Lighting circuit for vehicular discharge lamp

Abstract

Disclosed is a lighting circuit for a vehicular discharge lamp which controls current limitation in accordance with the status of the discharge lamp to thereby suppress an increase in consumption current when a source voltage drops. In the lighting circuit, when a lighting switch 5 is set on, a battery voltage is boosted in a DC booster circuit and is converted into an AC voltage by a DC/AC converter before it is applied to a metal halide lamp. The DC booster circuit has a chopper structure having an inductor and a field effect transistor (FET). A control signal from a control circuit is sent to the FET. The control circuit has a V (voltage)-I (current) controller for controlling the lighting and a PWM (Pulse Width Modulation) controller. The lighting circuit further has a current compensator to prevent an excessive current from flowing through the FET at the beginning of the lighting or when the battery voltage drops. This current compensator monitors the status of the lamp, produces a signal of a sawtooth wave corresponding to this status, and superimposes this signal on a current detection signal, thereby restricting the peak value of the current flowing through the FET and the average current with the help of the slope compensating function of the PWM controller. Thus, current limitation is controlled so that when the lamp current is large, the degree of current limitation is increased.