The present invention relates to End of Life detection and safety circuits and in particular to temperature independent End of Lamp Life safety circuits.
Low-pressure discharge lamps are driven by electronic ballasts that deliver preheating voltage for the lamp filaments, ignition voltage to ignite the lamps, and the lamp drive current in the steady state. Further, the electronic ballasts use a Power Factor Correction Stage to control the harmonic content of the input current. Moreover, the standards for electronic ballast manufacturing require inclusion of safety circuits for the detection of lamp End of Life (EOL).
At EOL the lamps develop a DC-Offset in the lamp-voltage. This offset is sensed by the resistors RL1, RL2, and the capacitor CAV. The circuit 10 detects EOL if voltage at node V1 increases or decreases to a level that Zener diodes DEOL2 or DEOL1 start conducting and the voltage on the input of the window comparator goes above 3V or below 1V. When the voltage goes outside this window, EOL NOT goes low, signifying end of life. The drawback of circuit 10 is that two Zener diodes are required for EOL detection. The Zener diodes have temperature dependency and a low accuracy at low currents.
It is an object of the present invention to provide an EOL circuit that is more accurate than the prior art circuit.
It is an object of the present invention to provide an EOL circuit that has little temperature dependency.
Provided is a circuit in an electronic ballast driving at least one low-pressure discharge lamp for providing safety during the End of Life (EOL) of the lamp. The circuit includes a comparator for comparing an input voltage to first and second threshold voltages and providing an EOL signal; a sensing circuit for sensing a DC offset in the lamp-voltage during the EOL of the lamp and a reference voltage setting circuit responsive to the DC offset including a reference diode for setting an adjustable reference voltage as said input voltage to the comparator.
Other features and advantages of the present invention will become apparent from the following description of the invention that refers to the accompanying drawings.
As illustrated in
During normal lamp behavior, when the lamp has no DC-Offset (
From the time t1 the lamp begins to exhibit EOL behavior. The rectification behavior of the lamp-voltage gives a positive offset on the lamp voltage at V1 and the reference voltage VDREF decreases (in response to the adjustment input terminal of DREF) below the voltage threshold TH2, indicating EOL-detection. EOL (inverse) goes low. Alternatively, if the rectification behavior of the lamp gives a negative offset on the lamp-voltage the reference voltage VDREF will increase above the voltage threshold TH1 (and EOL inverse also goes low).
An advantage of the circuit is that it is very accurate and it has nearly no temperature dependency.
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention not be limited by the specific disclosure herein.