Over-voltage protection and automatic re-strike circuit for an electronic ballast

1. An electronic ballast for a gas discharge lamp, comprising:

an inverter;

a shut-down circuit coupled to the inverter;

a safety circuit coupled to the inverter and to the shut-down circuit so that when a safety threshold is exceeded, the safety circuit is operable to instruct the shut-down circuit to disable the inverter until the ballast is reset;

a monitoring circuit operably coupled to the inverter, the safety circuit, and the shut-down circuit so that during normal inverter operating conditions the monitoring circuit is operable to prevent the safety circuit from disabling the inverter;

an overvoltage protection circuit coupled to the monitoring circuit, the inverter, and the shut-down circuit;

the overvoltage protection circuit is responsive to an overvoltage condition outside of the normal inverter operating conditions by causing the shut-down circuit to disable the inverter and the monitoring circuit until the overvoltage condition ends, and if the overvoltage condition cannot be corrected after a predetermined sequence, the safety circuit is operative to cause the shut-down circuit to disable the inverter until the ballast is reset; and

wherein the inverter is operable to attempt to re-strike the lamp following disablement of the inverter by the overvoltage protection circuit if the safety threshold has not been exceeded.

2. The ballast of claim 1 wherein the safety circuit comprises a capacitor having an associated charge level, and wherein when the charge level exceeds the safety threshold the safety circuit is operable to cause the shut-down circuit to disable the inverter.

3. The ballast of claim of 1 wherein the overvoltage protection circuit comprises a sensor and the inverter comprises a resonant circuit, and wherein the sensor is coupled to the resonant circuit so that the sensor can detect overvoltage conditions in the inverter.

4. The ballast of claim 1 further comprising:

a disabling node;

the inverter comprises an inverter power supply node having an operating supply potential;

the shut-down circuit comprises a first switch having first and second switch terminals;

the first switch terminal is coupled to the power supply node and second switch terminal is coupled to the disabling node; and

the disabling node has a potential lower than the supply node so that when the first switch is activated, the operating supply potential is pulled down to the disabling node potential and the inverter is disabled.

5. The ballast of claim 4 wherein the ballast includes an electrical ground and the safety circuit comprises a safety capacitor having a pair of terminals, with one of the safety capacitor terminals operably coupled to the electrical ground, and the first switch further comprises a control terminal coupled to the other of the safety capacitor terminals.

6. The ballast of claim 5 wherein the overvoltage protection circuit comprises a protecting capacitor having a first and second safety capacitor terminal, wherein the first safety capacitor terminal is operably connected to the control terminal and the second safety capacitor terminal is operably connected to the electrical ground.

7. The ballast of claim 6 wherein the monitoring circuit comprises a second switch having a monitoring terminal coupled to one of the safety capacitor terminals and to an enabling terminal coupled to the inverter power supply node.

8. An electronic ballast for a gas discharge lamp, comprising:

an inverter having a power supply node;

a shut-down circuit operatively coupled to the power supply node;

a safety circuit operably coupled to the inverter and to the shut-down circuit, wherein the safety circuit is responsive to overvoltage conditions in the inverter;

a monitoring circuit coupled to the inverter and to the safety circuit so that during normal operating conditions the monitoring circuit prevents the safety circuit from operating; and

an overvoltage protection circuit operably connected to the inverter, the shut-down circuit, and the monitoring circuit;

the overvoltage protection circuit is functional to detect overvoltage conditions beyond the normal operating conditions and, during the overvoltage conditions, to cause the shut-down circuit to disable the inverter and the monitoring circuit; and

if the overvoltage conditions are not corrected after a predetermined amount of time, the safety circuit is functional to instruct the shut-down circuit to disable the inverter until power to the ballast is cycled.

9. The ballast of claim 8 wherein:

the shut-down circuit comprises a shut-down switch having a first pair of terminals;

the monitoring circuit comprises a monitoring circuit switch having a second pair of terminals;

one of the first pair of terminals is coupled to one of the second pair of terminals, the other of the first pair of terminals is coupled to the safety circuit so that the safety circuit can activate the shut-down switch; and

the other of the second pair of terminals is coupled to the safety circuit so that during normal operating conditions the monitoring circuit switch is functional to prevent the safety circuit from activating the shut-down switch.

10. The ballast of claim 9 wherein the safety circuit comprises a safety capacitor coupled to the other of the first pair of terminals.

11. The ballast of claim 10 wherein the safety capacitor is coupled to the power supply node of the inverter and has a charging time, and wherein when the charging time exceeds the predetermined amount of time, the safety capacitor will activate the shut-down switch, and further wherein until the safety capacitor activates the shut-down switch, the inverter will attempt to re-strike the lamp following the overvoltage condition.

12. The ballast of claim 9 wherein the ballast comprises a power supply for the inverter and the one of the first pair and the one of the second pair of terminals are coupled to the power supply.

13. The ballast of claim 9 wherein the overvoltage protection circuit comprises an overvoltage detector inductively coupled to the inverter so that the overvoltage detector can sense overvoltage conditions.

14. The ballast of claim 13 wherein the overvoltage protection circuit further comprises an overvoltage capacitor responsive to overvoltage conditions detected by the overvoltage detector and the overvoltage capacitor is coupled to the shut-down switch so that the overvoltage capacitor can activate the shut-down switch during overvoltage conditions.

15. An electronic ballast for a gas discharge lamp, the ballast having an inverter, comprising:

a safety circuit coupled to the inverter;

a monitoring circuit coupled to the safety circuit and to the inverter so that during normal inverter operating conditions the monitoring circuit prevents the safety circuit from activating;

an overvoltage protection circuit engaged to the inverter and functional to detect an overvoltage condition outside of the normal inverter operating conditions; and

a shut-down circuit operably engaged to the safety circuit, to the monitoring circuit, to the overvoltage protection circuit, and to the inverter so that the shut-down circuit is responsive to the safety and overvoltage protection circuits and is functional to disable the inverter and the monitoring circuit;

wherein during the overvoltage condition, the overvoltage protection circuit is operative to cause the shut-down circuit to temporarily disable the inverter and the monitoring circuit and, if the overvoltage condition cannot be corrected after a predetermined sequence, the safety circuit is operative to cause the shut-down circuit to disable the inverter until the ballast is reset; and

the inverter is operative to attempt to re-ignite the lamp after the overvoltage protection circuit temporarily disables the inverter unless the safety circuit causes the shut-down circuit to disable the inverter.

16. The ballast of claim 15 further comprising:

an electrical ground and a power supply for the inverter;

the shut-down circuit comprises a shut-down switch responsive to the overvoltage condition detected by the overvoltage protection circuit;

the shut-down switch is operably connected between the power supply and the electrical ground so that in response to the overvoltage condition the shut-down switch activates and disables the inverter.

17. The ballast of claim 16 wherein the safety circuit comprises a safety capacitor operably connected to the power supply and to the shut-down switch so that if the power supply charges the safety capacitor to an activation level, the safety capacitor will activate the shut-down switch to disable the inverter until the ballast is reset.

18. The ballast of claim 17 wherein the monitoring circuit comprises a monitoring switch operably connected to the power supply and to the safety capacitor such that the monitoring switch prevents the safety capacitor from charging unless the overvoltage protection circuit instructs the shut-down circuit to temporarily disable the monitoring circuit.

19. The ballast of claim 18 wherein the overvoltage protection circuit comprises a sensor operably connected to the inverter and the monitoring switch so that the sensor can detect the overvoltage condition in the inverter and in response deactivate the monitoring switch.

20. The ballast of claim 19 wherein the inverter comprises a parallel resonant circuit and the sensor is inductively coupled to the parallel resonant circuit.