Restart circuit for multiple lamp electronic ballast

A ballast to power a first lamp and a second lamp, said first lamp having a first filament and a second filament, and said second lamp having a first filament and a second filament, said ballast comprising:

• a rectifier configured to receive alternating current power from a power supply and to provide a direct current (DC) voltage to a DC voltage bus;
• an inverter configured to receive power from the DC voltage bus and to provide AC power during execution of a switching operation;
• a resonant circuit configured to receive the AC power provided by the inverter, said resonant circuit comprising:
• a first output pair for connecting across the first filament of the first lamp,
• a second output pair for connecting across the second filament of the first lamp and the first filament of the second lamp, wherein the second filament of the first lamp is in series with the first filament of the second lamp, and
• a third output pair for connecting across the second filament of the second lamp;
• a controller configured to monitor a first current and a second current, wherein the first current is a DC current through at least one of the filaments of the lamps and the second current is a reference current, said controller further configured to determine a ratio of the first current to the second current, and to control the switching operation of the inverter as a function of the determined ratio, wherein:
• the controller inhibits the switching operation of the inverter when the determined ratio of the first current to the second current is less than or equal to a predetermined ratio,
• the controller drives the switching operation of the inverter when the determined ratio is more than the predetermined ratio, and
• the controller restarts the ballast in response to the ratio transitioning from below the predetermined ratio to above the predetermined ratio;
• and
• a dv/dt circuit configured to reduce the first current for a transient time period in response to a rapid voltage change at the second output pair;
• wherein the connection of at least one lamp filament to the resonant circuit causes the dv/dt circuit to reduce the first current for the transient time period, resulting in the determined ratio falling below the predetermined ratio for the transient time period.

A method of restarting a ballast that is powering a first lamp and a second lamp, in response to replacement of either lamp, each of said lamps having a first filament and a second filament, said method comprising:

• monitoring a first current and a second current, wherein the first current is a direct current (DC) current through at least one of the filaments of the lamps and the second current is a reference current;
• determining a ratio of the first current to the second current;
• controlling a switching operation of an inverter of the ballast via a controller of the ballast as a function of the determined current ratio, said controlling comprising:
• inhibiting the switching operation of the inverter when the determined ratio is less than or equal to a predetermined ratio;
• and
• driving the switching operation of the inverter when the determined ratio is higher than the predetermined ratio;
• reducing the first current for a transient time period in response to a rapid voltage change at the second filament of the first lamp, wherein the determined ratio falls below the predetermined ratio for the transient time period due to the reduction of the first current;
• and
• restarting the ballast in response to the determined ratio falling below the predetermined ratio and subsequently increasing to at least the predetermined ratio.