Current-mode direct-drive inverter

1. A current-mode inverter comprising:

a current regulator configured to accept a DC voltage and to output a regulated current;

a non-resonant switching network configured to produce an AC driving current by periodically alternating conduction paths for the regulated current; and

a closely-coupled output transformer configured to conduct the AC driving current in a primary winding and a corresponding load current in a secondary winding, wherein the closely-coupled output transformer has a magnetization inductance to leakage inductance ratio that is greater than 30:1.

2. The current-mode inverter of claim 1 , wherein the load current has a substantially identical wave shape as the AC driving current.

3. The current-mode inverter of claim 2 , wherein the amplitude of the load current is proportional to the amplitude of the AC driving current.

4. The current-mode inverter of claim 1 , further comprising a feedback circuit coupled to the non-resonant switching network on the primary side of the closely-coupled output transformer to sense the AC driving current and to generate a feedback signal for the current regulator.

5. The current-mode inverter of claim 1 , wherein the current regulator is a switching current regulator using hysteretic pulse width modulation, a switching current regulator using clocked pulsed width modulation or a linear current regulator.

6. The current-mode inverter of claim 1 , wherein the non-resonant switching network uses a push-pull topology with semiconductor switches coupled to respective opposite ends of the primary winding and the regulated current applied to a center tap of the primary winding.

7. The current-mode inverter of claim 2 , wherein the non-resonant switching network uses a full-bridge topology.

8. The current-mode inverter of claim 7 , wherein the full-bridge topology comprises a pair of p-type semiconductor switches coupled between an output of the regulated current and respective opposite terminals of the primary winding and a pair of n-type semiconductor switches coupled between the respective opposite terminals of the primary winding and the feedback circuit.

9. The current-mode inverter of claim 1 , wherein semiconductor switches in the non-resonant switching network are closed to conduct a predetermined idle current when the load current is substantially zero.

10. The current-mode inverter of claim 1 , wherein a fluorescent lamp is coupled across the secondary winding, and the voltage across the secondary winding automatically increases to ignite the fluorescent lamp.

11. A method to operate an inverter in current mode, the method comprising the steps of:

converting an input power source into a regulated current;

periodically alternating conduction paths for the regulated current using a non-resonant switching structure to generate an AC current source; and

coupling the AC current source to a lamp load with an output transformer, wherein the output transformer is a tightly-coupled transformer with a magnetization inductance to leakage inductance ratio that is greater than 30:1.

12. The method of claim 11 , wherein semiconductor switches in the non-resonant switching structure are closed to stop generating the AC current source if the lamp load is off, missing or faulty.

13. The method of claim 11 , further comprising sensing the AC current source to generate a feedback signal for controlling brightness of the lamp load.

14. The method of claim 11 , wherein the lamp load comprises a plurality of cold cathode fluorescent lamps coupled in series across a secondary winding of the output transformer.

15. The method of claim 11 , wherein the inverter operates in a single continuous mode for striking and regulating power to the lamp load.

16. The method of claim 11 , wherein the lamp load comprises one or more light sources for backlighting a liquid crystal display.

17. A current-mode inverter comprising:

means for generating a regulated current source;

means for periodically alternating conduction paths for the regulated current source with non-resonant switching to generate an AC driving current; and

output transformer means for coupling the AC driving current to a lamp structure, wherein the lamp structure conducts a lamp current with a substantially identical wave shape and proportional amplitude as the AC driving current, and wherein the output transformer means comprises a tightly-coupled transformer with a magnetization inductance to leakage inductance ratio that is greater than 30:1.

18. The current-mode inverter of claim 17 , wherein light intensity of the lamp structure is controlled by a combination of adjusting duty cycles or burst mode durations of the regulated current source and duty cycles or burst mode durations of the AC driving current.

19. The current-mode inverter of claim 17 , further comprising means for sensing the AC driving current to control brightness of the lamp structure by adjusting the regulated current source.

20. The current-mode inverter of claim 17 , wherein the lamp structure includes one or more fluorescent lamps used for backlighting a liquid crystal television, a desk top monitor, an automotive display, a notebook computer or a tablet computer.

21. The current-mode inverter of claim 17 , wherein the non-resonant switching is implemented by metal-oxide-semiconductor field-effect-transistors.