Part # Application Note Lighting Control BUL742C AN527 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: Electronic flourescent lamp ballast

Part Details:

APPLICATION NOTE ELECTRONIC FLUORESCENT LAMP BALLAST by A. Vitanza, R. Scollo, A. Hayes 1. INTRODUCTION the contacts in the starter is sufficient to ionise thegas there. This allows a current to flow in the metal Fluorescent lamps have applications in most areas strips and through the gas (and also through the of lighting, where they give longer lifetime and lower filaments of the main tube, which heats them and power consumption for equivalent light output facilitates the subsequent ionisation). The heat compared to filament bulbs, with the disadvantage generated by the current flow through the gas causes of higher initial cost. In operation, a voltage is applied the bimetallic strip to bend towards the other. When across the ends of a sealed glass tube containing the contacts finally touch, two things occur: firstly mercury vapour, causing the vapour to be ionised. the gas in the starter de-ionises, and so the bimetallic This ionised vapour radiates light in the ultra violet strip begins to cool. Secondly, as the impedance of part of the spectrum, which is converted to visible the circuit falls, the current through the ballast inductor light by a fluorescent coating on the inside of the and the filaments of the main tube increases. tube. A few tenths of a second later, the bimetallic strip 1.1 Driving a fluorescent tube has cooled sufficiently to bend back slightly, When the lamp is first turned on, the mains voltage reopening the gap. The sudden increase in across it is not sufficient to cause the initial ionisation impedance and consequent sharp reduction in of the vapour. A starting element is thus needed to inductor current causes a large overvoltage across provide a high voltage pulse across the tube to start the inductor. Given the correct conditions (see the process. section 2.2 below for a discussion of these) this Once the gas in the tube is ionised however, its overvoltage is large enough to cause ionisation of impedance becomes negative; that is to say the the gas in the main tube. At this point the impedance more current that flows through the lamp, the more of the fluorescent tube falls to a minimum, and the conductive it becomes (this is because the increased voltage drop across it falls to a level below that current increases the degree of ionisation of the required to ionise the gas in the starter contact gap. gas). Therefore, some form of limiter must be added The contacts thus remain open until the lamp is next to prevent the current increasing to a level where the turned on. lamp is destroyed. The current limiter is usuallyknown as a ballast. Figure 1. Simple conventional lamp ballast

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Application Note Lighting Control BUL742C AN527.pdf Datasheet