HID lamp ballast circuit

1. A ballast circuit operable to drive a high intensity discharge lamp comprises:

an energy conversion circuit operable to convert an input voltage into a bus voltage and to provide the bus voltage to a DC bus;

a first half bridge connected across the DC bus and operable to control an output voltage supplied to the lamp;

a control circuit operable to control the first half bridge such that a desired output voltage is provided to the lamp, the control circuit having a low voltage gate drive system to provide power factor correction;

a series inductor connected in series between the first half bridge and the lamp; and

a parallel capacitor connected across the lamp, wherein the control circuit operates the first half bridge at a high frequency for a set period of time such that a high voltage is built up across the parallel capacitor, and then reduces the frequency of the first half bridge until it approaches a resonance frequency which ignites the lamp.

2. The ballast circuit of claim 1 , wherein the resonance frequency is based on an inductance value of the series inductor and a capacitance value of the parallel capacitor.

3. The ballast circuit of claim 2 , wherein the energy conversion device is a flyback transformer, and wherein the control circuit provides zero crossing detection such that a time when all of the energy in the flyback transformer is transferred to the lamp is detected based on a drain voltage of a flyback MOSFET connected between the primary coil of the transformer and a common return of the ballast circuit.

4. The ballast circuit of claim 3 , wherein the control circuit controls the flyback MOSFET to provide power factor correction.

5. The ballast circuit of claim 2 , wherein the energy conversion device is a flyback transformer, and wherein the control circuit provides zero crossing detection such that a time when all of the energy in the flyback transformer is transferred to the lamp is detected based on a voltage at a node positioned between a secondary coil of the flyback transformer and a diode positioned between the flyback transformer and the DC bus.

6. The ballast circuit of claim 4 , further comprising a first current sense resistor positioned at a source of the flyback MOSFET and connected to the control circuit to provide an indication of current through the flyback switch, such that the control circuit detects faults in the primary coil and the flyback switch based on the current through the flyback switch.

7. The ballast circuit of claim 6 , further comprising a second current sense resistor operable to sense the lamp current through the lamp and to provide a signal indicative of the lamp current to the control circuit.

8. The ballast circuit of claim 7 , wherein the control circuit determines the power provided by to the lamp based on the signal indicative of the lamp current and the DC bus voltage to provide a signal indicating the power provided to the lamp, and wherein the control circuit controls the first half bridge to maintain a substantially constant lamp power.

9. The ballast circuit of claim 8 , wherein the control circuit reduces the frequency of the first half bridge a predetermined period of time after the lamp ignites until a desired low frequency operating frequency is reached.

10. The ballast circuit of claim 9 , wherein the control circuit turns the first half bridge off when the current through the flyback switch exceeds a second predetermined value for a second predetermined period of time.

11. The ballast circuit of claim 9 , wherein the control circuit turns the first half bridge off when a supply voltage drops below a set under voltage value.

12. The ballast circuit of claim 9 , wherein the control circuit turns the first half bridge off when an enable signal drops below a set enablement value.

13. The ballast circuit of claim 9 , further comprising a second half bridge, connected to an opposite side of the lamp such that the first half bridge and the second half bridge form a full bridge to drive the lamp.

14. A ballast circuit operable to drive a high intensity discharge lamp comprises:

a first half bridge connected across a DC bus having a bus voltage and operable to control an output voltage supplied to the lamp;

a control circuit operable to control the first half bridge such that a desired output voltage is provided to the lamp, the control circuit having a low voltage gate drive system to provide power factor correction;

a series inductor connected between the first half bridge and the lamp;

a parallel capacitor connected across the lamp, wherein the control circuit operates the first half bridge at a high frequency such that a high voltage is built up across the parallel capacitor, and then reduces the frequency of the first half bridge to approach a resonance frequency to ignite the lamp.

15. The ballast circuit of claim 14 , wherein the resonance frequency is based on an inductance value of the series inductor and a capacitance value of the parallel capacitor.

16. The ballast circuit of claim 14 , wherein the control circuit turns the first half bridge off when a lamp current exceeds a predetermined value for a predetermined period of time.

17. The ballast circuit of claim 14 , wherein the control circuit reduces the frequency of the first half bridge a predetermined period of time after the lamp ignites until a desired frequency is reached.

18. The ballast circuit of claim 14 , wherein the control circuit turns the first half bridge off when a supply voltage drops below a set under voltage value.

19. The ballast circuit of claim 14 , wherein the control circuit turns the first half bridge off when an enable signal drops below a set enablement value.

20. The ballast circuit of claim 14 , further comprising a second half bridge, connected to an opposite side of the lamp such that the first half bridge and the second half bridge form a full bridge to drive the lamp.