Lamp Current Regulation Research Articles

Digitally Controlled Integrated Electronic Ballast with Dimming and Power Factor Correction Features

This paper presents a digitally controlled integrated electronic ballast with dimming and power factor correction features. The control circuit is based on a low-cost PIC16C71 microcontroller where the different strategies for energysaving have been implemented. The ballast is operating in closed loop achieving tight lamp current regulation through a digital Proportional-Integral algorithm. The integrated power stage is based on a frequency-controlled single-switch boost rectifier plus a half-bridge series resonant parallel loaded inverter sharing one power switch. The single-switch boost rectifier works in discontinuous inductor current mode with automatic power factor correction. Detailed analysis of the power stages and experimental results using 42-watt electronic ballast are presented.

Low Frequency Architecture for Multi-Lamp CCFL Systems With Capacitive Ignition

A low frequency architecture is proposed for driving parallel cold cathode fluorescent lamps (CCFLs) in large screen liquid crystal display (LCD) TV backlighting applications. Key to the architecture is a proposed capacitive coupling approach for aiding lamp ignition. A dc voltage is applied to the lamp electrodes while an ac voltage is applied to an external plate for capacitive coupling. The result is reliable, simultaneous ignition of parallel lamps with a required applied dc voltage near the lamp steady-state operating voltage. The complete system architecture includes a single high voltage converter, a pulse lamp ignition circuit, current control circuits and a single backlight controller. The topology is capable of driving a large number of parallel lamps with independent lamp current regulation, while avoiding ac coupling losses in steady-state operation and achieving significant reduction in reactive components when compared to typical high frequency ac ballast designs. Experimental results are presented for a system of four parallel 250 mm length lamps, demonstrating simultaneous parallel lamp ignition and dc current regulation.

Electronic Ballast Control IC With Digital Phase Control and Lamp Current Regulation

A digital control architecture is presented for electronic ballasts that provides a phase sweep for reliable, soft lamp ignition and a smooth transition to lamp current regulation mode. The controller is based on an inner phase loop for fast regulation of the resonant tank operating point and an outer current loop for lamp current regulation. The inner loop operates on a simple digital control law that computes the required gate timing relative to the inductor current positive zero crossing. Phase control provides reliable drive of the resonant converter in the presence of large dynamic changes in the load impedance during lamp ignition and warm up and natural tracking of component variations with temperature and time. The primarily digital approach provides programmability for broad application, insensitivity to process and temperature variations, realization in low cost CMOS processes and few external components. Experimental results are presented for an integrated ballast controller fabricated in a 0.8 mum CMOS process used in a 400 W, 150 kHz HID electronic ballast.

26.1: Low-Frequency Square-Wave Drive for Large Screen LCD-TV Backlighting Systems

paper presents a low-frequency square-wave drive, consisting of a single high voltage converter, an ac lamp ignition circuit, current control devices and a single backlight controller, capable of driving an arbitrary number of parallel cold cathode fluorescent lamps (CCFLs) with independent accurate lamp current regulation. Key to the architecture is a proposed capacitive coupling approach for ac lamp ignition that results in reliable, simultaneous ignition of parallel lamps with a maximum ignition voltage near the normal lamp operating voltage. A brief summary of the lamp model and behavior is presented to explain the findings during the capacitive ignition and low-frequency operation. Experimental results are presented demonstrating parallel lamp ignition and current regulation for four, 250 mm CCFLs. 1. Introduction screen LCD TV backlighting systems, generally consisting of 16 or more cold cathode fluorescent lamps (CCFLs), require a ballast capable of driving all or groups of lamps in parallel with accurate current control and regulation. The most common electronic ballasts, based on high frequency LCC resonant inverters, are capable of driving at most four lamps in parallel and require complex multi-winding transformers and specialized control circuitry. This solution suffers from several disadvantages, the major limitation being the inability to simultaneously maintain high efficiency, proper lamp ignition, and individual lamp current regulation (1, 2). We present a suitable architecture, capable of driving a large parallel CCFL array with high efficiency, accurate lamp current control and near operating voltage lamp ignition. The system block diagram, shown in Fig. 1, is based on low-frequency square-wave (LFSW) drive and removes many of the drawbacks associated with a high frequency drive, including energy loss through capacitive coupling, the thermometer effect luminance uniformity degradation and electromagnetic interference (EMI). The proposed architecture is capable of driving a large parallel CCFL array with only a single high voltage and high efficiency converter, resulting in reduced size, weight and cost over existing designs. Reduced ignition voltage (at near operating voltage) is achieved by a unique capacitive coupling approach, based on the similar principles to normal operation of electrodeless or external electrode fluorescent lamps. However, the displacement current is only required in our proposed architecture as a short high frequency pulse during cold lamp ignition, which triggers simultaneous lamp ignition of all parallel lamps. High impedance current source circuit (based on MOSFET current mirror) is connected in series with each lamp to ensure individual lamp current regulation and control. In this paper, we discuss a suitable architecture for large CCFL arrays and study new approaches for lamp ignition to enable low frequency drive.