LED backlight device

• US 8,686,650 B2
• Filed: 11/08/2011
• Issued: 04/01/2014
• Est. Priority Date: 11/09/2010
• Status: Active Grant

• Current Assignee: Samsung Display Company Limited (Samsung Electronics Co. Ltd.)
• Original Assignee: Samsung Display Company Limited (Samsung Electronics Co. Ltd.)
• Inventors: Shimura, Tatsuhisa
• Primary Examiner(s): Richardson, Jany
• Application Number:
  US13/291,720
• Publication Number:
  US 20120112649A1
• Time in Patent Office: 875 Days
• Field of Search: None
• US Class Current: 315/188
• CPC Class Codes: H05B 45/35 Balancing circuits
• CPC Class Codes: H05B 45/39 Circuits containing inverter bridges
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines

Abstract

An LED backlight device includes an inverter having an input connected to a DC power supply to provide an output AC current. A plurality of transformers are each configured to drop AC current input from the inverter. Input sides of the transformers are connected in series to an output of the inverter and output sides of the transformers are disposed in parallel. A plurality of full-wave rectification circuits are respectively connected to the output sides of the transformers and full-wave rectify the dropped AC currents, respectively. A plurality of smoothing circuits are respectively connected to outputs of the full-wave rectification circuits, and are configured to smooth the full-wave rectified currents to output DC currents, respectively. A plurality of LED strings are respectively connected to the outputs of the smoothing circuits and each of the LED strings have a plurality of LEDs.

Claims

1. An LED backlight device comprising:

an inverter having an input connected to a DC power supply and configured to output an AC current;

a plurality of transformers each configured to drop the AC current input from the inverter, input sides of the transformers being connected in series with respect to an output of the inverter and output sides of the transformers being disposed in parallel;

a plurality of full-wave rectification circuits respectively connected to the output sides of the transformers, and configured to full-wave rectify the dropped AC currents, respectively;

a plurality of smoothing circuits respectively connected to outputs of the plurality of full-wave rectification circuits, and configured to smooth the full-wave rectified currents to output DC currents, respectively; and

a plurality of LED strings respectively connected to outputs of the smoothing circuits and each of the LED strings having a plurality of LEDs connected in series cathode to anode respectively,

wherein respective turn ratios of the input and output sides of the transformers are the same as one another.

2. The LED backlight device of claim 1, wherein each of the full-wave rectification circuits comprises a diode bridge circuit, and cathode sides of the LED strings and anode sides of the full-wave rectification circuits are connected to a common ground, one end of the serially connected input sides of the transformers being connected to the common ground and the other end of the serially connected input sides of the transformers being connected to the output of the inverter.

3. The LED backlight device of claim 1, wherein inductance values of the output sides of the transformers are set to be larger than impedance values of the LED strings.

4. The LED backlight device of claim 1, further comprising:

a DC converter having an input connected to the DC power supply and an output connected to a common node of the full-wave rectification circuits and configured to output a DC voltage for compensating for a voltage corresponding to a difference between forward voltages of the LED strings.

5. The LED backlight device of claim 4, wherein the DC converter is further configured to set the DC voltage to a maximum voltage by which the LED strings are not switched on.

6. The LED backlight device of claim 4, wherein each of the full-wave rectification circuits comprises a diode bridge circuit, and cathode sides of the LED strings and anode sides of the full-wave rectification circuits are connected to a common ground, one end of the serially connected input sides of the transformers being connected to the common ground and the other end of the serially connected input sides of the transformers being connected to the output of the inverter.

7. An LED backlight device comprising:

an inverter having an input connected to a DC power supply and configured to output an AC voltage;

a plurality of transformers each configured to output AC currents, input windings of the transformers being connected in series to form a loop, output windings of transformers being disposed in parallel, and one end of the output windings of the transformers being connected in parallel to an output of the inverter;

a plurality of rectification circuits respectively connected to the other ends of the output windings of the transformers, and configured to rectify the AC currents from the transformers to output positive currents and negative currents, respectively;

a plurality of smoothing circuits respectively connected to outputs of the rectification circuits, and configured to smooth the positive and negative currents to output positive and negative DC currents, respectively; and

a plurality of LED strings respectively connected to outputs of the smoothing circuits and each of the LED strings having a plurality of LEDs.

8. The LED backlight device of claim 7, wherein each of the rectification circuits includes a first rectification element configured to output a positive current and a second rectification element configured to output a negative current, and the LED strings includes first LED strings and second LED strings, anode sides of the first LED strings being connected to outputs of first rectification elements of the rectification circuits and the smoothing circuits, respectively, and cathode sides of the second LED strings being connected to outputs of second rectification elements of the rectification circuits and the smoothing circuits, respectively.

9. The LED backlight device of claim 8, wherein cathode sides of the first LED strings and anode sides of the second LED strings are connected to a common ground.

10. The LED backlight device of claim 7, wherein inductance values of the input windings of the transformers and inductance values of the output windings of the transformers are adjusted such that the same input AC currents flow into the input windings of the transformers and the same output AC currents flow into the output windings of the transformers.

11. An LED backlight device comprising:

an inverter having an input connected to a DC power supply and configured to output an AC voltage;

a transformer having an input winding and an output winding wound in a reverse direction and configured to output an AC current, one end of the input winding and one end of the output winding being connected in parallel to an output of the inverter;

a plurality of rectification circuits respectively connected to the other end of the input winding and the other end of the output winding, and configured to rectify AC currents output from the other end of the input winding and the other end of the output winding to output positive currents and negative currents, respectively;

a plurality of smoothing circuits respectively connected to outputs of the rectification circuits, and configured to smooth the positive and negative currents to output positive and negative DC currents, respectively; and

a plurality of LED strings respectively connected to outputs of the smoothing circuits and each of the LED strings having a plurality of LEDs.

12. The LED backlight device of claim 11, wherein each of the rectification circuits includes a first rectification element outputting a positive current and a second rectification element outputting a negative current, and the LED strings includes first LED strings and second LED strings, anode sides of the first LED strings being connected to outputs of first rectification elements of the rectification circuits and the smoothing circuits, respectively, and cathode sides of the second LED strings being connected to outputs of second rectification elements of the rectification circuits and the smoothing circuits, respectively.

13. The LED backlight device of claim 12, wherein cathode sides of the first LED strings and anode sides of the second LED strings are connected to a common ground.

14. A backlight device for a liquid crystal display device, comprising:

a plurality of light emitting diode strings, each light emitting diode string comprising a series of diodes coupled in series, one end of each diode string coupled to ground;

an inverter configured to convert a DC voltage supplied from a DC power source into an AC voltage having a predetermined frequency and amplitude;

one or more transformers configured to receive the AC voltage;

a plurality of rectification circuits whose respective input sides are configured to receive an AC voltage from a respective transformer; and

a plurality of capacitors, each capacitor coupled between a non-grounded end of a respective light emitting diode string and an output of a respective rectification circuit,

wherein one end of a first light emitting diode string of a pair of light emitting diode strings has its anode coupled to ground and one end of a second light emitting diode string of the pair of light emitting diode strings has its cathode coupled to ground.

15. The backlight device of claim 14, wherein the rectification circuits comprise full-wave rectification circuits whose outputs are connected to respective non-grounded ends of the light emitting diode strings.

16. The backlight device of claim 14,

wherein the rectification circuits comprise diode pairs, an anode of one of diode pairs being coupled to the non-grounded end of the first light emitting diode pair and a cathode of the second one of the diode pairs being coupled to the non-grounded end of the second light emitting diode string pair.

17. The backlight device of claim 14, wherein the DC power source is a DC power supply.

18. The backlight device of claim 14, wherein the DC power source comprises:

an AC/DC rectification unit configured to receive an AC power supply voltage from an AC outlet and to provide an input DC voltage; and

a DC/DC converter configured to convert the input DC voltage from the AC/DC rectification unit into the DC voltage received by the inverter.