High-efficiency LED driver without electrolytic capacitor for street lighting

High-Brightness Light Emitting Diodes (HB-LEDs) are considered as a remarkable lighting device due to their high reliability, chromatic variety and increasing efficiency. As a consequence, a high number of solutions for supplying LED strings are coming out. One-stage solutions are cost-effective, but their efficiency is low as they have to fulfill several purposes with only one converter: Power Factor Correction (PFC), galvanic isolation (in some cases) and current regulation. Two-stage and three-stage solutions have higher efficiency as each stage is optimized for just one or two tasks and they are the preferred option when supplying several strings at the same time. Nevertheless, due to their higher cost in comparison to one-stage solutions, they are used when high-efficiency, high-performance and the possibility of supplying several strings are the main concerns. Besides, they are also used when high reliability is needed and electrolytic capacitors cannot be used. In this paper, a three-stage solution and its complete design guideline for LED-based applications is proposed. PFC is achieved by a Boost converter while the galvanic isolation is provided by an Electronic Transformer (second stage). The third stages (one for each LED string) are designed following the TIBuck schematic, but taking advantage of the load characteristics (i.e., the high value of the LED string knee voltage, approximately equal to half the string nominal voltage). Besides, a variation of the analog driving technique is also proposed. Experimental results obtained with a 160-W prototype show an efficiency as high as 93% for the whole topology and 95% for the cascade connection of the second and third stage

A very simple analog control for QSW-ZVS source/sink buck converter with seamless mode transition

2018 IEEE 19th Workshop on Control and Modeling for Power Electronics (COMPEL), 25-28 junio, Padua (Italia)A simple, analog, control circuit is proposed for seamless transition between source and sink modes in a Quasi- Square-Wave Zero Voltage Switching (QSW-ZVS) source/sink buck converter. The inductor current is controlled by a variablewidth hysteretic current mode control. The upper and lower bounds of the hysteretic band are clamped to ensure QSWZVS operation with a single current command from the control loop and independently from the power flow direction. This enables the control of any PWM converter able to operate in QSW-ZVS with a single control loop. If the proposed circuit is used, using a complex multi-mode or look-up-table based digital control is no longer required, simplifying the implementation and lowering the cost of the converter. A 50 W buck converter is built to demonstrate the proposed control circuit and experimental measurements are shown to verify its correct operatio

Optimization procedure of source/sink converters for DC power distribution nano-grids

2018 IEEE 19th Workshop on Control and Modeling for Power Electronics (COMPEL), 25-28 junio, Padua (Italia)DC-Power Distribution Systems require DC-DC converters to interface all their elements. These converters should provide a high power quality and be efficient, compact and inexpensive. Furthermore, the characteristics of the loads and sources connected to these converters are not known beforehand and may change widely (there is no regulation about the DC loads, such as IEC61000-3-2 in AC grids). As a consequence, the design procedure is neither standard nor obvious. This work proposes an optimized design procedure for any Bus Provider in a DC-Power Distribution System. It is based on models, hence extensible to different Bus-Provider topologies, and takes into account the main issues in DC-Power Distribution Systems, especially the design conditions imposed by the wide variety of loads, or even sources, which may be connected to the output bus. Experimental results obtained from three designs show a close match with the analytical models and verify that the proposed procedure minimizes converter losses and complies with the requirement

Convertidores electrónicos de potencia para alimentar LEDs de iluminación desde tensiones muy bajas

XXIV Seminario Anual de Automática, Electrónica Industrial e Instrumentación 2017 (SAAEI'17), Valencia (España)Este artículo describe y valora distintas alternativas para alimentar diodos emisores de luz (LEDs) blancos, de los usados habitualmente para iluminación, desde tensiones mucho menores que la tensión de codo del propio dispositivo emisor de luz. En resumen, se va a estudiar cómo alimentar un LED con tensión de codo entorno de los 3 V desde una única celda de batería de tensión comprendida entre 1,2 y 2 V. En el artículo se valorarán y compararán dos tipos de convertidores para esta aplicación: convertidores CC/CC estándar con diodo Schottky (en vez de diodo PN) y una nueva familia de convertidores CC/CC expresamente pensados para ser usados con LEDs en los que el diodo de la topología es el propio LED. Ambos tipos de soluciones han sido experimentadas, buscando establecer comparaciones entre ellas desde distintos puntos de vista, como son el coste, el rendimiento eléctrico y el rendimiento lumínic

Modular three-phase ac-dc LED driver based on summing the light output of each phase

IEEE Applied Power Electronics Conference and Exposition, San Antonio, (Estados Unidos de Norteamérica), marzo de 2018This work proposes the driving of high power Light-Emitting Diodes (LEDs) luminaires in three phase power grids using each phase to drive an independent LED load. The driving is done by means of a boost converter in charge of controlling the current across its LED load. Moreover, each boost converter needs to achieve Power Factor Correction (PFC) to comply with IEC 61000-3-2 Class C requirements. Hence, achieving unity Power Factor (PF) and low Total Harmonic Distortion (THD). Taking advantage of the light properties, the output of the LED driver will be the sum of the light of each of the phases. Therefore, a theoretical study will be carried out to observe the feasibility of removing the electrolytic capacitor while guaranteeing a flicker free behaviour of the LED luminaire, even if the current of each string is pulsating at twice the mains frequency. In order to validate the study, a prototype has been built comprised of three PFC boost converters disposing of their electrolytic capacitors. The designed prototype operates in the full range of the European three-phase line voltage, which varies between 380 V and 420 V, and it supplies an output light of 42.000 lm at a maximum power of 300 W while achieving an electrical efficiency of 97.5

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

Optimized design of a high input-voltage-ripple-rejection converter for LED lighting

The asymmetrical half-bridge (AHB) converter has been deeply analyzed as second stage in ac–dc light emitting diode (LED) drivers. Galvanic isolation, high reliability and high efficiency are their main advantages while a limited duty cycle range (i.e., 0–0.5) and a poor dynamic behavior are their main disadvantages. As second stage of an LED driver, the most demanding requirement for its control loop is cancelling the low-frequency ripple introduced by the first stage. According to its limited maximum attainable bandwidth, this is normally achieved by implementing a feedforward loop. Nonetheless, the static gain of the standard AHB converter presents a nonlinear relation between the output voltage and the duty cycle. As a consequence, the effectiveness of this feedforward loop is limited. In this paper, the use of the zeta AHB converter as second stage of an LED driver is analyzed and an optimized design is proposed. Regarding its advantages, it presents an extended duty cycle range (theoretically, 0–1.0). Besides, it presents a linear relation between the output voltage and the duty cycle. Therefore, although its dynamic behavior is still limited, it can perfectly cancel the low-frequency ripple introduced by the first stage of the LED driver by means of a straightforward feedforward loop. The optimized design proposed in this paper is focused on minimizing the losses in the magnetic components (transformer and inductor) by wisely choosing the value of the two turns ratios of the transforme

Energization and Start-up of Modular Three-stage Solid State Transformers

Workshop on Control and Modeling for Power Electronics, COMPEL (9th, 2028, Padua

The voltage-controlled compensation ramp: A new waveshaping technique for power factor correctors

IEEE Applied Power Electronics Conference (APEC) (27. 2008. Austin, Texas)This paper deals with a new control method for Power Factor Correctors. Control is carried out by a standard IC controller for peak current-mode dc-dc converters, with only an additional compensation ramp generator and peak detector. Neither an analog multiplier nor an input voltage sensor is needed to achieve quasi-sinusoidal line waveforms, which makes this method very attractive. The method is similar to the one-cycle control method, but can be very easily adapted for use with topologies different to the boost converter, i.e. flyback, buck-boost, SEPIC Cuk and Zeta topologies. Moreover, as the line current is cycle-by-cycle controlled, the resulting input current feedback loop is extremely fast, thus allowing the use of this type of control with high frequency line