Patent ID: 8803436

Claim:
A dimmable ballast circuit for a fluorescent lamp comprising: a bus-voltage-generating circuit receiving an input from an AC voltage supply and producing a DC bus voltage characterized by a low-frequency voltage ripple; an inverter circuit receiving said DC bus voltage and generating a variable frequency square-wave output signal, said inverter circuit having a control input provided with a drive control signal that determines the frequency of operation of said inverter circuit and thus the frequency of said square-wave output signal; an output filter circuit receiving said square wave output signal and generating a substantially sinusoidal lamp current through said lamp at said frequency; a control circuit producing said drive control signal provided at said control input of said inverter circuit to control the frequency of said square-wave output signal and thus the intensity level of said lamp, said control circuit receiving a first signal related to a target intensity level of said lamp and a second signal related to the actual intensity level of said lamp, said control circuit comprising an error amplifier circuit for comparing said first and second signals to produce said drive control signal to cause said inverter circuit to operate at a frequency to illuminate said lamp at said target intensity level; and a non-linear amplifier circuit for amplifying said second signal such that for first values of lamp current below a current threshold, said second signal is amplified by a first amount and for second values of lamp current above said current threshold, said second signal is amplified by a second, smaller amount, thereby increasing responsiveness of said error amplifier circuit to changes in lamp intensity levels at low lamp intensity levels; wherein said bus-voltage-generating circuit produces said DC bus voltage to define an envelope for said sinusoidal lamp current, and said error amplifier circuit operates in a frequency range such that said error amplifier circuit is responsive to said second signal at the frequency of said second signal thereby to adjust the frequency of operation of said inverter circuit to reduce variation in said envelope of said sinusoidal lamp current.