Electronic ballast having a partially self-oscillating inverter circuit

A multi-switch power converter for an electronic ballast, the power converter comprising:

• a main transformer having a primary winding for producing an oscillating output voltage;
• first and second semiconductor switches electrically coupled to the primary winding of the main transformer for conducting current through the primary winding on an alternate basis;
• and
• first and second drive circuits for controlling the first and second semiconductor switches, respectively, on a cycle-by-cycle basis in response to first control signals derived from the main transformer and second control signals received from an external control circuit.

An electronic ballast for driving a gas discharge lamp comprising:

• a bus capacitor for producing a substantially DC bus voltage;
• an inverter circuit for converting the DC bus voltage to a high-frequency AC voltage for driving the lamp, the inverter circuit comprising a main transformer having a primary winding for producing the high-frequency AC voltage, first and second semiconductor switches electrically coupled to the primary winding of the main transformer for conducting current through the primary winding on an alternate basis, and first and second drive circuits for controlling the first and second semiconductor switches, respectively, on a cycle-by-cycle basis;
• and
• a control circuit coupled to the first and second drive circuits of the inverter circuit for controlling the first and second semiconductor switches;
• wherein the first and second drive circuits control the respective first and second semiconductor switches in response to first control signals derived from the main transformer and second control signals received from the control circuit.

A method of driving a gas discharge lamp from an electronic ballast having an inverter circuit and a control circuit, the inverter circuit comprising a main transformer having a primary winding coupled across an output of the inverter circuit, first and second semiconductor switches electrically coupled to the primary winding of the main transformer, and first and second drive circuits coupled to the first and second semiconductor switches, respectively, the method comprising the steps of:

• producing a high-frequency AC voltage across the primary winding of the main transformer;
• deriving first control signals from the main transformer;
• receiving second control signals from the control circuit;
• and
• controlling the first and second semiconductor switches on a cycle-by-cycle basis to conduct current through the primary winding on an alternate basis in response to the first and second control signals.

An inverter circuit comprising:

• a bus capacitor connected across a DC bus voltage;
• 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 said center tap and a common point;
• first and second controlled switches, said first switch being coupled between said common point and said first terminal of said primary winding, said second switch being coupled between said common point and said second terminal of said primary winding;
• and
• a control circuit for controlling the conduction state of said first and second switches, such that a current flows from said bus capacitor alternately through said first and second winding portions thereby generating a substantially square-wave voltage having a magnitude approximately twice said DC bus voltage across said primary winding, said control circuit having first and second drive circuits, one for each switch, coupled to control inputs of said first and second switches, respectively, said first and second drive circuits receiving respective first and second control signals;
• wherein said transformer has first and second magnetically-coupled drive windings, one for each switch, said first and second drive windings conducting current into said first and second drive circuits, respectively, to alternately turn on said first and second switches, said first and second control signals additionally rendering said first and second switches non conductive prior to said currents from said first and second drive windings rendering said first and second switches, respectively, conductive.

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;
• and
• a resonant tank circuit for receiving said square-wave voltage and generating a sinusoidal voltage for driving said lamp;
• wherein said inverter circuit comprises:
• 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;
• 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, such that a current flows from said bus capacitor alternately through said first and second winding portions thereby generating said substantially square-wave voltage across said primary winding, said control circuit having first and second drive circuits, one for each switch, coupled to control inputs of said first and second switches, respectively, said first and second drive circuits receiving respective first and second control signals;
• wherein said transformer has first and second magnetically coupled drive windings, one for each switch, said first and second drive windings conducting current into said respective first and second drive circuits to alternately turn on said first and second switches, said first and second control signals additionally rendering said first and second switches non conductive prior to the currents from said first and second drive windings rendering said first and second switches, respectively, conductive.

An electronic ballast for driving a gas discharge lamp, such that a lamp current flows through the lamp, the ballast comprising:

• a bus capacitor for producing a substantially DC bus voltage;
• an inverter circuit for converting the DC bus voltage to a high-frequency AC voltage for driving the lamp, the inverter circuit comprising a main transformer having a primary winding for producing the high-frequency AC voltage, first and second semiconductor switches electrically coupled to the primary winding of the main transformer for conducting current through the primary winding on an alternate basis, and first and second drive circuits controlling the first and second semiconductor switches, respectively, on a cycle-by-cycle basis;
• a lamp current measurement circuit operable to generate a lamp current control signal representative of the magnitude of the lamp current;
• and
• a control circuit operable to receive the lamp current control signal, the control circuit coupled to the first and second drive circuits of the inverter circuit for controlling the first and second semiconductor switches in response to the magnitude of the lamp current;
• wherein the first and second drive circuits control the first and second semiconductor switches, respectively, in response to a first control signal derived from the main transformer and a second control signal received from the control circuit.

A method of controlling a switching power converter for an electronic ballast, the power converter having an energy storage capacitor and at least one semiconductor switch for conducting a converter current, the method comprising the steps of:

• scaling the converter current to produce a scaled current;
• integrating the scaled current to generate an integral control signal representative of the scaled current;
• comparing the integral control signal to a threshold voltage;
• and
• rendering the semiconductor switch non-conductive in response to the integral control signal reaching the threshold voltage.

An electronic ballast for driving a gas discharge lamp comprising:

• a bus capacitor for producing a substantially DC bus voltage;
• an inverter circuit for converting the DC bus voltage to a high-frequency AC voltage for driving the lamp, the inverter circuit comprising a semiconductor switch adapted to conduct a converter current;
• and
• a control circuit operable to scale the converter current to produce a scaled current, integrate the scaled current to generate an integral control signal representative of the scaled current, compare the integral control signal to a threshold voltage, and render the semiconductor switch non-conductive in response to the integral control signal reaching the threshold voltage.

An inverter circuit for an electronic ballast, the inverter circuit comprising:

• a bus capacitor connected across a DC bus voltage;
• 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 said center tap and a common point;
• first and second controlled switches, said first switch being coupled between said common point and said first terminal of said primary winding, said second switch being coupled between said common point and said second terminal 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 respective switches, whereby said first and second switches are alternately rendered conductive to generate a substantially square wave voltage having a magnitude approximately twice said DC bus voltage across said primary winding;
• wherein said control circuit scales the current drawn through said first and second switches to produce a scaled current signal, integrates said scaled current signal to produce an integrated signal, and renders said switches non conductive in response to said integrated signal reaching a threshold voltage.

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;
• and
• a resonant tank circuit for receiving said square-wave voltage and generating a sinusoidal voltage for driving said lamp;
• wherein said inverter circuit comprises:
• 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 said center tap and a common point;
• 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 respective switches, whereby said first and second switches are alternately rendered conductive to generate a substantially square wave voltage having a magnitude approximately twice said DC bus voltage across said primary winding;
• wherein said control circuit scales the current drawn through said first and second switches to produce a scaled current signal, integrates said scaled current signal to produce an integrated signal, and renders said switches non conductive in response to said integrated signal reaching a threshold voltage.