| 摘要 |
PURPOSE:To suppress an unstable discharge and an increase of stress to switching devices of an inverter at the start of the on-period by providing a selecting means selecting a duty ratio control means to perform a dimming action for a shallow dimming level and selecting a power control means to perform a dimming action for a deep dimming level. CONSTITUTION:When the voltage 8a of a reference voltage source 8 is lowered, the value of a variable resistor 72 is also decreased accordingly, the output frequency of an oscillating circuit 7 rises, the current amplitude of a discharge lamp 3 is decreased, and the dimming level is gradually made deep. When the dimming level becomes deep, a dimming means selected by a dimming means selecting circuit 9 is switched from a duty ratio control means 5 to a power control means in the middle, and the value of the variable resistor 72 is set not to generate a step in the brightness of the discharge lamp 3 at the time of this switching. An unstable discharge and an increase of stress to switching devices 21, 22 of an inverter 2 at the start of the on-period can be suppressed during dimming by duty ratio control. |
