Analysis and applications of current-source-mode converters in light-emitting-diode lighting systems

Abstract

Light-emitting-diodes (LEDs), which have been extensively applied in various lighting situations, are solid-state devices with specifc v-i characteristics. The LED is becoming a promising new generation light source because of the advantages of environmental safety, long life expectancy and high efficiency. Based on an LED's v-i curve, the LED can be regarded as a non-ideal voltage source. Therefore, for LED drivers, the importance of consideration of the termination type and the corresponding choice of converter type should be fully reinforced. In this thesis, LED drivers are studied from the perspective of basic circuit theory. Current-source-mode (CSM) converters are derived from voltage-source-mode (VSM) converters based on the duality principle. This thesis aims to present a comprehensive comparison of the key performance areas in relation to LED lighting applications. A CSM single-inductor multiple-output (SIMO) converter with simple control strategy and the least number of inductors is constructed by using CSM converters for driving LEDs. In order to improve the efficiency of the CSM SIMO LED driver, the adaptive current bus technique is applied to the proposed CSM SIMO converter. First, we study the basic requirement of the driving circuits and discuss the proper approach to driving LEDs in view of their characteristics. We compare voltage source driving and current source driving, and discuss their relative advantages and constraints. We specifically introduce the use of circuit duality principle for developing CSM drivers which are less known but are theoretically more versatile compared to their conventional VSM counterparts. The study highlights the effects of the choice of driving circuits in terms of the number and size of circuit components used, duty cycle variation, sensitivity of control, nonlinearity and control complexity of LED drivers. We present a systematic and comparative exposition of the circuit theory of driving LEDs, with two examples supporting the high value and huge potential use of CSM converters in lighting systems. SIMO LED drivers have the advantages of being compact, efficient and low cost. However, the voltage-to-current transfer function of each output of the SIMO converter is not independent, with significant cross regulation issues that necessitate the use of complex closed-loop control for achieving independent dimming function. Based on the duality principle, this thesis proposes a CSM SIMO dc-dc converter which is shown to be more suited for LED driving due to widened control range of the duty cycle and inherently inductor-less LED driving topology. The outputs of the CSM SIMO dc­dc converter are inherently independent, resulting in very simple control requirement involving only one closed-loop controller for providing the constant current feeder, and several simple open-loop controllers for independent dimmable driving of LEDs. Simultaneous voltage step-up and step-down of multiple outputs is an added feature that simplifies the input power source requirement especially for portable applications. The whole system is simple, reliable and low cost. Taking the CSM single-inductor dual-output (SIDO) dc-dc converter as an example, we illustrate the circuit operation and establish a small-signal model for facilitating the design of the feedback control. A direct duty-cycle dimming method is described. Finally, an attempt is made to improve the efficiency of the CSM SIMO LED driver. This LED driver has many advantages, however, the low efficiency is a serious constraint for the widespread use of the CSM SIMO dc-dc converter. Another limiting factor is that the low-frequency pulse-width modulation (PWM) dimming method can not be extended to three or more outputs in this CSM SIMO dc-dc converter. We apply the adaptive current bus technique to the CSM SIMO dc-dc converter for improving the efficiency. The reason for the increase in efficiency is explained in terms of the energy flow path and the power consumption of devices. The problem of using low-frequency PWM dimming method is analyzed and solved. Regarding the voltage stress of switches, we compare the traditional series-input-connected structure with the CSM SIMO dc-dc converter, and conclude that the CSM SIMO dc-dc converter is more suited for high-step-down applications without the use of transformers.