Measurement circuit for an electronic ballast

A lamp current measurement circuit for an electronic ballast for driving a gas discharge lamp having first and second electrodes, such that a lamp current flows from the first electrode to the second electrode the lamp and a lamp voltage is produced across the lamp, the ballast comprising an inverter circuit operable to convert a substantially DC bus voltage to a high-frequency AC voltage, and a resonant tank circuit having an output and operable to couple the high-frequency AC voltage to the lamp, the lamp current measurement circuit comprising:

• a current transformer having first and second primary windings, and first and second secondary windings magnetically coupled to the first and second primary windings, the first primary winding adapted to be coupled in series electrical connection between the output of the resonant tank circuit and the first electrode of the lamp, the second primary winding adapted to be coupled in series electrical connection between the output of the resonant tank circuit and the second electrode of the lamp, the first and second primary windings coupled such that differential-mode currents in the electrodes are added and common-mode currents in the electrodes are subtracted, the first and second secondary windings operable to conduct respective first and second currents representative of the lamp current;
• and
• a capacitor coupled to conduct the first and second currents of the first and second secondary windings of the current transformer, such that the first current flows into the capacitor when the lamp current is positive, and the second current flows out of the capacitor when the lamp current is negative, the capacitor only conducting the first and second currents of the first and second secondary windings during every other half-cycle of the lamp voltage;
• wherein a voltage produced across the capacitor is representative of the magnitude of the lamp current that is in-phase with the lamp voltage.

A method of measuring a lamp current in an electronic ballast for driving a gas discharge lamp having first and second electrodes, the lamp current flowing through the lamp, the method comprising the steps of:

• measuring a first current in the first electrode of the lamp;
• measuring a second current in the second electrode of the lamp;
• adding the first and second currents in such a way that differential-mode currents in the electrodes are added and common-mode currents in the electrodes are subtracted;
• and
• determining the lamp current in response to the step of adding the first and second currents, the lamp current not dependent upon the common-mode currents.

An electronic ballast for driving a gas discharge lamp, said ballast comprising:

• a bus capacitor connected across a DC bus voltage;
• an inverter circuit for receiving said DC bus voltage and for generating a substantially square-wave voltage having a magnitude approximately twice said DC bus voltage, said inverter circuit comprising a transformer having a primary winding comprising first and second winding portions connected at a center tap and having first and second terminals, said bus capacitor being connected between a common point and said center tap, said inverter circuit further comprising first and second switches coupled between said common point and said respective first and second terminals of said primary winding, and a control circuit for controlling the conduction state of said first and second switches, said control circuit providing first and second control signals to control inputs of said first and second switches, respectively, whereby said first and second switches are alternately rendered conductive to generate said substantially square wave voltage across said primary winding;
• a resonant tank circuit for receiving said square-wave voltage and generating a substantially sinusoidal voltage for driving said lamp;
• and
• a measurement circuit for determining the magnitude of the current flowing from said inverter circuit to said lamp;
• wherein said measurement circuit senses said lamp current during alternate half-cycles of said lamp current, thereby measuring only a real component of said lamp current and ignoring a reactive component.

An electronic ballast for driving a gas discharge lamp, the ballast comprising:

• an inverter circuit operable to convert a substantially DC bus voltage to a high-frequency AC voltage provided at an output of the inverter circuit, the inverter circuit including a main transformer having a primary winding coupled across the output of the inverter circuit;
• a resonant tank circuit operable to couple the high-frequency AC voltage to the lamp, the resonant tank circuit including a resonant inductor adapted to be coupled between the output of the inverter circuit and the lamp;
• a first winding magnetically coupled to the primary winding of the main transformer of the inverter circuit, the first winding operable to generate a first voltage representative of the magnitude of the high-frequency AC voltage at the output of the inverter circuit;
• and
• a second winding magnetically coupled to the resonant inductor of the resonant tank circuit, the second winding operable to generate a second voltage representative of the magnitude of a voltage across the resonant inductor;
• wherein the first and second windings are coupled in series to generate a third voltage representative of the lamp voltage measured across the lamp.

A method of measuring a lamp voltage in an electronic ballast for driving a gas discharge lamp having first and second electrodes, the lamp voltage measured across the first and second electrodes of the lamp, the ballast comprising an inverter circuit operable to convert a substantially DC bus voltage to a high-frequency AC voltage provided across a primary winding of a transformer, and a resonant tank circuit operable to couple the high-frequency AC voltage to the lamp and including a resonant inductor adapted to be coupled between the primary winding of the transformer of the inverter circuit and the lamp, the method comprising the steps of:

• magnetically coupling a first winding to the primary winding of the transformer of the inverter circuit;
• magnetically coupling a second winding to the resonant inductor of the resonant tank circuit;
• generating a first voltage across the first winding, the first voltage representative of the magnitude of the high-frequency AC voltage of the inverter circuit;
• generating a second voltage across the second winding, the second voltage representative of the magnitude of a voltage across the resonant inductor;
• and
• electrically connecting the first and second windings so as to add the first and second voltages to produce a third voltage representative of a lamp voltage measured across the lamp.

An electronic ballast for driving a gas discharge lamp, said ballast comprising:

• a bus capacitor connected across a DC bus voltage;
• an inverter circuit for receiving said DC bus voltage and for generating a substantially square-wave voltage having a magnitude approximately twice said DC bus voltage, said inverter circuit comprising a transformer having a primary winding comprising first and second winding portions connected at a center tap and having first and second terminals, said bus capacitor being connected between a common point and said center tap, said inverter circuit further comprising first and second switches coupled between said common point and said respective first and second terminals of said primary winding, and a control circuit for controlling the conduction state of said first and second switches, said control circuit providing first and second control signals to control the conduction of said first and second switches, respectively, whereby said first and second switches are alternately rendered conductive to generate said substantially square wave voltage across said primary winding;
• a resonant tank circuit for receiving said square-wave voltage and generating a substantially sinusoidal voltage for driving said lamp;
• and
• a measurement circuit for determining the magnitude of a lamp voltage across said lamp, said measurement circuit determining the magnitude of said lamp voltage by sensing said square-wave voltage and a voltage across said resonant inductor, and determining the magnitude of said lamp voltage as a difference between the magnitude of said square-wave voltage and the magnitude of said voltage across said resonant inductor.