Dimming ballast for compact fluorescent lamps

A fluorescent lamp ballast circuit comprising:

• a phase cut dimmer connectable to a source of AC power, the phase cut dimmer comprising a user selectable control for setting a brightness level of the lamp and an electronic switch having a trigger voltage and a conduction period, the conduction period of the electronic switch being controlled by the user selectable control whereby to pass a phase cut signal comprising a selectable portion of a half cycle of the AC power from a firing angle when the electronic switch triggers to the end of the half cycle;
• a rectifying circuit coupled to the output of the phase cut dimmer for providing a DC voltage across a DC bus;
• a charging circuit for charging an energy storage device from the selectable portion of the AC power and for providing a continuous current draw from said phase cut dimmer during the selectable portion of the halfcycle of the AC power;
• a filter circuit receiving the output of said phase cut dimmer and converting the output to a DC level signal related to the firing angle of the electronic switch;
• a ballast control circuit for driving an output stage comprising at least one electronic switching device, the at least one electronic switching device of the output stage being coupled to said DC bus and being controlled by the ballast control circuit to provide a pulsed power signal to the fluorescent lamp adapted to be coupled hereto, the ballast control circuit including a control input for changing a frequency of said pulsed power signal, the control input being coupled to said DC level signal, the DC level signal varying in accordance with the selectable portion of the AC power provided by said phase cut dimmer, thereby varying the brightness level of said fluorescent lamp.

A method of dimming a fluorescent lamp comprising:

• applying an AC power source waveform to a phase cut dimmer;
• triggering on an electronic switch of the phase cut dimmer at a selected point in a half cycle of the AC power source waveform, thereby providing a phase cut signal, the selected point being related to a desired dimming level of the fluorescent lamp;
• rectifying said phase cut signal and charging a storage device to thereby generate a DC power supply voltage across a DC bus;
• continuously drawing current from said electronic switch of the phase cut dimmer from the selected point in the half cycle of the AC power source waveform to the end of the half cycle;
• filtering at least a portion of the phase cut signal to provide a control signal related to the desired dimming level;
• providing the control signal to a ballast control circuit for driving at least one output electronic switch to provide a pulsed power signal comprising pulses of said DC power supply voltage;
• providing said pulses of said DC power supply voltage to said fluorescent lamp.