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 and small-signal models are 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. I.

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 only one or two tasks and they are the preferred option 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 is focused this paper, is the Asymmetrical Half Bridge (AHB). Its design has been optimized taking into account the needs and characteristic 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 transitions allows us to optimize the duration of the dead times 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.Index Terms--Asymmetrical Half-Bridge dc-dc converter, Complementary control Half-Bridge dc-dc converter, LED's driver, street lighting based on LEDs.

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Abstract-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.Index Terms-AC-DC power converters, electronic transformer, high efficiency, light-emitting diode (LED) lighting, threestage topology, two-input buck.

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