Discharge lamp ignition apparatus and discharge lamp ignition method

1. A discharge lamp ignition apparatus for igniting a discharge lamp, the discharge lamp ignition apparatus comprising:

a direct-current power supply generation circuit unit for supplying a direct-current power supply voltage;

a resonant circuit unit constituted from an inductor and a capacitor, for applying a resonant voltage generated by the inductor and the capacitor to the discharge lamp;

a voltage detection circuit unit for detecting the resonant voltage being applied to the discharge lamp;

an inverter circuit unit for, based on the direct-current power supply voltage supplied by the direct-current power supply generation circuit unit, alternating-current driving the resonant circuit unit at a drive frequency; and

a control circuit unit for monitoring the resonant voltage detected by the voltage detection circuit unit and controlling the inverter circuit unit based on the resonant voltage, wherein

the control circuit unit:

lowers the drive frequency from a predetermined initial frequency and when the resonant voltage reaches an ignition voltage at which the discharge lamp ignites, establishes the drive frequency for the ignition voltage as a discharge-lamp-ignition drive frequency being the frequency for igniting the discharge lamp; and

if the resonant voltage does not reach the ignition voltage even though the drive frequency is lowered from the predetermined initial frequency to the resonant frequency, establishes as the discharge-lamp-ignition drive frequency a frequency that is a predetermined value higher than the drive frequency at the resonant-voltage peak voltage; and

the inverter circuit unit alternating-current drives the resonant circuit unit at the discharge-lamp-ignition drive frequency established by the control circuit unit.

2. The discharge lamp ignition apparatus according to claim 1 , wherein

the control circuit unit controls the inverter circuit unit at the discharge-lamp-ignition drive frequency.

3. The discharge lamp ignition apparatus according to claim 2 , wherein

the control circuit unit establishes the discharge-lamp-ignition drive frequency continuously for a predetermined time period.

4. The discharge lamp ignition apparatus according to claim 2 , wherein

the control circuit unit periodically establishes the discharge-lamp-ignition drive frequency.

5. A discharge lamp ignition method performed by a discharge lamp ignition apparatus for igniting a discharge lamp, the method comprising the steps of:

setting a drive frequency at which an inverter circuit is driven to a predetermined initial frequency;

lowering the drive frequency gradually from the initial frequency at predetermined intervals;

detecting whether a resonant voltage in a resonant circuit constituted from an inductor and a capacitor has reached a preset ignition voltage for igniting the discharge lamp;

obtaining, if the resonant voltage does not reach the ignition voltage, a drive frequency at a resonant-voltage peak voltage that is lower than the ignition voltage and establishing, as a discharge-lamp-ignition drive frequency, the drive frequency a predetermined value higher than the resonant frequency;

obtaining the drive frequency at the ignition voltage if the resonant voltage reaches the ignition voltage;

setting the drive frequency as the discharge-lamp-ignition drive frequency; and

performing a control operation so as to drive the inverter circuit at the discharge-lamp-ignition drive frequency.