| 摘要 |
PURPOSE:To prevent extinction of a discharge lamp in the final period of lifetime and also its running temperature by controlling the duty ratio of the lighted period and turned-off period so that the mean value of the power supplied to the discharge lamp becomes constant. CONSTITUTION:The switch element current sensing voltage generated in a current sensing resistance 10 is passed through a low pass filter 11 to determine the mean value of the switch element current, and the obtained value is compared with the target voltage value Vref by a comparator 12. When the reference value Vref is exceeded, the output voltage value of the comparator 12 is lowered, and the duty ratio of the output signal of a PWM circuit 13 is lessened. In other case, the duty ratio is enlarged. By an AND gate 15, the output frequency of an oscillator 14 is supplied to a switch element driving circuit 16 only when a PWM signal is Hi, and current flows to switch elements 2-5 during the Hi period of PWM signal, and the mean value is controlled constant. Because the supply voltage is constant, the power to the lamp 9 connected to the secondary side of trunks is proportional to the constant primary current I1 of the trunks, so that supply voltage to the lamp 9 becomes also constant. |
