Electronic ballast having a symmetric topology

An electronic ballast for driving a gas discharge lamp having first and second electrodes, the ballast comprising:

• an inverter circuit having an input for receiving a substantially DC bus voltage, the inverter circuit operable to convert the bus voltage to a high-frequency AC voltage;
• and
• a split resonant inductor having first and second windings magnetically coupled together, the first winding adapted to be electrically coupled between the inverter circuit and the first electrode of the lamp, the second winding adapted to be electrically coupled between the inverter circuit and the second electrode of the lamp, the first and second windings adapted to couple the high-frequency AC voltage of the inverter circuit to the electrodes of the lamp;
• wherein the improvement comprises first and second capacitors coupled in series electrical connection between the electrodes of the lamp, the junction of the first and second capacitors coupled to the DC bus voltage at the input of the inverter circuit.

An electronic ballast for driving a gas discharge lamp having first and second electrodes, the ballast comprising:

• a rectifier circuit for receiving a phase-controlled AC voltage and to generate a rectified voltage;
• a charge pump circuit coupled to the rectifier circuit for receiving the rectified voltage, the charge pump circuit comprising two series-connected diodes;
• a push-pull converter having an input coupled to the charge pump circuit for receiving a substantially DC bus voltage, the push-pull converter operable to generate a high-frequency AC voltage and to provide the high-frequency AC voltage at an output, the push-pull converter further comprising a bus capacitor coupled across the input and a main transformer having a primary winding coupled across the output, the primary winding having a center tap coupled to the DC bus voltage, the push-pull converter further comprising first and second semiconductor switches electrically coupled to the primary winding of the main transformer for conducting an inverter current through the primary winding on an alternate basis;
• and
• a split resonant inductor having first and second windings magnetically coupled together, the first winding adapted to be electrically coupled between the output of the push-pull converter and the first electrode of the lamp, the second winding adapted to be electrically coupled between the output of the push-pull converter and the second electrode of the lamp, the first and second windings adapted to couple the high-frequency AC voltage of the inverter circuit to the electrodes of the lamp;
• wherein the charge pump circuit further comprises a capacitor and an inductor coupled in series between the junction of the two series-connected diodes and the output of the push-pull converter.

A circuit for driving a gas discharge lamp from an AC power source, said circuit comprising:

• a dimmer switch adapted to be connected to said AC source and producing a phase-controlled voltage;
• and
• an electronic dimming ballast connected to a dimmer output of said dimmer switch and having a ballast output adapted to be connected to said gas discharge lamp, said ballast comprising:
• a rectifier circuit for producing a rectified voltage having a magnitude related to said phase-controlled output voltage;
• an inverter circuit connected to said rectified voltage and producing a square wave output voltage having a period related to said rectified voltage;
• and
• a resonant tank circuit comprising an inductor assemblage and a capacitor assemblage connected in parallel with said inductor assemblage for converting said square wave input voltage to a generally sinusoidal output voltage which is coupled across said lamp, said inductor assemblage comprising first and second inductor windings, which are magnetically coupled together, said capacitor assemblage comprising first and second capacitors connected in series at a common node, said common node connected to said rectified voltage, said first and second inductor windings having first terminals connected in series with said square wave voltage and second terminals connected to said first and second capacitors, respectively.

A resonant tank circuit for an electronic ballast for a gas discharge lamp comprising:

• an inductor assemblage comprising first and second inductor windings magnetically coupled by a common magnetic core;
• and
• a parallel-connected capacitor assemblage comprising first and second series-connected capacitors having first terminals connected at a common node and second terminals, respectively;
• wherein first terminals of said first and second windings of said inductor define input terminals of said resonant tank circuit, and second terminals of said first and second windings define output terminals of said resonant tank circuit, said second terminals of said first and second windings connected to said second terminals of said first and second capacitors.