capable to change color in real time, less dissipative, and more durable are finding more applications than conventional light bulbs in domestic, commercial, and industrial environments. However, requirements such as high-power factor, long lifetime, accurate current control, and high-efficiency pose challenges to the design of LED ballast circuits. This paper proposes an LED ballast with a dual noncascading structure. The first-stage noncascading structure is an isolated current-fed power factor correction (PFC) preregulator. In the proposed design, the short-lifetime high-voltage storage capacitor at the primary is replaced by a long-lifetime low-voltage capacitor at the secondary, thus extending the overall system lifetime. The PFC is programmed by the conventional averaged current-mode control for high-power-factor applications. Furthermore, the high-voltage stress on the main switch, which is typical in current-fed converters, is reduced substantially by appropriately exploiting the transformer leakage inductance. The design uses two secondary transformer windings and an LED current driver to form a second noncascading structure to improve efficiency. Multiple noncascading structures can be used for LED lamps for instant independent brightness control. Analysis, design example, and prototype verification are given for the LED ballast.

Color Control System for RGB LED Light Sources Using Junction Temperature Measurement

The efficiency of LED lights is approaching that of fluorescent lamps. LED light sources are finding more applications than conventional light bulbs due to their compactness, lower heat dissipation, and most importantly, real-time color changing capability. Accurate control of colors for RGB LED lights is a challenging task, which includes optical color mixing, color light intensity control and color point maintenance due to LED junction temperature change and device aging. In this paper , we present a LED junction temperature measurement technique for a pulse width modulation (PWM) diode forward current controlled RGB LED lighting system. The technique can control the color effectively without the need for using expensive feedback systems involving light sensors. Performance of chromaticity and luminance stability for a temperature compensated RGB LED system will be presented.

Hybrid IPT Topologies With Constant Current or Constant Voltage Output for Battery Charging Applications

The inductive power transfer (IPT) technique in battery charging applications has many advantages compared to conventional plug-in systems. Due to the dependencies on transformer characteristics, loading profile, and operating frequency of an IPT system, it is not a trivial design task to provide the battery the required constant charging current (CC) or constant battery charging voltage (CV) efficiently under the condition of a wide load range possibly defined by the charging profile. This paper analyzes four basic IPT circuits with series-series (SS), series-parallel (SP), parallel-series (PS), and parallel-parallel (PP) compensations systematically to identify conditions for realizing load-independent output current or voltage, as well as resistive input impedance. Specifically, one load-independent current output circuit and one load-independent voltage output circuit having the same transformer, compensating capacitors, and operating frequency can be readily combined into a hybrid topology with fewest additional switches to facilitate the transition from CC to CV. Finally, hybrid topologies using either SS and PS compensation or SP and PP compensation are proposed for battery charging. Fixed-frequency duty cycle control can be easily implemented for the converters.

Temperature Measurement Technique for Stabilizing the Light Output of RGB LED Lamps

The efficiency of light-emitting-diode (LED) lights approaches that of fluorescent lamps. LED light sources find more applications than conventional light bulbs due to their compactness, lower heat dissipation, and real-time color-changing capability. Stabilizing the colors of red-green-blue (RGB) LED lights is a challenging task, which includes color light intensity control using switching-mode power converters, color point maintenance against LED junction temperature change, and limiting LED device temperature to prolong the LED lifetime. In this paper, we present a LED junction temperature measurement technique for a pulsewidth modulation diode forward current controlled RGB LED lighting system. The technique has been automated and can effectively stabilize the color without the need for using expensive feedback systems that involve light sensors. Performance in terms of chromaticity and luminance stability for a temperature-compensated RGB LED system will be presented.

Isolated PFC Pre-Regulator for LED Lamps

The efficiency and lifetime of light-emitting-diode (LED) lamps are approaching and exceeding that of fluorescent lamps. LED light sources are finding more applications than conventional light bulbs due to their compactness, lower heat dissipation, real-time color changing capability and most importantly, long life which is much longer life than that of conventional offline power converters. Conventional power converters normally use high voltage electrolytic capacitors as energy storage element which could have the shortest lifetime when compared to other components of the LED lighting system. In this paper, a new isolated PFC power pre-regulator is proposed. The pre-regulator allows the charge storage capacitor to be positioned on the secondary side of the isolation transformer. Longer lifetime low voltage and high capacitance value capacitors can therefore be used to extend the overall lifetime of the LED lighting system. Steady-state state-space averaging analysis is preformed for designing the converter. A prototype converter is built to verify performance of the proposed PFC LED preregulator.

A Current Balancing Scheme With High Luminous Efficacy for High-Power LED Lighting

The imbalance in the currents of strings of LEDs will cause fast degradation or even failure in some LEDs. Such current imbalance should be avoided. To balance the common average current, a small duty cycle may necessitate a large LED turn-on current amplitude, which may cause temporal overheat and low luminous efficacy of the LEDs. This paper presents a current balancing method based on pulse-width modulation of a common bus voltage to each LED string to achieve the intended average current. An optimal feedback control scheme is proposed to maximize the duty cycles and minimize the bus voltage. As a result, at least one of the LED strings is operating at unity duty cycle. The analysis, implementation, and verification are detailed in this paper.

An Improved LCLC Current-Source-Output Multistring LED Driver With Capacitive Current Balancing

Passive or active current balancing circuits are usually used to mitigate current imbalance in driving multiple light-emitting-diode (LED) strings. Passive current balancing schemes adopting capacitors with high reliability, small size and low cost are very popular in many applications. However, the high reactive power of the capacitive balancing scheme with variable frequency control will bring high power stress on the VA rating of the main switches which drive this passive current balancing circuit and decrease the overall efficiency. Fixed frequency control does not permit zero-voltage switching (ZVS) under load variations. Hence, this paper proposes a current-source-output LED driver based on LCLC resonant circuit to provide a constant output current regardless of variations in LED parameters. In the LCLC circuit, the number of additional capacitors is scalable with the number of LED strings for current balancing. Moreover, the input impedance of the improved LCLC circuit is designed to be resistive at the operating frequency to minimize reactive power. The conventional duty cycle control can easily incorporate ZVS. The analysis, implementation and verification are detailed in this paper.

Higher Order Compensation for Inductive-Power-Transfer Converters With Constant-Voltage or Constant-Current Output Combating Transformer Parameter Constraints

Compensation is crucial for improving performance of inductive-power-transfer (IPT) converters. With proper compensation at some specific frequencies, an IPT converter can achieve load-independent constant output voltage or current, near zero reactive power, and soft switching of power switches simultaneously, resulting in simplified control circuitry, reduced component ratings, and improved power conversion efficiency. However, constant output voltage or current depends significantly on parameters of the transformer, which is often space constrained, making the converter design hard to optimize. To free the design from the constraints imposed by the transformer parameters, this paper proposes a family of higher order compensation circuits for IPT converters that achieves any desired constant-voltage or constant-current (CC) output with near zero reactive power and soft switching. Detailed derivation of the compensation method is given for the desired transfer function not constrained by transformer parameters. Prototypes of CC IPT configurations based on a single transformer are constructed to verify the analysis with three different output specifications.

Design of a Current-Source-Output Inductive Power Transfer LED Lighting System

Inductive power transfer (IPT) light-emitting diode (LED) lighting systems have many advantages in commercial, industrial, and domestic applications. Due to the large leakage inductance, primary and secondary capacitor compensations are necessary to improve the power transfer capability. To offer a direct current source output for LED driving, topology with series-series (SS) compensation is chosen and analyzed in this paper. Using a properly designed operating frequency, the SS compensation can be the best topology for the voltage-source-input and current-source-output applications. To design an efficient IPT system, the loosely coupled transformer is the key component whose design and interaction with other system components can be difficult to formulate. In this paper, a systematic design methodology for an efficient IPT LED lighting system is proposed. The analysis, implementation, and verification are detailed.