Analysis and development of improved converters for led drivers with special focus on efficiency and dimming

Resumen

Light-emitting-diodes (LED) were first developed in 1960s. Nowadays, they have become one of the most popular lighting sources in a wide variety of applications. This is owing to the following advantages shown by LEDs: longer lifetime, higher efficacy, smaller size, fast response, robustness, reliability, and high color rendering index. The only drawback is that LEDs cannot be connected directly to the mains. Thus, it is fundamental to drive the LEDs through a current-controlled power supply. The aim of this driver is not only to drive the LEDs, but also to fulfill all required standards. Numerous types of drivers have been presented in the literature. Single-Stage drivers used as power factor correction (PFC) and constant power control (PC) simultaneously. Moreover, for the sake of a better performance the two-stage LED drivers are proposed. The first stage operates as a PFC stage, while the second stage operates as a PC stage. A promising solution for a compact driver keeping the operation of the two stage converters is the integrated converters. Integrated converters operate similarly to a two-stage converter but using a single switch. This thesis investigates certain solutions to overcome the issues that the LED driver poses. Firstly, a study of the losses in the integrated buck flyback converter used as high PF LED driver is presented. The study proposes a technique to increase the efficiency of the integrated buck flyback converter by redesigning the converter parameters. Furthermore, it presents a case of study with a step-by-step efficiency enhancement process of an existing driver. The new design shows an improvement of the efficiency from 82 \% in the old design to 89 \% in the proposed one. Finally, the presented methodology is explained in detail so that it can easily be applied to other integrated converters. Secondly, a high PF, low THD LED driver with dimming capability is presented. The driver is implemented by using an interleaved capacitor, which is placed between the rectifier and the integrated buck flyback converter. In this way, the line current conduction angle is increased, which in return increases the PF and decreases the THD. The operation of the proposed converter ensures that one diode of the conventional converter will not conduct, so that it can be removed. Moreover, owing to the continuous power flow, the proposed technique makes a significant reduction of the converter output ripple. Thirdly, an improved dimming technique, to enhance the operation by improving the LED current waveform under PWM dimming is presented. The idea is accomplished by the new hybrid series parallel PWM dimming technique, through passive and active techniques. The passive technique is made by adding a resistive branch. While, the active method is made through a new double integrated converter, the integrated buck flyback buck converter. It ensures a constant output current regulation as well as high PF at any dimming ratio. Fourthly, a high-power-density off-line LED driver is proposed. The proposed AC-DC driver is the novel integrated buck and boost converter. Besides the high-power-density, the converter shows good PF and THD. For a further increase of the power-density, a magnetic integration is made by integrating the two inductors of both the buck and boost converters in one core. Thus, the proposed converter has been named as fully integrated buck and boost converter. In addition, a comparison between two prototypes is presented, one for the integrated buck and boost converter and another for the fully integrated converter. A patent application has been registered for this converter in China, Taiwan and Europe. Lastly, a dynamic characterization of Organic Light Emitting Diodes (OLED) aging process is presented. The aim of this study is to establish a dynamic electrical model illustrating the OLED characterization over time. In addition, the model is used to examine the control behavior using simulation, showing its behavior during both transient and steady-state. Furthermore, it is possible to predict the drift of the control system with aging. This ensures better performance for the OLEDs over its whole lifetime, which in return increases its lifetime. Moreover, it is presented a methodology to predict the lifetime of the OLED just by measuring the voltage across the OLED at rated current. Moreover, the study presents the effect of aging on each parameter of the dynamic model.