Discharge lamp lighting device

Abstract

A discharge lamp lighting device is provided with an inverter circuit 7 having a coupling capacitor C2 connected to the middle point of MOS-FETs Q2 and Q3 serially connected to each other and a choke coil T2 connected to the coupling capacitor C2; a protection circuit 8, having a positive potential peak detection circuit 11 for detecting the peak voltage of positive potential and a negative potential peak detection circuit 12 for detecting the peak voltage of negative potential both of which are connected to the mid point of the coupling capacitor C2 and the choke coil T2, and a lifetime end stage detection circuit 14 for detecting the lifetime end stage of the discharge lamp La, in response to the detected peak voltages of positive potential and negative potential; and a control circuit 9 for controlling the output of the inverter circuit 7, based on a protective signal. The difference between the voltages of a detecting capacitor C4 in normal lighting and the lifetime end stage is increased and the lifetime end stage can be detected, without being affected by the state of the lifetime end stage, and the lifetime end stage of a discharge lamp is capable of surely detected.

Description

technical field [0001] The present invention relates to a protection circuit for detecting the end of life of a discharge lamp and protecting an inverter circuit of a discharge lamp lighting device. Background technique [0002] For example, Patent Document 1 discloses a method for detecting voltage fluctuation of a DC cut-off capacitor (referred to as a coupling capacitor in this application) due to half-wave discharge at the end of the life of the discharge lamp load. The average value reduces or stops the output of the inverter circuit to protect the circuit. [0003] [Patent Document 1] Japanese Unexamined Patent Publication No. 2002-83699 (paragraph "0026", line 1 to line 14) [0004] The existing discharge lamp lighting device has the advantage that by making the resistance of the detection circuit of the DC cut-off capacitor and the time constant of the DC component detection capacitor sufficiently long relative to the operating frequency, the DC component detection ...

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Problems solved by technology

[0005] However, since the average value of the voltage fluctuation of the capacitor for DC cutoff is detected, the output of the inverter circuit is reduced or the output is stopped to protect the circuit, so the voltage of the capacitor for DC component detection during normal lighting is related to the end of life There is little difference in the voltage of the DC component detection capacitor, and there is a possibility that detection cannot be performed from the state at the end of life (especially the initial state at the end of life)

[0006] In addition, when the resistance of the detection circuit and the time constant for DC component detection are sufficiently long relative to the operating frequency, it may take a long time until the voltage fluctuation of the DC cut-off capacitor is detected.

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