Patent ID: 7291990

Claim:
A discharge lamp lighting circuit comprising a discharge lamp, a DC-AC conversion circuit having a plurality of switching elements and a series resonance circuit, and a control circuit for preventing continuation of a situation in which a drive frequency of the switching element is less than its specified minimum frequency, wherein the control circuit comprises a driving situation detection circuit for detecting the phase difference between a signal for driving the switching element, the output of the DC-AC conversion circuit or a signal that corresponds to the voltage of the discharge lamp and a signal that corresponds to the current of the discharge lamp; wherein when the discharge lamp is lit, the switching element is driven at a frequency in a frequency range which is higher than a resonant frequency of the series resonance circuit, and a driving situation of the switching element is monitored by the driving situation detection circuit, and wherein, upon detection that the phase of the signal that corresponds to the current of the discharge lamp is leading the phase of any of the signal for driving the switching element, the output of the DC-AC conversion circuit or a signal that corresponds to the voltage of the discharge lamp, the drive frequency is increased.