Patent ID: 6856102

Claim:
A three-stage electronic ballast for metal halide lamps, comprising: a filter and rectification circuit connected to an input voltage source terminal for suppressing electromagnetic disturbances and rectifying an input voltage signal; a step-up converter disposed after said filter and rectification circuit, said step-up converter further comprising a push-pull transistor, a step-up inductor and a rectification diode and a filter capacitor; a step-down converter disposed after said step-up converter, said step-down converter further comprising an inductor, a diode, a capacitor and a transistor switch, said step-down converter controlling the current in said inductor at the boundary of continuity and discontinuity so as to lessen power loss and enhance operation efficiency; a DC-AC converter disposed after said step-down converter for providing an alternating square-wave voltage to a metal halide lamp; an ignition circuit using circuit elements including a step-up transformer and a capacitor to generate an electric voltage up to 3 kilo volts for discharging said metal halide lamp; and a micro processor for controlling the conducting rate of said step-down converter according to the product of output voltage and current of said step-down converter, whereby power regulation can be achieved; whereby said three-stage electronic ballast for metal halide lamps can force said inductor in said step-down converter to operate in a continuous boundary current mode, whereby the peak value of said inductor current will be decreased, and whereby said transistor switch switches under zero current, significantly reducing power loss in switching.