| 摘要 |
PURPOSE:To attain dimming to a continuously dark dimming level by adjusting length of an on-period to dim a discharge lamp, so that a switching loss of a switching element is reduced at the time of full lighting. CONSTITUTION:A switching element Q1 is controlled by a control signal from a control circuit 4, and an on-period of this switching element is adjusted by a variable resistor in the control circuit 4 to dim a discharge lamp 3. At the time of full lighting in the on-period of the element Q1, leakage inductance of a drive transformer T1 and a capacitor C2 are in resonance to smoothly adjust inter-drain source voltage and a drain current at switching time. Accordingly, bouncing voltage of the inter-drain source voltage is also not generated to reduce a switching loss, and lighting can be maintained. At the time of minimum level dimming, decrease of a lamp current increases equivalent resistance of the discharge lamp 3, and the bouncing voltage appears by weakening the above-mentioned resonance condition. This bouncing voltage serves as low beam lighting maintenance voltage, and the discharge lamp 3 can be dimmed to a deep level. |
