Patent ID: 7564200
Filing Date: 2009-07-21
Classification: H05B

Abstract:
1. A discharge lamp lighting circuit comprising: a power supplying portion which comprises an inverter circuit comprising a switching element; a series resonant circuit comprising a capacitor, and at least one of an inductor and a transformer; and a driving circuit which drives said switching element, said power supplying portion converting DC power to AC power and supplying the AC power to a discharge lamp; and a controlling portion which produces a frequency control signal for controlling a frequency of a drive signal output from said driving circuit, the controlling portion comprising: a phase difference detecting portion which detects a phase difference between an input voltage and an input current that are supplied from said inverter circuit to said series resonant circuit; and a control signal producing portion which produces the frequency control signal in accordance with the phase differences, wherein said phase difference detecting portion comprises: a first phase difference detecting circuit which, when a phase of the input voltage leads a phase of the input current, produces an inductive detection signal having a pulse width that is proportional to the phase difference; and a second phase difference detecting circuit which, when the phase of the input voltage lags the phase of the input current, produces a capacitive detection signal having a pulse width that is proportional to the phase difference, wherein said control signal producing portion comprises: a detection capacitor, one end of which is set to a first voltage; a charging circuit which is coupled to another end of said detection capacitor, said charging circuit supplying a current to said another end of said detection capacitor in accordance with one of the inductive detection signal and the capacitive detection signal; a discharging circuit which is coupled to said another end of said detection capacitor, and which sinks a current from said another end of said detection capacitor in accordance with the other one of the inductive detection signal and the capacitive detection signal; and a signal producing circuit which detects a voltage across said detection capacitor, and which produces the frequency control signal so as to control the frequency of the drive signal in accordance with a voltage across said detection capacitor, wherein the first voltage is set to a value less than a power source voltage supplied to said charging circuit, and greater than a power source voltage supplied to said discharging circuit.