Electronic ballast
View Patent ↗An electronic ballast for driving a gas discharge lamp includes a rectifier, a valley-fill circuit, an inverter having first and second series-connected controllably conductive switches having complementary duty cycles, a control circuit for controlling the controllably conductive switches, and an independent cat ear power supply to provide power to the ballast control circuits. The result is a ballast having substantially improved power factor, THD, and current crest factor. In a preferred embodiment, the valley-fill circuit includes an energy storage device that stores energy in response to a controllably conductive switch. In an especially preferred embodiment, the controllably conductive switch of the valley-fill circuit is also one of the switches of the inverter.
1. An electronic ballast for driving at least one gas discharge lamp from a source of AC power which has a substantially sinusoidal line voltage at a given line frequency, comprising:
a rectifying circuit having AC input terminals and DC output terminals; said AC input terminals connectable to the source of AC power; said rectifying circuit producing a rectified output voltage at its said DC output terminals when said AC input terminals are energized by said source of AC power;
an inverter circuit having input terminals connected to said output terminals of said rectifying circuit and producing a high frequency drive voltage for driving a lamp current through said at least one gas discharge lamp when said AC input terminals are energized by said source of AC power;
and a cat ear circuit connected to said source of AC power, said cat ear circuit being adapted to conduct current for a first relatively short time following a first zero crossing of said line voltage and for a second relatively short time prior to the next zero crossing of said line voltage thereby to reduce the total harmonic distortion of the current drawn from said source of AC power below that which would occur in the absence of said cat ear circuit.
2. The electronic ballast of claim 1 , wherein said cat ear circuit further comprises a cat ear power supply.
3. The electronic ballast of claim 1 , wherein said cat ear circuit draws current from said source of AC power only when the instantaneous value of said line voltage is less than a predetermined absolute value.
4. The electronic ballast of claim 1 , wherein said cat ear circuit draws current from said source of AC power only when the current drawn by said inverter circuit from said source of AC power is substantially zero.
5. The electronic ballast of claim 1 , wherein said cat ear circuit draws current from said source of AC power at least when the current drawn by said inverter circuit from said source of AC power is substantially zero.
6. The electronic ballast of claim 2 , wherein said electronic ballast includes an auxiliary circuit coupled thereto which has an auxiliary circuit power supply input terminal; said cat ear circuit coupled to and driving said auxiliary circuit power supply input terminal.