| 摘要 |
PURPOSE:To hold the lamp current constant at a desired value by providing pulse width modulation means in a feedback control system and outputting a PWM signal to a semiconductor switching device in response to the result of comparison for controlling it. CONSTITUTION:The desired value of a lamp Current flowing through an electric discharge lamp 2 is set by a reference power supply 10. On the other hand, during the operation of the circuit, a switching ripple current, accompanied by the charging and discharging of a capacitor C, corresponding to the lamp current is continually detected via a detecting winding 5a by a detecting circuit 5b, and the detected current value is supplied to the input of an error amplifier 7. If there is a difference between the detected current value and the desired value, an error signal is produced from the output of the error amplifier 7, and a pulse width modulation(PWM) signal is produced from the output of a pulse width modulator 8 for correcting the error so that the switching duty ratio of the switching transistor Q3 or Q4 is automatically controlled. The lamp current is thus held constant at a desired value. |
