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AN2747 Datasheet(PDF) 7 Page - STMicroelectronics |
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AN2747 Datasheet(HTML) 7 Page - STMicroelectronics |
7 / 44 page AN2747 Lamp description 7/44 Figure 6. Instantaneous lamp power with sinusoidal and square waveforms For ideal square waveforms, the instant power delivered to the lamp is constant and not modulated by any frequency. This driving plus the low frequency driving allows obtaining a system that is immune to the problem of acoustic resonance. For this reason HID lamp applications using square wave current techniques are the best solution. 1.1 Lamp phases 1.1.1 Ignition phase The ignition voltage for HID lamps is higher with respect to the mains voltage. The peak voltage value to initiate the discharge is very high. Typically this is about 3 kV ÷ 5 kV. The voltage level at which an HID lamp ignites is called "ignition voltage". This voltage level is referred to the lamp when it is cold but this value increases with lamp temperature. To ensure immediate HID lamp restart, called “hot restrike”, very high re-ignition peaks are necessary (about 20 kV). Standard luminaries are not designed for hot restrike voltages. For this reason in order to obtain hot lamp re-ignition, several minutes must elapse for the lamp to cool. The restart time depends on lamp temperature. 1.1.2 Warm-up When the discharge has been initiated by the starting gas and the lamp still does not burn properly, a warm-up time is required. During the warm-up time the gas temperature increases, increasing the light output. The lamp voltage starts approximately from a quarter of Vlamp and increases up to Vlamp. Instantaneous Power Voltage Current Instantaneous Power Voltage Current |
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