Lighting circuit

Abstract

A lighting circuit includes a DC/DC converting circuit for boosting an input DC voltage so as to produce boosted DC power, a detecting circuit for producing a detection signal in response to the output voltage of the DC/DC converting circuit, a DC/AC converting circuit for producing AC power to be supplied to the discharge tube from the boosted DC power, a starting circuit for starting up the discharge tube, and a control circuit for receiving the detection signal from the detecting circuit. The control circuit contains a capacitor which is charged based upon the detection signal, a first discharging unit for discharging the capacitor, a second discharging unit for providing a discharging path for the capacitor in response to turning-OFF of the input DC voltage, and a signal producing unit for producing a control signal used to adjust the output of the DC/DC converting circuit in response to a terminal voltage of the capacitor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit block diagram showing an arrangement of a lighting circuit according to an exemplary embodiment of the present invention.

FIG. 2 is a circuit block diagram showing an arrangement of a power control unit.

FIG. 3 is a circuit block diagram showing one example of a lighting initial voltage detecting and holding unit, and a difference signal producing unit.

FIG. 4 is a circuit block diagram representing an arrangement of a second control unit.

FIG. 5 is a circuit block diagram showing an arrangement of a charging unit in connection with a major unit of the second control unit.

FIG. 6 is a diagram showing waveforms of respective circuit portions when a discharge tube is lighted by the lighting circuit of this exemplary embodiment.

FIG. 7 is a circuit diagram showing an arrangement of a second discharging unit according to a first exemplary embodiment of the present invention.

FIG. 8 is a diagram showing waveforms of respective circuit portions when a discharge tube is re-lighted by the lighting circuit of this exemplary embodiment.

FIG. 9 is a diagram showing waveforms of respective circuit portions when the discharge tube is re-lighted by a lighting circuit which is not equipped with the second discharging unit of this exemplary embodiment.

FIG. 10 is a circuit diagram showing an arrangement of a second discharging unit according to a second exemplary embodiment of the present invention.

FIG. 11 is a circuit diagram showing an arrangement of a second discharging unit according to a third exemplary embodiment of the present invention.

FIG. 12 is a circuit diagram showing an arrangement of a second discharging unit according to a fourth exemplary embodiment of the present invention.

FIG. 13 is a diagram representing the waveforms of the respective circuit portions when the discharge tube is lighted by the conventional lighting circuit.

Claims

1. A lighting circuit which lights a vehicle discharge tube, the lighting circuit comprising: a DC/DC converting circuit which boosts an input DC voltage so as to produce boosted DC power;a detecting circuit coupled to said DC/DC converting circuit, which produces a detection signal in response to an output voltage of said DC/DC converting circuit;a DC/AC converting circuit which receives said boosted DC power and produces AC power to be supplied to said discharge tube;a starting circuit coupled to said DC/AC converting circuit, which starts up said discharge tube; anda control circuit coupled to said DC/DC converting circuit, which receives said detection signal from said detecting circuit,whereinsaid control circuit includes:a difference signal producing unit which produces a difference signal indicative of a difference between said detection signal and an initial value of said detection signal;a charging unit which receives said difference signal and supplies an electric charge in a case where said difference is greater than or equal to a predetermined threshold value;a capacitance element coupled to an output of said charging unit via a resistive circuit;a first discharging unit which is coupled to the output of said charging unit and to said capacitance element via said resistive circuit;a second discharging unit coupled to said capacitance element, which provides a discharging path for said capacitance element in response to a turning-OFF of said input DC voltage; anda signal producing unit which produces a control signal used to adjust the output of said DC/DC converting circuit in response to a terminal voltage of said capacitance element.

2. A lighting circuit as claimed in claim 1, wherein said second discharging unit includes: a diode having an anode coupled to said capacitance element;a first resistor element coupled between a cathode of said diode and a first power supply line; anda second resistor element coupled between the cathode of said diode and a second power supply line.

3. A lighting circuit as claimed in claim 1, wherein said second discharging unit includes: a diode having an anode coupled to said capacitance element; anda resistor element coupled between a cathode of said diode and a power supply line.

4. A lighting circuit as claimed in claim 1, wherein said second discharging unit includes:a diode having an anode coupled to said capacitance element and a cathode coupled to a first power supply line; anda resistor element coupled between the cathode of said diode and a second power supply line.

5. A lighting circuit as claimed in claim 4, further comprising: an interrupting circuit interposed between said first power supply line and the cathode of said diode, which interrupts a connection between said first power supply line and the cathode of said diode when said input DC voltage is turned OFF.