Method and driving unit for driving a gas-discharge lamp

A driving unit (10) for driving a gas-discharge lamp (1) comprising:

• a commutation unit (12) for applying an alternating current to the gas-discharge lamp (1),
• a wave forming unit (14) for forming the current wave-shape applied to the gas-discharge lamp (1),
• an arc-length monitoring unit (17), for monitoring an arc-length control value, indicating the current length of a discharge arc of the gas-discharge lamp (1),
• a control unit (16),
• which control unit (16) controls the wave forming unit (14) such that
• the lamp (1) is driven in a first operation mode (OMp) with a first current wave-shape when the arc-length control value indicates that the arc-length is higher than a switching threshold, which first current wave-shape is selected to result in a tip (31) growing on an electrode (30) of the lamp (1),
• and the lamp (1) is driven in a second operation mode (OMn) with a second current wave-shape when the arc-length control value indicates that the arc-length is lower than a switching threshold, which second current wave-shape is selected such that the tip (31) on the electrode (30) is at least partly melted back,
• and which control unit (16) controls the commutation unit (12) such that
• an operating frequency of the lamp (1) is switched from a first frequency value (f1) to a second frequency value (f2), which second frequency value (f2) is higher than the first frequency value (f1), if a first time criterion pertaining to a duration of driving of the lamp (1) in the second operation mode (OMn) is fulfilled
• and the operating frequency is switched from the second frequency value (f2) to the first frequency value (f1), if a second time criterion pertaining to a duration of driving of the lamp (1) in the first operation mode (OMp) is fulfilled.