Multi-lamp backlight system

A multi-lamp backlight system. The system includes a core, a first coil set wrapped around the core, to which a first AC voltage is applied, a second and third coil sets wrapped around the core and respectively disposed on two sides of the first coil set, on which a second and third AC voltage are induced by the first voltage signal applied to the first coil set respectively, wherein the numbers of coils of the second and third coil sets are substantially the same, and first and second lamps are supplied with power by the second and third AC voltages respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backlight system and particularly to a multi-lamp backlight system with only a single transformer, with driving currents of the lamps inherently balanced.

2. Description of the Prior Art

Present displays, such as LCD, require a backlight system of small size and high power efficiency. Thus, CCFL (Cold Cathode Fluorescent Lamp) is popularly used in the backlight system. Moreover, backlight systems with only a single lamp are not suitable for large size displays requiring much higher illumination. Only multi-lamp backlight systems have illumination performance to satisfy this requirement.

FIG. 1is a diagram showing a conventional multi-lamp backlight system. It includes a driving circuit11, a transformer composed of a primary coil set121, a secondary coil set122and a core123, capacitors C1and C2, current balancing circuits13, and lamps151and152. The driving circuit11provides a low AC voltage applied to the primary coil set121of the transformer12so that a high AC voltage is induced on the secondary coil set122, by which the lamps151and152are supplied with power. Since the resistances or parasitic capacitances of lines connecting the transformer12to the lamps151and152can be different, the magnitudes of thus generated driving currents of the lamps151and152can also differ. This results in illumination difference between the lamps151and152, which degrades the performance of the display. The current balancing circuits13eliminate the current difference of the lamps151and152.

FIG. 2is a diagram showing another conventional multi-lamp backlight system. For sake of clarity, the same elements inFIGS. 1 and 2refer to the same symbols. The systems shown inFIGS. 1 and 2are similar, except that the current balancing circuits23inFIG. 2are coupled between the lamps151and152, and ground.

FIG. 3is a diagram showing the structure of the transformer12used in conventional multi-lamp backlight systems. The primary and secondary coil set121and122are both wrapped around and respectively disposed on two sides of the core123.

With the conventional transformer structure, the lamps in the backlight system must be connected in series or parallel when using one single transformer, or connected to one transformer each when using multiple transformers since there is only one secondary coil set. However, the transformer suffers much high voltage on its secondary coil set with the lamps serially connected, additional current balancing circuits are needed when the lamps are connected in parallel, and a backlight system with multiple transformers has a high cost and occupies a large space in the display.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a multi-lamp backlight system with only a single transformer, with driving currents of the lamps inherently balanced.

The present invention provides a multi-lamp backlight system comprising a core, a first coil set wrapped around the core, to which a first AC voltage is applied, a second and third coil sets wrapped around the core and respectively disposed on two sides of the first coil set, in which a second and third AC voltage are induced by the first voltage signal applied to the first coil set respectively, wherein the numbers of coils of the second and third coil sets are substantially the same, and a first and second lamp supplied with power by the second and third AC voltage respectively.

The present invention further provides a multi-lamp backlight system comprising a core, a first coil set wrapped around the core, to which a first AC voltage is applied, a plurality of second coil sets wrapped around the core and symmetrically disposed on two sides of the first coil set, on each of which a second AC voltage is induced by the first voltage signal applied to the first coil set, wherein the numbers of coils of the second coil sets are substantially the same, and a plurality of lamps are supplied with power by the second AC voltages.

The present invention also provides a transformer for multi-lamp backlight system, the system comprising a core, a first coil set wrapped around the core, and a second and third coil sets wrapped around the core and respectively disposed on two sides of the first coil set, wherein the numbers of coils of the second and third coil sets are substantially the same.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4is a diagram showing a multi-lamp backlight system according to a first embodiment of the invention. It includes a driving circuit41, a transformer42composed of a primary coil set421, secondary coil sets422and423, and a core424, capacitor Cl and C2, and discharge lamps431and432. The primary coil set421is wrapped around the core424with a low AC voltage from the driving circuit41applied thereto. The secondary coil sets422and423are also wrapped around the core424, on each of which a high AC voltage is induced by the low AC voltage on the primary coil set421. The lamps431and432are supplied with power by the high AC voltage on the secondary coil sets422and423respectively. The discharge lamps431and432may be CCFLs. Each of the secondary coil sets422and423has two ends of, respectively, first and second polarities. The lamps431and432are coupled to the ends of the first polarity of the secondary coil sets422and423respectively. The numbers of coils of the secondary coil sets422and423are substantially the same.

FIG. 5is a diagram showing a multi-lamp backlight system according to a second embodiment of the invention. It includes transistors51and52, diodes53and54, capacitors55, C1and C2, a transformer56composed of primary coil sets561and562, secondary coil sets563and564, and a core565, and discharge lamps571and572. The primary coil set561is wrapped around the core565and has one end coupled to receive an AC voltage Vin. The other primary coil set562is wrapped around the core565and has one end coupled to receive the AC voltage Vin. The transistor51has a drain coupled to one end of the primary coil set561, and a gate coupled to receive a square wave signal. The transistor52has a drain coupled to one end of the primary coil set562, a gate coupled to receive another square wave signal and a bulk coupled to ground. The diodes53and54are respectively coupled between the source and drain of the transistor51, and the source and drain of the transistor52. The capacitor55is coupled between one end of the primary coil set561and the bulk of the transistor51. The secondary coil sets563and564are wrapped around the core565, on which high AC voltages are induced by the low AC voltages applied to the primary coil sets561and562. The lamps571and572are supplied with power by the high voltages on the secondary coil sets563and564. The capacitors C1and C2are respectively coupled between the lamp571and the secondary coil set563, and the lamp572and the secondary coil set564. The discharge lamps571and572may be CCFLs. Each of the secondary coil sets563and564has two ends of, respectively, first and second polarities. The capacitors C1and C2are coupled to the ends of the first polarity of the secondary coil sets563and564respectively. The numbers of coils of the secondary coil sets563and564are substantially the same.

FIG. 6is a diagram showing a multi-lamp backlight system according to a third embodiment of the invention. It includes transistors61and62, resistors63and64, an inductor65, capacitors C1and C2, a transformer66composed of primary coil sets661,662and663, secondary coil sets664and665, and a core666, and discharge lamps671and672. The primary coil sets661,662and663are wrapped around the core666. One end of the primary coil set661is coupled to one end of the primary coil set662. The inductor65has one end coupled to receive an AC voltage Vin and the other end to the primary coil set661. The transistor61has a drain coupled to one end of the primary coil set661, a source coupled to ground and a gate coupled to one end of the primary coil set663. The transistor62has a drain coupled to one end of the primary coil set662, a source coupled to ground and a gate coupled to one end of the primary coil set663. The resistors63and64are respectively coupled between the gate of the transistor62and one end of the primary coil set661, and the gate of the transistor61and one end of the primary coil set661. The secondary coil sets664and665are wrapped around the core666, on which high AC voltages are induced by the low AC voltages applied to the primary coil sets661,662and663. The lamps671and672are supplied with power by the high voltages on the secondary coil sets664and665. The capacitors C1and C2are respectively coupled between the lamp671and the secondary coil set664, and the lamp672and the secondary coil set664. The discharge lamps671and672may be CCFLs. Each of the secondary coil sets664and665has two ends of, respectively, first and second polarities. The capacitors C1and C2are coupled to the ends of the first polarity of the secondary coil sets664and665respectively. The numbers of coils of the secondary coil sets664and665are substantially the same.

FIG. 7is a diagram showing a multi-lamp backlight system according to a fourth embodiment of the invention. For sake of clarity, the same elements shown inFIGS. 4 and 7refer to the same symbols. The systems shown inFIGS. 4 and 7are similar, except that the system inFIG. 7has an additional feedback network71to further stabilize the driving currents of the lamps431and432by feeding back the AC voltages to the driving circuit41.

FIG. 8is a diagram showing a multi-lamp backlight system according to a fifth embodiment of the invention. For sake of clarity, the same elements shown inFIGS. 4 and 8refer to the same symbols. The systems shown inFIGS. 4 and 8are similar, except that the system inFIG. 8has four secondary coil sets822˜825respectively to drive four lamps431˜434. Each of the secondary coil sets822˜825has two ends of, respectively, first and second polarities. The lamps431˜434are coupled to the ends of the first polarity of the secondary coil sets822˜825respectively. The numbers of coils of the secondary coil sets822˜825are substantially the same.

FIG. 9is a diagram showing the structure of the transformers used in the backlight system shown inFIG. 4˜8. All the primary coil sets are wrapped around and disposed on the center portion912of the core424(565,666) while all the secondary coil sets are wrapped around and disposed on the lateral portions913and914so that the primary coil sets are disposed between the secondary coil sets.

The present invention has the advantages of multiple lamps driven by only a single transformer, saving space and cost, inherently balanced currents with no need for any current balancing circuit, and a current flow through two secondary coil sets, which increases the endurance of the transformer.

In conclusion, the present invention provides a multi-lamp backlight system using only a single transformer with multiple secondary coil sets to drive multiple lamps. The driving currents are inherently balanced since the numbers of the coils of the secondary coil sets are substantially the same. There is neither need for an additional current balancing circuit nor for multiple transformers in the multi-lamp backlight system.