Diego G. Lamar

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.

A Single-Stage High-Power-Factor Electronic Ballast Based on Integrated Buck Flyback Converter to Supply Metal Halide Lamps

In this paper, a novel single-stage electronic ballast with a high power factor is presented. The ballast circuit is based on the integration of a buck converter to provide the power factor correction, and a flyback converter to control the lamp power and to supply the lamp with a low-frequency square-waveform current. Both converters work in discontinuous conduction mode, which simplifies the control. In spite of being an integrated topology, the circuit does not present additional stress of voltage or current in the main switch, which handles only the flyback or buck current, depending on the operation mode. To supply the lamp with a low-frequency square-wave current to avoid acoustic resonances, the flyback has two secondary windings that operate complementarily at a low frequency. The design procedure of the converters is also detailed. Experimental results from a 35-W metal halide lamp are presented, where the proposed ballast reached a power factor of 0.95, a total harmonic distortion of 30% (complying with IEC 61000-3-2), and an efficiency of 90%.

Different Purpose Design Strategies and Techniques to Improve the Performance of a Dual Active Bridge With Phase-Shift Control

This paper addresses the performance of the bidirectional Dual Active Bridge (DAB) converter. One of the advantages of the DAB is the possibility to achieve Zero Voltage Switching (ZVS) operation in all the switches of this converter. However, the ZVS operation range can be lost for light loads, especially if high voltage is required in at least one of the DAB ports and the phase-shift control is used to regulate the power processed by the converter. Theoretically simple averaged model is presented for the DAB converter. Using the study presented in this paper, the boundaries of ZVS operation can be easily evaluated. The proposed models and analysis of the ZVS boundaries allow the proposal and evaluation of two different design strategies with different purposes: on the one hand, increasing the ZVS operation range and, on the other, improving efficiency at full load. Moreover, some techniques are presented for increasing the ZVS operation range and improving the efficiency of the DAB at full load (both using phase-shift control) employing the aforementioned analysis to obtain certain design criteria and conclusions. Finally, the proposed models, design strategies and techniques to improve the performance of the DAB are experimentally tested using a 1kW prototype with input and output voltages of 48V and 400V, respectively.

An Insight into the Switching Process of Power MOSFETs: An Improved Analytical Losses Model

The piecewise linear model has traditionally been used to calculate switching losses in switching mode power supplies due to its simplicity and good performance. However, the use of the latest low voltage power MOSFET generations and the continuously increasing range of switching frequencies have made it necessary to review this model to account for the parasitic inductances that it does not include. This paper presents a complete analytical switching loss model for power MOSFETs in low voltage switching converters that includes the most relevant parasitic elements. It clarifies the switching process, providing information about how these parasitics, especially the inductances, determine switching losses and hence the final converter efficiency. The analysis presented in this paper yields two different types of possible switching situations: capacitance-limited switching and inductance-limited switching. This paper shows that, while the piecewise linear model may be applied in the former, the proposed model is more accurate for the latter. Carefully-obtained experimental results, described in detail, support the analytical results presented.

An overall study of a Dual Active Bridge for bidirectional DC/DC conversion

The increase demand of an intermediate storage of electrical energy in battery systems, in particular due to the use of renewable energy, has resulted in the need of bidirectional DC/DC power converters with galvanic isolation. Uninterruptible Power Supplies (UPS), battery charging systems, photovoltaic equipment and auxiliary power supplies in traction applications are examples of some fields of application of this kind of converters. A Dual Active Bridge (DAB) bidirectional DC/DC converter is a topology with the advantages of decreased number of devices, soft-switching commutations, low cost, and high efficiency. The use of this topology is proposed for applications where the power density, cost, weight, and reliability are critical factors. In the present paper the steady-state analysis of the converter has been carried out, giving some guidelines for the design (considering soft switching limits and the amount of reactive current) and a small-signal model of the topology. Simulations and experimental results are also presented.

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.

Steady-State Analysis and Modeling of Power Factor Correctors With Appreciable Voltage Ripple in the Output-Voltage Feedback Loop to Achieve Fast Transient Response

The classical design of an active power factor corrector (PFC) leads to slow transient response in this type of converter. This is due to the fact that the compensator placed in the output-voltage feedback loop is usually designed to have narrow bandwidth to filter the voltage ripple of twice the line frequency coming from the PFC output. This feedback loop is designed with this filtering effect because a relatively high ripple would cause considerable distortion in the reference of the line current feedback loop, and hence in the line current. However, the transient response of the PFC can be substantially improved if the bandwidth of this compensator is relatively wide, thus permitting certain distortion in the line current that leads to a tradeoff between transient response (and hence voltage ripple at the output of the compensator) and harmonic content in the line current. As a consequence of the voltage ripple at the output of the compensator (which is considered the control signal), both the static and the dynamic behaviors of the PFC change in comparison with the standard case, i.e., with no voltage ripple on the control signal. The static behavior of a PFC with appreciable voltage ripple in the output-voltage feedback loop is studied in this paper using two parameters: the amplitude of the relative voltage ripple on the control signal and its phase lag angle. The total power processed by the PFC depends on these parameters, which do not vary with the load and which determine the total harmonic distortion and the power factor at the input of the PFC. Furthermore, these parameters also determine the maximum power that can be processed by the converter while still complying with EN 61000-3-2 regulations for Class A and Class B equipment. When the converter comply with the aforementioned regulations for Class C or Class D equipment, however, the compliance does not depend on the power processed by the PFC. In the case of Class C equipment, not all the possible combinations of the relative ripple of the control signal and its phase lag angle manage to comply with these regulations. Finally, the study was verified by simulation and in a real prototype.

An estimator of luminous flux for enhanced control of high brightness LEDs

Usually, PWM operation is selected to drive High Brightness LEDs, because the dimming behavior is more linear than in DC operation. Nevertheless, to obtain an enhanced operation of the device (full output light control), the luminous flux should be measured. This paper proposes a control method based on an estimator of the luminous flux emitted by the LED. Firstly, the LEDs are characterized. Based on these characterization, an estimator of the flux emitted by the LED is defined. The estimator provides this flux value from only two temperature values (the case temperature and the ambient temperature). The estimator is defined and validated in steady state as well as in the transient stage. The flux estimator is then validated with actual measurements. Once the estimator is defined and validated, the electronic driver to supply the LEDs as well as the digital control scheme are presented. Finally, a built prototype is presented, and the experimental results are shown. Conclusions of this work show that with the presented estimator, both the steady state and the transient luminous flux can be accurately estimated. Thus, the output light control of the LEDs can be accomplished by sensing temperature rather than luminous flux. The final output characteristic of the system shows linearity between the output flux and the reference value, with a DC operation of the LEDs.

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.