Multiple lamp ballast control circuit

A ballast control circuit for a plurality of parallel connected lamps, the circuit comprising:

• a control circuit for driving two series connected switches of a lamp ballast connected across a supply potential and having a switched node between the switches, the switched node adapted to be connected to the plurality of parallel connected lamps, the control circuit comprising an oscillator;
• an output circuit comprising the plurality of parallel connected lamps including inductive and capacitive components and having a resonance frequency that is dependent on the number of lamps in the output circuit, an output voltage being developed across the output circuit;
• a circuit for reducing hard switching when a lamp is removed from the output circuit;
• and
• a feedback circuit comprising a voltage sensing circuit coupled across the output circuit and an oscillator control circuit receiving an output of the voltage sensing circuit for generating the output voltage to increase the oscillator frequency when the output voltage increases above a threshold thereby to maintain a substantially constant voltage across said output circuit, wherein the feedback circuit decreases the oscillator frequency to increase the output voltage each time a lamp ignites, and the output voltage decreases, thereby increasing the output voltage until the next lamp ignites.

A ballast control circuit for multiple lamps comprising:

• a control circuit for driving two series connected switches of a lamp ballast connected across a supply potential and having a switched node between the switches;
• the switched node adapted to be connected to an output circuit comprising a plurality of parallel connected lamps;
• the control circuit comprising an oscillator,
• the output circuit comprising the plurality of parallel connected lamps including inductive and capacitive components and having a resonance frequency that is dependent on the number of lamps in the output circuit;
• an output voltage being developed across the output circuit;
• further comprising:
• a circuit for reducing hard switching when a lamp is removed from the output circuit, said circuit for reducing hard switching comprising a circuit for sensing when non-zero voltage switching of said switches occurs, said sensing circuit monitoring a potential on said switched node when one of said switches comprising a low side switch is turned on, said sensing circuit coupled to said oscillator and operating to increase the frequency of said oscillator above the resonance frequency when non-zero voltage switching occurs thereby to achieve zero voltage switching.

A ballast control integrated circuit for driving two series connected switches of a lamp ballast connected across a supply potential and having a switched node between the switches;

• the switched node adapted to be connected to each of a plurality of parallel connected lamps, the control integrated circuit comprising:
• an oscillator;
• an output circuit comprising the plurality of parallel connected lamps including inductive and capacitive components and having a resonance frequency that is dependent on the number of lamps in the output circuit, a lamp output voltage being developed across the output circuit;
• and
• a feedback circuit comprising a circuit monitoring the output voltage for driving the lamp output voltage to a substantially constant voltage, the feedback circuit comprising an oscillator control circuit generating the output voltage to increase the oscillator frequency when the output voltage increases above a threshold thereby to maintain a substantially constant voltage across said output circuit,
• wherein the feedback circuit decreases the oscillator frequency to increase the output voltage each time a lamp ignites, and the output voltage decreases, thereby increasing the output voltage until the next lamp ignites.

A ballast control integrated circuit for driving two series connected switches of a lamp ballast connected across a supply potential and having a switched node between the switches;

• the switched node adapted to be connected to an output circuit comprising a plurality of parallel connected lamps;
• the control integrated circuit comprising:
• an oscillator,
• the output circuit comprising the plurality of parallel connected lamps including inductive and capacitive components and having a resonance frequency that is dependent on the number of lamps in the output circuit;
• an output voltage being developed across the output circuit;
• and
• a circuit for reducing hard switching when a lamp is removed from the output circuit, said circuit for reducing hard switching comprising a circuit for sensing when non-zero voltage switching of said switches occurs, said sensing circuit monitoring a potential on said switched node when one of said switches comprising a low side switch is turned on, said sensing circuit coupled to said oscillator and operating to increase the frequency of said oscillator above the resonance frequency when non-zero voltage switching occurs thereby to achieve zero voltage switching.

A ballast control circuit for a plurality of parallel connected lamps, the circuit comprising:

• a control circuit for driving two series connected switches of a lamp ballast connected across a supply potential and having a switched node between the switches, the switched node adapted to be connected to each of the plurality of parallel connected lamps, the control circuit comprising an oscillator
• an output circuit comprising the plurality of parallel connected lamps including inductive and capacitive components and having a resonance frequency that is dependent on the number of lamps in the output circuit, a resonant output voltage being developed across the output circuit;
• a feedback circuit comprising a circuit monitoring the resonant output voltage for driving the lamp output voltage to a substantially constant voltage, the feedback circuit converting the resonant output voltage to an AC voltage;
• and
• a circuit for maintaining a substantially constant current to each lamp including when a lamp is removed, said circuit providing a DC voltage proportional to lamp current, and further wherein said DC voltage proportional to lamp current and the AC voltage from said feedback circuit are superimposed to provide a single feedback signal for controlling the frequency of said oscillator, whereby when the lamp current increases, said output of the feedback circuit decreases, thereby reducing the output voltage and reducing the current in each lamp to maintain each lamp at a substantially constant current.