Experimental results

AN3032

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Doc ID 16165 Rev 2

Figure 11.

Lamp voltage and current in run mode condition

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 12 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

again triggered, the L6585DE stops.

Figure 12.

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 one of the lamp’s tubes is broken: the lamp cannot ignite and

the lamp voltage must be limited. Figure 13 shows how the dual lamp ballast ignites when

one lamp is broken.

AM05141v1

AM05142v1