Patent ID: 8760083

Claim:
A ballast for gas discharge lamps, comprising: a power factor corrector for receiving an AC input voltage and converting the AC input voltage into a DC voltage with a corrected power factor; a DC/DC converter connected to the power factor corrector and having a switch located at a low-voltage side of the DC/DC converter for converting the DC voltage with a corrected power factor into a DC output voltage according to switching operations of the switch; a controller connected to a control terminal of the switch in the DC/DC converter for sending a switching control signal to control the switching operations of the switch; and a control device for open-circuit voltage including a detecting circuit for detecting a voltage associated with an open-circuit voltage of the ballast, and a switching control device connected to the detecting circuit for driving the controller to regulate a duty ratio or a pulse density or a switching frequency of the switching control signal according to results of detection by the detecting circuit for controlling the open-circuit voltage of the ballast; wherein the switching control device includes: a control switch having a pair of current-conducting terminals and a control terminal, wherein the control terminal is connected to the detecting circuit for receiving a sampled voltage; and a third resistor connected to one of the current-conducting terminals of the control switch.