Circuit for powering a high intensity discharge lamp

1. A circuit for powering a high intensity discharge (HID) lamp, the circuit comprising:

first and second input terminals for receiving a source of substantially direct current (DC) voltage;

first and second output terminals adapted for coupling to an HID lamp;

a first inverter switch coupled between the first input terminal and a first node;

a second inverter switch coupled between the first node and circuit ground, wherein circuit ground is coupled to the second input terminal;

a buck inductor coupled between the first node and a second node;

a first buck capacitor coupled between the first input terminal and the second node;

a second buck capacitor coupled between the second node and circuit ground;

a first low-side capacitor coupled between the first input terminal and a third node, wherein the third node is coupled to the second output terminal;

a second low-side capacitor coupled between the third node and circuit ground;

an ignitor coupled between the second node and the first output terminal; and

wherein the circuit is characterized by further comprising:

a diode subcircuit coupled between the first and second input terminals; and

a secondary winding that is magnetically coupled to the buck inductor, wherein the secondary winding is coupled between the diode subcircuit and the third node.

2. The circuit of claim 1 , wherein the diode subcircuit comprises a series combination of two diodes.

3. The circuit of claim 1 , wherein the diode subcircuit comprises:

a first diode having an anode coupled to a fourth node and a cathode coupled to the first input terminal; and

a second diode having an anode coupled to circuit ground and a cathode coupled to the fourth node.

4. The circuit of claim 3 , wherein the secondary winding is coupled to the diode subcircuit at the fourth node.

5. The circuit of claim 1 , wherein the first and second low-side capacitors each have a capacitance that is at least several orders of magnitude less than about 68 microfarads.

6. The circuit of claim 1 , wherein:

the first and second low-side capacitors each have a first capacitance;

the first and second buck capacitors each have a second capacitance; and

the first capacitance and the second capacitance are on the same order of magnitude.

7. The circuit of claim 6 , wherein the first and second capacitances are about equal.

8. The circuit of claim 1 , wherein the first and second low-side capacitors each have a capacitance that is on the order of about 47 nanofarads.

9. The circuit of claim 1 , wherein the buck inductor and the secondary winding are loosely coupled.

10. The circuit of claim 1 , wherein the buck inductor and the secondary winding have an unequal number of wire turns.

11. A circuit for powering a high intensity discharge (HID) lamp, the circuit comprising:

first and second input terminals ( 202 , 204 ) for receiving a source of substantially direct current (DC) voltage (V RAIL );

first and second output terminals ( 206 , 208 ) adapted for coupling to an HID lamp ( 10 );

a first inverter switch (SW 1 ) coupled between the first input terminal ( 202 ) and a first node ( 210 );

a second inverter switch (SW 2 ) coupled between the first node ( 210 ) and circuit ground ( 20 ), wherein circuit ground ( 20 ) is coupled to the second input terminal ( 204 );

a first buck capacitor (C BUCK1 ) coupled between the first input terminal ( 202 ) and a second node ( 212 );

a second buck capacitor (C BUCK2 ) coupled between the second node ( 212 ) and circuit ground ( 20 );

a first low-side capacitor (C LOWSIDE1 ) coupled between the first input terminal ( 202 ) and a third node ( 214 ), wherein the third node ( 214 ) is coupled to the second output terminal ( 208 );

a second low-side capacitor (C LOWSIDE2 ) coupled between the third node ( 214 ) and circuit ground ( 20 );

an ignitor ( 216 ) coupled between the second node ( 212 ) and the first output terminal ( 206 );

a first diode ( 250 ) having an anode ( 252 ) coupled to a fourth node ( 218 ) and a cathode ( 254 ) coupled to the first input terminal ( 202 );

a second diode ( 260 ) having an anode ( 262 ) coupled to circuit ground ( 20 ) and a cathode ( 264 ) coupled to the fourth node ( 218 );

a transformer ( 240 ), comprising:

a primary winding ( 242 ) coupled between the first node ( 210 ) and the second node ( 212 ); and

a secondary winding ( 244 ) coupled between the fourth node ( 218 ) and the third node ( 214 ).

12. The circuit of claim 11 , wherein the first and second low-side capacitors (C LOWSIDE1 , C LOWSIDE2 ) each have a capacitance that is at least several orders of magnitude less than about 68 microfarads.

13. The circuit of claim 12 , wherein the capacitance of the first and second low-side capacitors (C LOWSIDE1 , C LOWSIDE2 ) is on the order of about 47 nanofarads.

14. The circuit of claim 11 , wherein:

the first and second low-side capacitors (C LOWSIDE1 , C LOWSIDE2 ) each have a first capacitance;

the first and second buck capacitors (C BUCK1 , C BUCK2 ) each have a second capacitance; and

the first capacitance and the second capacitance are on the same order of magnitude.

15. The circuit of claim 14 , wherein the first and second capacitances are about equal.

16. The circuit of claim 15 , wherein the primary and secondary windings ( 242 , 244 ) of the transformer ( 240 ) are loosely coupled.

17. The circuit of claim 15 , wherein the primary and secondary windings ( 242 , 244 ) of the transformer ( 240 ) have an unequal number of wire turns.