Lamp end of life (EOL) detection circuit

A method of detecting an end of life (EOL) condition for one or more lamps connected to a ballast and energized by the ballast, the ballast having a transformer, the method comprising:

• detecting a voltage signal across a primary winding of the transformer;
• determining whether the voltage signal includes an even harmonic having a magnitude that exceeds a threshold value;
• and
• shutting down an inverter circuit of the ballast when the voltage signal is determined to include an even harmonic having a magnitude that exceeds the threshold value.

A lamp system comprising:

• an electromagnetic interference filter configured to receive alternating current (AC) voltage from a power source;
• a rectifier connected to the electromagnetic interference filter to convert the alternating current (AC) voltage to direct current (DC) voltage;
• a power factor control circuit connected to the rectifier to produce a DC voltage output;
• a DC voltage bus connected to the power factor control circuit to receive the DC voltage output from the power factor control circuit;
• an inverter circuit to selectively energize one or more lamps, the inverter circuit having a transformer to provide voltage to the one or more lamps, the transformer having a primary winding connected to a direct current (DC) voltage bus and a secondary winding to connect to one or more lamps;
• a filter connected to the primary winding to receive a primary winding signal representative of the voltage across the primary winding, wherein the primary winding signal has a frequency spectrum and the filter detects a particular characteristic of the frequency spectrum of the primary winding signal, and wherein the particular characteristic of the frequency spectrum is indicative of an end of life (EOL) condition of the one or more lamps;
• and
• a control circuit connected to the inverter circuit and to the filter, wherein the control circuit is configured to shut off the inverter circuit when the particular characteristic of the frequency spectrum of the primary winding signal is detected by the filter.