1. Field of the Invention
The present invention relates to lighting apparatuses for lighting high-voltage discharge lamps used for, for example, front projectors.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2000-123989 describes a lighting apparatus for lighting a high-voltage discharge lamp used for, for example, a front projector.
The apparatus described in Japanese Unexamined Patent Application Publication No. 2000-123989 will now be described with reference to FIG. 1. A lamp 5 is, for example, a 150-watt 75-volt metal halide lamp. A DC power source 1 requires an output open-circuit voltage (OCV) higher than a voltage required by the lamp for maintaining the discharge before the lamp breaks down. This output open-circuit voltage can be preferably 200 to 400V.
The voltage of the DC power source 1 is supplied to a step-down chopper circuit 2. The step-down chopper circuit 2, including a switching element 11, a diode 12, and an inductor 13, is of a typical switching power-supply type. The inductance of the inductor 13 is, for example, 0.39 mH.
A voltage detecting circuit 7 that detects the output voltage of the step-down chopper circuit 2 and a current detecting circuit 6 that detects the output current are disposed downstream of the step-down chopper circuit 2. The output voltage and the output current are input to an operating circuit 8, and fed back to the switching element 11 via a pulse-width modulating circuit 9.
The step-down chopper circuit 2 is switched on/off at a high frequency (tens to hundreds of kilohertz). The pulse-width modulating circuit 9 controls the pulse width of the switching element 11 in the step-down chopper circuit 2 such that a fixed power is supplied after the lamp lighting is stabilized on the basis of the detected output voltage and output current.
Although the power supplied from the step-down chopper circuit 2 is mainly used for power control after arc discharge occurs, the pulse-width modulating circuit 9 controls the step-down chopper circuit 2 such that appropriate energy can be supplied also during a phase of glow discharge. A glow current of, for example, several hundreds of milliamperes is supplied.
A switching circuit 16 insulates a smoothing capacitor 14 from the output of the step-down chopper circuit before arc discharge occurs so that the lamp can be appropriately lit, and connects the capacitor to the step-down chopper circuit 2 after arc discharge occurs so that ripples are removed and so that the lamp stably operates without acoustic resonance.
A timer 20 is set to, for example, about three seconds, and the switching circuit 16 insulates the capacitor during the time set by the timer 20 after an igniter 4 starts operating so that the discharge state of the lamp immediately switches to arc discharge.
When the discharge lamp 5 breaks down and the load is momentarily short-circuited while the lamp is lit, the control of the step-down chopper circuit 2 cannot respond to the rate at which inrush current momentarily passes through the discharge lamp 5 since the step-down chopper circuit 2 and the igniter 4 are independently controlled. That is, a few pulses of inrush current with a high peak value as shown in FIG. 2 pass through the lamp although the inrush current is suppressed by the switching circuit 16 that insulates the large-capacitance capacitor 14 in the circuit and only energy charged in a small-capacitance capacitor 15 is supplied to the discharge lamp 5.
Since the inrush current passing through the discharge lamp during the breakdown affects the lifetime of the discharge lamp, the peak value and the time period during which the current is applied need to be suppressed.
However, the lighting apparatus used for the high-voltage discharge lamp described in Japanese Unexamined Patent Application Publication No. 2000-123989 uses the small-capacitance capacitor 15 and the large-capacitance capacitor 14, and connects or disconnects the large-capacitance capacitor 14 using the switching circuit 16 in order to realize smooth switching of the discharge state of the lamp from glow discharge to arc discharge and stable operation during steady lighting after arc discharge occurs. This disadvantageously leads to an increase in cost and in the space taken up by parts.
Moreover, even though the apparatus uses a small-capacitance capacitor, the control of the control circuit may not be able to respond to a high breakdown speed when the discharge lamp breaks down. Since energy supplied from the capacitor 15 to the discharge lamp cannot be reduced by shortening the on-duty cycle of the switching element 11, the small-capacitance capacitor requires a certain degree of capacitance. Therefore, inrush current is not necessarily minimized.