Thermal protection for lamp ballasts

1. A circuit for controlling output current from a ballast to a lamp comprising:

a) a temperature sensor thermally coupled to the ballast to provide a temperature signal having a magnitude indicative of ballast temperature, Tb; and

b) a programmable controller operable to cause the ballast to enter a current limiting mode when the magnitude of the temperature signal indicates that Tb has exceeded a predetermined ballast temperature, T 1 ;

wherein the programmable controller causes the output current to be responsive to the temperature signal according to one of (i) a step function or (ii) a combination of step and continuous functions, while continuing to operate the ballast.

2. The circuit of claim 1 , wherein the programmable controller comprises one of a microcontroller, a microprocessor, a programmable logic device, and an application specific integrated circuit.

3. The circuit of claim 1 , further comprising:

a low-pass filter operable to receive the temperature signal and to provide a filtered temperature signal to the programmable controller.

4. The circuit of claim 3 , wherein the low-pass filter comprises a resistor and a capacitor.

5. The circuit of claim 1 , further comprising:

a ballast drive circuit responsive to a pulse-width modulated signal from the programmable controller, the pulse-width modulated signal resulting in a lamp current corresponding to a current level set by a dimmer control signal or a software high end clamp value.

6. The circuit of claim 1 , wherein the programmable controller comprises:

a processor for executing a software program to input and process a dimmer control signal and a temperature signal;

at least one analog-to-digital converter for sampling the temperature signal; and

a pulse width modulated digital output signal.

7. The circuit of claim 6 , wherein the software program comprises:

instructions for processing multiple consecutive samples of the temperature signal; and

instructions for calculating a software high end clamp value to limit a current to the lamp.

8. The circuit of claim 7 , wherein the instructions for processing multiple consecutive samples of the temperature signal comprise a recursive digital filter.

9. The circuit of claim 1 , wherein the programmable controller reduces a maximum permissible output current in response to the temperature signal.

10. A thermally protected ballast comprising:

a) a front end AC-to-DC converter for receiving a supply voltage;

b) a back end DC-to-AC converter coupled to the front end AC-to-DC converter for providing output current to a load;

c) a temperature sensor adapted to provide a temperature signal having a magnitude indicative of a temperature of the ballast, Tb; and

d) a programmable controller responsive to the temperature signal and operable to cause the DC-to-AC converter to adjust the output current;

wherein the temperature signal causes the programmable controller to adjust the output current in response to a detected over-temperature condition, according to one of (i) a step function or (ii) a combination of step and linear functions, while continuing to operate the ballast.

11. The thermally protected ballast of claim 10 , further comprising:

a hardware low-pass filter operable to receive the temperature signal and to provide a filtered temperature signal to the programmable controller.

12. The thermally protected ballast of claim 10 , wherein the programmable controller comprises:

a processor executing instructions to process a dimmer control signal and a temperature signal to control the output current, wherein the processor is responsive to the dimmer control signal to operate at a first current level until a temperature is reached having a corresponding lower current level, wherein a reduction to the lower current level is asserted.

13. The thermally protected ballast of claim 12 , wherein the instructions executed by the processor comprise a recursive digital filter for filtering information from the temperature sensor.

14. A method of controlling a ballast comprising the steps of:

a) determining a temperature Tb of the ballast;

b) comparing the temperature Tb to a first reference temperature T 1 ; and

c) controlling an output current provided by the ballast according to one of (i) a step function or (ii) a combination of a step and continuous functions, while continuing to operate the ballast, in accordance with the result of step (b).

15. The method of claim 14 , further comprising the step of:

acquiring a temperature signal representative of the temperature Tb of the ballast.

16. The method of claim 15 , wherein acquiring the temperature signal comprises sampling the temperature signal using a hardware low pass filter.

17. The method of claim 15 , wherein the step of controlling an output current comprises:

acquiring multiple samples of the temperature Tb with an analog-to-digital converter;

applying the samples to a digital filter;

determining if the digital filter output exceeds the first temperature T 1 ;

if the digital filter output exceeds the first temperature T 1 , calculating a high end current value corresponding to operation of the ballast at the temperature T 1 , wherein the calculation is one of (i) a step function or (ii) a combination of a step and continuous functions; and

adjusting the output current to correspond to the calculated high end current value.

18. The method of claim 15 , further comprising the step of:

acquiring a dimmer control signal representative of a desired lamp illumination level, the dimmer control signal acquired using a programmable controller which is responsive to the dimmer control signal to operate the ballast at a first current level until the temperature signal indicates an elevated ballast temperature; and

upon determination of an elevated ballast temperature, reducing the output current according to a temperature-versus-current profile of the programmable controller.

19. The method of claim 15 , further comprising the step of:

comparing the temperature Tb to a second reference temperature T 2 greater than the first reference temperature T 1 ;

wherein the step of controlling an output current further comprises the steps of:

controlling the output current provided by the ballast linearly with respect to the temperature Tb when the temperature Tb is between the first reference temperature T 1 and the second reference temperature T 2 ; and

controlling the output current provided by the ballast in accordance with a step function when the temperature Tb is greater than second reference temperature T 2 .

20. The method of claim 15 , further comprising the steps of:

comparing the temperature Tb to a second reference temperature T 2 , greater than the first reference temperature T 1 ; and

comparing the temperature Tb to a third reference temperature T 3 , greater than the first reference temperature T 1 and less than the second reference temperature T 2 ;

wherein the step of controlling an output current further comprises the steps of:

controlling the output current provided by the ballast linearly with respect to the temperature Tb when the temperature Tb is between the first reference temperature T 1 and the second reference temperature T 2 ;

controlling the output current provided by the ballast in accordance with a step function to a first magnitude when the temperature Tb is greater than the second reference temperature T 2 ; and subsequently controlling the output current provided by the ballast in accordance with a step function to a second magnitude greater than the first magnitude, when the temperature Tb is less than the third reference temperature T 3 .