| 摘要 |
A lighting system for a discharge lamp includes an inverter circuit (5, 5a, 5b, 5c or 5d) to which is connected a load circuit (6, 6a or 6b) including a resonant circuit of an inductor (12) and a capacitor (11) in serial connection, with a discharge lamp (13) connected in parallel with the capacitor. An inversely frequency dependent voltage is applied between the lamp electrodes according to a predefined resonance characteristic such that the resonance frequency (fo) is less than a discharge start frequency (f2) at which the lamp is to start glowing. For lighting up the lamp the frequency of the inverter output voltage is changed from a first frequency (f1) that is higher than the discharge start frequency to a second frequency (f3) that is less than the resonance frequency. If the lamp accidentally goes off, the current flowing through the load circuit will advance out of phase with the inverter output voltage, possibly resulting in the destruction of inverter switch or switches (Q1 and Q2) due to overcurrent. This danger is precluded by constantly monitoring the phase of the load current and, in event the load current is found to be in phase advance, by making the inverter output frequency higher than the resonance frequency of the resonant circuit and thereby delaying the phase of the load current. <IMAGE> |
