Anti-striation circuit for current-fed ballast

An electronic ballast for providing an asymmetric time-varying electrical output signal to drive at least one lamp, the electronic ballast comprising:

• an input circuit with first and second input terminals receiving an input from a power source, the input circuit including a first capacitor coupled between the first input terminal and an intermediate node, a second capacitor having a first terminal coupled to the intermediate node and a second terminal, a first inductor coupled between the first input terminal and a first bus node, and a second inductor coupled between the second input terminal and a second bus node;
• a switching circuit operatively coupled between the bus nodes and an output terminal to selectively couple alternate ones of the bus nodes with the output node to create a time-varying output signal at the output node, the switching circuit including:
• a first switching device with a first switch terminal coupled with the first bus node, a second switch terminal coupled with the output terminal, and a control terminal actuated to render the first switching device conductive in one half-cycle of the switching circuit operation, and
• a second switching device with a first switch terminal coupled with the output terminal, a second switch terminal coupled with the second bus node, and a control terminal actuated to render the first switching device conductive in another half-cycle of the switching circuit operation;
• a resonant circuit including:
• a transformer primary winding coupled between the output terminal and the second terminal of the second capacitor, and
• an anti-striation circuit separate from the control terminals of the switching devices and coupled between the output terminal and at least one of the first bus terminal, the second bus terminal, and the second terminal of the second capacitor, the anti-striation circuit including at least one anti-striation component active to change a resonant frequency of the resonant circuit in a first portion of a resonant cycle and inactive in a second portion of the resonant cycle to cause the switching circuit to generate an asymmetric time-varying output signal at the output node;
• and
• an output circuit for outputting the asymmetric output signal to a lamp system.

An electronic ballast for providing an asymmetric time-varying electrical output signal to drive at least one lamp, the electronic ballast comprising:

• first and second DC power sources coupled in series, the first and second power sources having unequal voltages;
• an input circuit with first and second input terminals receiving an input from the first and second series connected power sources, the input circuit including a first inductor coupled between the first input terminal and a first bus node, and a second inductor coupled between the second input terminal and a second bus node;
• a switching circuit operatively coupled between the bus nodes and an output terminal to selectively couple alternate ones of the bus nodes with the output node to create a time-varying output signal at the output node, the switching circuit including:
• a first switching device with a first switch terminal coupled with the first bus node, a second switch terminal coupled with the output terminal, and a control terminal actuated to render the first switching device conductive in one half-cycle of the switching circuit operation, and
• a second switching device with a first switch terminal coupled with the output terminal, a second switch terminal coupled with the second bus node, and
• a control terminal actuated to render the first switching device conductive in another half-cycle of the switching circuit operation;
• a resonant circuit including:
• a transformer primary winding coupled between the output terminal and an intermediate node between the first and second power sources, and
• a resonant capacitor coupled in parallel with the transformer primary winding between the output terminal and the intermediate node;
• and
• an output circuit for outputting the asymmetric output signal to a lamp system.

A method of providing an asymmetric time-varying electrical output signal to drive at least one lamp, the method comprising:

• inputting power to an input section of the electronic ballast;
• generating a resonant signal;
• creating a time-varying output signal at an output node by selectively actuating control terminals of first and second switching devices of a switching circuit in individual half-cycles of the switching circuit operation;
• altering the resonant signal using at least one anti-striation component active to change a resonant frequency of the resonant signal in a first portion of a resonant cycle and inactive in a second portion of the resonant cycle to cause the switching circuit to generate an asymmetric time-varying output signal at the output node;
• and
• outputting the asymmetric output signal to a lamp system to generate an asymmetric current lamp signal.