Manuel Arias

Design of a Soft-Switching Asymmetrical Half-Bridge Converter as Second Stage of an LED Driver for Street Lighting Application

High-brightness LEDs are considered remarkable lighting devices due to their high reliability, chromatic variety, and increasing efficiency. As a result, a high number of solutions for supplying LED strings are emerging. One-stage solutions are cost-effective, but their efficiency is low because 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 because each stage is optimized for only one or two tasks and they are the preferred options when supplying several strings at the same time. In this paper, a two-stage solution is proposed. The first stage is the well-known PFC boost converter. The second stage, on which this paper is focused, is the asymmetrical half bridge (AHB). Its design has been optimized based on the needs and characteristics of LED-based street lighting applications. The proposed transformer design (with asymmetrical secondary windings) minimizes the conduction losses while the model of the converter during the dead times optimizes their duration, reducing switching losses in the MOSFETs and diodes. Experimental results obtained with a 40-W prototype show an efficiency as high as 94.5% for this second stage and validate the proposed design procedure and model.

High-Efficiency Asymmetrical Half-Bridge Converter Without Electrolytic Capacitor for Low-Output-Voltage AC–DC LED Drivers

Due to their high reliability and luminous efficacy, high-brightness light-emitting diodes are being widely used in lighting applications, and therefore, their power supplies are required to have also high reliability and efficiency. A very common approach for achieving this in ac-dc applications is using a two-stage topology. The power factor corrector boost converter operating in the boundary conduction mode is a very common converter used as first stage. It is normally designed without electrolytic capacitors, improving reliability but also increasing the low-frequency ripple of the output voltage. The asymmetrical half-bridge (AHB) is a perfect option for the second stage as it has very high efficiency, it operates at constant switching frequency, and its output filter is small (i.e., it can be also easily implemented without electrolytic capacitors). Moreover, the AHB is an excellent candidate for self-driven synchronous rectification (SD-SR) as its transformer does not have dead times. However, the standard configuration of the SD-SR must be modified in this case in order to deal with the transformer voltage variations due to the input voltage ripple and, more important, due to the LED dimming state. This modification is presented in this paper. Another important issue regarding the AHB is that its closed-loop controller cannot be very fast and it cannot easily cancel the previously mentioned low-frequency ripple. In this paper, a feed-forward technique, specifically designed to overcome this problem, is also presented. The experimental results obtained with a 60-W topology show that efficiency of the AHB may be very high (94.5%), while the inherent control problems related to the AHB can be overcome by the proposed feed-forward technique.

High-Efficiency LED Driver Without Electrolytic Capacitor for Street Lighting

High-brightness light-emitting diodes (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. In addition, 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 two-input buck 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). Moreover, 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 stages.

EMI Radiated Noise Measurement System Using the Source Reconstruction Technique

One of the requirements that electronics circuits must satisfy comprises conducted and irradiated noise specifications. Whereas conducted noise is well covered in the literature, radiated noise is not. Radiated noise regulations impose limits on the noise measured 3 or 10 m away from electronic equipment. These measurements are usually made in anechoic rooms, which are very expensive. Moreover, the measurement procedure is not a ldquoplug-and-playrdquo feature, but requires a strict measuring protocol. Once the electronic circuit has been tested, the designer remains ignorant of the source of the problem should the regulation not be met. Hence, the procedure to make an electronic circuit comply with regulations is usually one of trial-and-error, in which the experience of the designer is essential. A new radiated noise measurement technique is proposed in this paper with a twofold objective: to simplify the measurement procedure and to obtain more information about noise sources. The main idea is to scan the electric/magnetic field at two arbitrary although known distances. From these measurements, the source reconstruction technique enables the identification of the noise sources in the surface of the circuit and the field estimation at any distance and the assessment of compliance with regulations. Moreover, if regulations are not met, the effect of modifying the noise source can be tested in order to ascertain how the circuit should be modified to comply with regulations.

Single Stage Inverter for a Direct AC Connection of a Photovoltaic Cell Module

There are several possibilities to transfer the energy taken from a photovoltaic cell to the grid. In the past, several cell modules used the same centralized inverter to transfer the energy to the grid. Nowadays, decentralized modules are preferred because of their versatility to harvest a larger amount of power, even when some of the modules of the total surface are covered and do not receive the same solar irradiation. This paper presents an inverter topology that can be used to connect a photovoltaic cell module directly to the grid. The inverter is based on a single stage Flyback topology and it is quite simple. Furthermore, a maximum power point tracking algorithm is proposed. The system can be implemented in a very simple microcontroller and the output current, although it is not sinusoidal, its harmonic content is lower than the limit imposed by IEC 61000-3-2 regulations. A prototype of the topology has been built and tested.

An Overview of the AC-DC and DC-DC Converters for LED Lighting Applications

High-Brightness Light Emitting Diodes (HB-LEDs) are considered the future trend in lighting not only due to their high efficiency and high reliability, but also due to their other outstanding characteristics: chromatic variety, shock and vibration resistance, etc. Nevertheless, they need the development of new power supplies especially designed for boosting and taking advantage of their aforementioned characteristics. Besides, their behaviour is completely different from the rest of lighting devices and, consequently, it should be also taken into account in the design of the converters used to drive them. As a result, many well-known topologies have been optimized or redesigned in order to be used in LED—lighting applications and many new topologies have come up in the recent years with the same purpose. In this paper, the main HB-LED characteristics will be explained, highlighting how they influence the design of their power supplies. After, the main topologies will be presented from the simplest to the most complex ones, analysing their advantages and disadvantages.

A Unity Power Factor Correction Preregulator with Fast Dynamic Response Based on a Low-Cost Microcontroller

Low cost passive Power Factor Correction (PFC) and Single-Stage PFC converters cannot draw a sinusoidal input current and are only suitable solutions to supply low power levels. PFC preregulators based on the use of a multiplier solve such drawbacks, but a second stage DC/DC converter is needed to obtain fast output voltage dynamics. The output voltage response of PFC preregulators can be improved by increasing the corner frequency of the output voltage feedback loop. The main drawback to obtaining a faster converter output response is the distortion of the input current. This paper describes a simple control strategy to obtain a sinusoidal input current. Based on the static analysis of output voltage ripple, a modified sinusoidal reference is created using a low cost microcontroller in order to obtain a input sinusoidal current. This reference replaces the traditional rectified sinusoidal input voltage reference in PFC preregulators with multiplier control. Using this circuitry, PFC preregulator topologies with galvanic isolation are suitable solutions to design a power supply with fast output voltage response (10 ms or 8.33 ms) and low line current distortion. Finally, theoretical and simulated results are validated using a 500 W prototype.

Reduction of the output capacitor in Power Factor Correctors by distorting the line input current

Active Power Factor Correctors (PFCs) are needed to design ac-dc power supplies with universal input voltage range and sinusoidal input current. The classical method to control PFCs consists in two feedback loops and an analog multiplier. Hence, the input current is sinusoidal and it is in phase with the input voltage. However, a bulk capacitor is needed to balance the input power and the output power. Due to its high capacitance, an electrolytic capacitor is traditionally used as a bulk capacitor in PFCs. As a consequence, the lifetime of the ac-dc power supply is limited by the electrolytic capacitors, which becomes insufficient to some applications (e.g. High-Brightness Light Emitting Diodes, HB-LEDs). This paper proposes a reduction of the output voltage ripple (which allows reduction of the output capacitance) by distorting the input current, but maintaining the harmonic continent compatible with EN 61000-3-2 regulations. Also, a control strategy with a low-cost microcontroller is developed to put the proposed study into practice. Finally, the theoretical results are validated in a 500 W prototype.

Practical Application of the Wave-Trap Concept in Battery–Cell Equalizers

The use of battery-cell equalizers is mandatory in order to assure that all the cells connected in series are charged to its maximum capacity, even when they present small differences in this parameter due to several factors, such as aging, manufacturing, or temperature. Active equalizers, with a higher efficiency in comparison to passive ones, have the disadvantage of using a considerable number of components. Moreover, in the case of active equalizers with very high performance, this number can be even higher. In this paper, the use of the wave-trap concept, widely used in telecommunication systems, is studied. This concept allows the battery-cell equalizer to use its switching frequency as the control variable that decides which cell is being charged. Hence, it is not necessary to use a complex system based on a high number of controlled switches in order to determine which cell is being charged. In this way, the number of switches (and the corresponding driving signals) can be strongly minimized without reducing the performance of the system. In order to proof the validity of this concept (i.e., wave traps) in the design of battery-cell equalizers, a topology based on a half-bridge structure is also proposed in this paper. It uses only two controlled switches in order to decide which cell is charged. Experimental results are provided for a four-cell equalizer as a proof of concept.

Elimination of the Transfer-Time Effects in Line-Interactive and Passive Standby UPSs by Means of a Small-Size Inverter

The main drawback of line-interactive and passive standby uninterruptible power supplies (UPS) is the transfer time when changing from normal mode of operation to battery mode of operation. During this time, the critical loads protected by the UPS are under the influence of the grid disturbance which forced the mode change. On the other hand, equipment protected by a double conversion UPS is never affected by any disturbance in the mains. Nevertheless, the efficiency of this topology is lower and its size and cost are considerably higher. Hence, the elimination of the transfer process between modes in line-interactive and passive standby UPSs would make them more competitive than double conversion topology in many situations. In this paper, a method for eliminating the transfer time is proposed. It is based on a half-bridge inverter with a hysteretic controller whose output voltage is only applied to the load during the transfer time. Considering the random nature of grid failures and the short duration of the transfer time, the design and selection of the components is nothing but typical. This also leads to a non-standard hysteretic controller that needs to be fully analyzed. Experimental results are provided for a 750-W line-interactive UPS with a transfer time of 4 ms.