Electronic ballast having a triac dimming filter with preconditioner offset control

A ballast for use with a phase control dimmer having phase controlled AC mains supply voltage output, said ballast comprising:

• mains input terminals for receiving the phase controlled AC mains supply voltage output by the phase angle dimmer, said phase controlled AC voltage having a characteristic frequency;
• rectifier means receptive of said phase controlled AC voltage from said input terminals for providing a rectified DC voltage,
• filter means including capacitive means for exhibiting capacitive characteristics, said capacitive means being charged by said rectified DC voltage;
• power factor correction means for controlling the power factor of the power drawn from the input terminals, said power factor correction means including (i) inductive means for exhibiting inductive characteristics, said inductive means being connected to said capacitive means of said filter means, (ii) a switch switchable between a first switch state which provides a discharge path for said inductive means and said capacitive means and a second switch state, and (iii) control means for controlling the switching of said switch between said first and second switch states at frequencies substantially higher than the characteristic frequency of said phase controlled AC voltage, said control means including means for sensing the voltage of the rectified DC voltage and having the characteristic of reducing the time that said switch is in said first switch state when said rectified DC voltage is at or near zero;
• ballasting means coupled to said power factor correction means for powering a gas discharge lamp;
• and
• capacitive loading means for increasing the duration of time that said switch is in said first switch state when said rectified DC voltage is at or near zero, to thereby increase the discharging of said capacitive means.

A ballast for use with a phase angle dimmer having a phase controlled AC mains supply voltage output, said ballast comprising:

• mains input terminals for receiving the phase controlled AC mains supply voltage output by the phase angle dimmer, said phase controlled AC voltage having a characteristic frequency;
• a rectifier having a pair of rectifier outputs, said rectifier being connected to said mains input terminals for providing a full wave rectified DC output voltage at said rectifier outputs, said rectified output voltage including the conduction phase angle of the phase controlled AC mains voltage;
• filtering means for suppressing high frequency harmonics from entering the mains supply, said filtering means including a filter capacitor connected across said rectifier outputs and charged by said rectified output voltage;
• a preconditioner connected to said outputs of said rectifier means, said pre-conditioner comprising
• (i) a boost inductor connected in series with said filter capacitor,
• (ii) a switch switchable between a conducting and a non-conducting state for controlling the current through said inductor, the conducting state of said switch providing a discharge path for said filter capacitor through said inductor and switch,
• (iii) control means for controlling the switching of said switch to control the current through said inductor for (1) controlling the power factor of the electrical power drawn from said mains supply voltage and (2) to increase the level of the DC voltage from the output of said rectifier, said control means including a control input for sensing a voltage representing the instantaneous magnitude of the DC rectified voltage, said control means controlling the switching of said switch in dependence on the voltage sensed at said input, said control means preventing current from flowing through said inductor from said filter capacitor when the sensed voltage is below a predetermined voltage level at said control input, and
• iv) means for applying an offset voltage at said control input to prevent the sensed voltage from falling below a predetermined level despite said instantaneous rectified DC voltage being at or near zero;
• ballasting means for providing electrical power to a gas discharge lamp, said ballasting means including (i) a DC input receptive of the DC output voltage of said preconditioner, (ii) a dim input separate from said DC input for receiving a dimming signal and (iii) means for controlling the electrical power supplied to the gas discharge lamp at a level corresponding to a characteristic of the dimming signal;
• and
• dim signal deriving means for deriving the dimming signal from the phase controlled rectified output voltage and for supplying the dimming signal to said dim input of said ballasting means.

A ballast for use with a phase angle dimmer having a phase controlled AC mains supply voltage output, said ballast comprising:

• mains input terminals for receiving the phase controlled AC mains supply voltage output by the phase angle dimmer;
• converting means connected to said mains input terminals for converting the phase controlled AC voltage to a DC voltage;
• ballasting means connected to said converting means for supplying electric power to a gas discharge lamp;
• filtering means for suppressing high frequency harmonics, said filtering means including capacitive means exhibiting capacitive characteristics, said capacitive means being charged by said DC voltage from said converting means, said filtering means having a peak overshoot less than said DC voltage input to said ballasting means.