AN2870 datasheet(5/41 Pages) STMICROELECTRONICS

5 / 41 page

AN2870

Introduction

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After a good preheating, the voltage across the lamp is suddenly increased in order to

generate a strike inside the tube and ignite the lamp. This phase should last between 10 ms

and 100 ms.

The strike voltage depends on various parameters, many of which cannot be exactly

evaluated: preheating energy, remaining lamp life, number and efficiency of the past

ignitions. An insufficient preheating causes greater ignition voltage and a subsequent stress

of the cathodes that lose small amounts of material that darken the region of the tube near

to the cathode itself (sputtering).

Lamp ageing is related with the symmetrical or, more often, asymmetrical increasing of the

cathodes resistance. A symmetrically aged lamp absorbs more power causing hard

switching and over-current. Asymmetrically aged lamps experience a current that is more

intense in one direction than in the other. This implies that the current flowing into the lamp

has positive or negative mean value (DC component). This effect can be detected

measuring the mean values of the lamp voltage that should be zero in normal lamps. The

worst case of rectifying effect causes the current flowing only in one direction: the voltage

across the resonant capacitor can reach very high values and heavy hard switching is

detected.

When symmetrical or asymmetrical ageing of the lamp reaches a value indicated in

international norms, the lamp reaches its end of life (EOL).

AN2870 Arkusz danych(PDF) 5 Page - STMicroelectronics