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AN3040 Datasheet(PDF) 17 Page - STMicroelectronics |
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AN3040 Datasheet(HTML) 17 Page - STMicroelectronics |
17 / 27 page AN3040 Experimental results Doc ID 16181 Rev 2 17/27 4.2 Protections With old lamps, abnormal behavior may occur during run mode as a result of the rectifying effect. This effect relates to a differential increase of the ohmic resistance of the two cathodes. The lamp equivalent resistance is therefore higher when the lamp current flows in one direction than in the other. The current waveform is distorted and the mean value of the lamp current is no longer zero. Figure 8 shows the behavior of a dual lamp ballast during a rectifying effect. In the EOL pin, as soon as the internal window comparator is triggered by a voltage variation due to the rectifying effect, the Tch cycle starts, and if at its end the comparator is again triggered, the L6585DE stops. Figure 8. Run mode, rectifying effect When an old lamp is connected to the ballast, the strike voltage is higher than the nominal voltage and may also be higher than the safety threshold. In this case, the lamp can take longer than usual to ignite or may not ignite at all. In both cases, because of the frequency drop, the voltage at the output of the ballast can easily reach dangerous values during this ignition time. The same problem occurs if any one of the lamp’s four tubes is broken: the lamp cannot ignite and the lamp voltage must be limited. Figure 9 and Figure 10 show how the four-lamp ballast ignites when two lamps are broken. When the preheating time Tpre = tTch is finished, the L6585DE detects the lost ignition of one of the two lamps and starts reducing the preheating time tTch,reduced. At the end of this time, if one of the four lamps is not ignited, the IC is latched. |
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