LED light source for backlighting with integrated electronics

• US 20070046485A1
• Filed: 08/26/2005
• Published: 03/01/2007
• Est. Priority Date: 08/26/2005
• Status: Active Grant

• Current Assignee: Signify North America Corporation (Signify N.V.)
• Original Assignee: Lumileds Lighting US LLC (Apollo Global Management, Inc.)
• Inventors: Stassar, Emanuel, Grootes, Pieter, Hendriks, Robert, Schrama, Charles, Kaito, Noboru, Claessens, Dennis
• Granted Patent: US 7,317,403 B2
• Time in Patent Office: Days
• US Class Current: 340/815.450
• CPC Class Codes: G02F 1/133603 with LEDs
• CPC Class Codes: G02F 1/133609 including means for improving the color mixing, e.g. white
• CPC Class Codes: H05B 45/3725 Switched mode power supply [SMPS]
• CPC Class Codes: H05B 45/40 Details of LED load circuits
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines
• CPC Class Codes: H05B 45/48 having LEDs organised in strings and incorporating parallel shunting devices
• CPC Class Codes: H05K 1/181 associated with surface mounted components
• CPC Class Codes: Y02B 20/30 Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]
• CPC Class Codes: Y10S 362/80 Light emitting diode

Abstract

An LED backlight structure and technique for setting the voltages and currents for the LEDs are described. In one embodiment, red LEDs are connected in series between a first voltage regulator and a first controllable current source, green LEDs are connected in series between a second voltage regulator and a second controllable current source, and blue LEDs are connected in series between a third voltage regulator and a third controllable current source. The current sources may linear current regulators. After all the LEDs are mounted on a printed circuit board, each voltage regulator is controlled so that there is a minimum voltage drop across the current source to minimize energy dissipation by the current source. Also, after all the LEDs are mounted on the printed circuit board, the current sources are controlled to balance the three colors to achieve a target light output of the board using a light detection chamber. In another embodiment, a constant current source is connected to the series string of LEDs. PWM controllers control transistor switches connected in parallel across groups of LEDs, where the duty cycles set the average current through each group of LEDs. The control values used to achieve the target light characteristics are then stored in a memory on the board. With such a technique, LEDs do not need to be binned to match their efficiencies, since the controllable voltage and current sources compensate for the variety of LED efficiencies. Accordingly, all boards will have the same target light output using the particular values stored on each board. In other embodiments, not all the LEDs of a particular color are connected in series. The brightness levels of the LEDs may be further controlled by a PWM controller. The backlight may be for an LCD television or other color display.

Claims

1. A light emitting diode (LED) light source for use in a backlight for a display comprising:

at least one LED of a first color;

at least one LED of a second color;

at least one LED of a third color;

a plurality of current controllers, each controller connected to at least one LED of the first color, at least one LED of the second color, and at least one LED of the third color for controlling a brightness level of each color;

the at least one LED of the first color, the at least one LED of the second color, the at least one LED of the third color, and the plurality of controllers being mounted on a same circuit board; and

a memory mounted on the circuit board, the memory storing values for controlling an output current magnitude of each of the controllers so that a combined light output of the light source has characteristics set by the values stored in the memory.

2. The light source of claim 1 where the values in the memory control the controllers to achieve a target white point.

3. The light source of claim 1 where the values in the memory control the controllers such that light characteristics of the board are substantially the same as light characteristics of another board despite the LEDs on the two boards having different operating characteristics.

4. The light source of claim 1 where the first color is red, the second color is green, and the third color is blue, wherein the values in memory control the controllers to set the relative brightness of the red, green, and blue colors to achieve a desired overall light output from the board.

5. The light source of claim 1 where each controller is a current source connected to a cathode end of a string of LEDs connected in series.

6. The light source of claim 5 further comprising a first voltage regulator connected to an anode end of the at least one LED of the first color, a second voltage regulator connected to an anode end of the at least one LED of the second color, and a third voltage regulator connected to an anode end of the at least one LED of the third color.

7. The light source of claim 6 wherein the first voltage regulator, the second voltage regulator, and the third voltage regulator are adjusted to provide a voltage drop across a respective current source of 1 volt or less.

8. The light source of claim 1 wherein the at least one LED of the first color comprises a plurality of LEDs of the first color connected in series.

9. The light source of claim 1 wherein the at least one LED of the second color comprises a plurality of LEDs of the second color connected in series.

10. The light source of claim 1 wherein the at least one LED of the third color comprises a plurality of LEDs of the third color connected in series.

11. The light source of claim 1 further comprising a reflective box at least partially surrounding the at least one LED of the first color, at least one LED of the second color, and at least one LED of the third color to mix the first color, second color, and third color.

12. The light source of claim 1 wherein the at least one LED of a first color comprises a plurality of LEDs of the first color, the at least one LED of a second color comprises a plurality of LEDs of the second color, and the at least one LED of a third color comprises a plurality of LEDs of the third color, wherein the LEDs of the first color, second color, and third color are arranged in clusters of at least one LED of the first color, at least one LED of the second color, and at least one LED of the third color, each cluster of LEDs being controlled by a respective controller for each color such that at least one cluster is controlled by at least three controllers to have a desired combined color point and brightness.

13. The light source of claim 12 wherein the clusters are housed in a reflective box at least partially surrounding the clusters, a cluster proximate to an edge of the box being controlled to have a brightness level lower than a brightness level of a cluster further from the edge of the box.

14. The light source of claim 1 wherein the at least one LED of the first color comprises a plurality of series strings of LEDs of the first color connected in parallel.

15. The light source of claim 1 wherein the plurality of controllers comprises a plurality of controllable linear current sources.

16. The light source of claim 1 further comprising a current level controller on the circuit board for receiving digital values from the memory and converting the digital values to signals for controlling the plurality of controllers.

17. The light source of claim 1 wherein the at least one LED of the first color comprises a plurality of LEDs of the first color connected in series, and wherein each current controller comprises:

a constant current source connected to the plurality of LEDs of the first color connected in series;

a transistor switch connected in parallel across at least one LED in the plurality of LEDs of the first color connected in series; and

a pulse width modulated (PWM) controller connected to the transistor switch for setting a duty cycle of the switch to cause an average current magnitude, determined by the duty cycle, to pass through the at least one LED in the plurality of LEDs of the first color connected in series.

18. The light source of claim 17 wherein the transistor switch is connected in parallel across a group of LEDs in the plurality of LEDs of the first color connected in series.

19. The light source of claim 17 further comprising one or more additional transistor switches, each switch connected in parallel across at least one LED in the plurality of LEDs of the first color connected in series, and each switch being controlled by an associated PWM controller.

20. The light source of claim 17, wherein the PWM controller controls the duty cycle to achieve a target brightness of the first color.

21. The light source of claim 17 further comprising a PWM controller for blanking the output of the PWM controller at a duty cycle to control an overall brightness of the first color.

22. The light source of claim 17 further comprising a PWM controller for controlling the constant current source to apply either a full current or no current to the plurality of LEDs of the first color connected in series at a duty cycle to control an overall brightness of the first color.

23. The light source of claim 17 further comprising a controller for controlling a magnitude of the constant current source to control an overall brightness of the first color.

24. The light source of claim 17 wherein the at least one LED of the second color comprises a plurality of LEDs of the second color connected in series, and wherein each current controller further comprises:

a constant current source connected to the plurality of LEDs of the second color connected in series;

a transistor switch connected in parallel across at least one LED in the plurality of LEDs of the second color connected in series; and

a pulse width modulated (PWM) controller connected to the transistor switch for setting a duty cycle of the switch to cause an average current magnitude, determined by the duty cycle, to pass through the at least one LED in the plurality of LEDs of the second color connected in series.

25. A method for producing a light source for use as a backlight comprising:

providing on a circuit board at least one LED of a first color, at least one LED of a second color, and at least one LED of a third color;

providing on the circuit board a plurality of current controllers, each controller connected to at least a cathode end of a respective one of the at least one LED of the first color, at least one LED of the second color, and at least one LED of the third color;

providing on the circuit board a memory, values stored in the memory determining a current output of each of the controllers;

controlling the plurality of controllers to vary brightness levels of the at least one LED of the first color, at least one LED of the second color, and at least one LED of the third color while detecting a combined light output of the at least one LED of the first color, at least one LED of the second color, and at least one LED of the third color;

determining control signals for the plurality of controllers needed to obtain a target light output of the board from the combination of the first color, second color, and third color; and

storing in the memory values corresponding to the control signals needed to obtain the target light output.

26. The method of claim 25 wherein the at least one LED of a first color comprises a plurality of LEDs of the first color, the at least one LED of a second color comprises a plurality of LEDs of the second color, and the at least one LED of a third color comprises a plurality of LEDs of the third color, wherein the LEDs of the first color, second color, and third color are arranged in clusters of at least one LED of the first color, at least one LED of the second color, and at least one LED of the third color, wherein each cluster of LEDs is controlled by a respective controller for each color such that a first cluster is controlled by at least three controllers to have a desired brightness level and color point, the method further comprising storing in the memory values for setting brightness levels of the first cluster and the second cluster to be different.

27. The method of claim 25 further comprising:

downloading the values stored in the memory to control the plurality of controllers; and

energizing the at least one LED of a first color, at least one LED of a second color, and at least one LED of a third color to obtain the same target light output achieved when determining control signals for the plurality of controllers needed to obtain the target light output.

28. The method of claim 27 wherein the first color is red, the second color is green, and the third color is blue, wherein downloading the values in memory causes the controllers to control the relative brightness of the red, green, and blue colors to achieve the target light output.

29. The method of claim 25 wherein the at least one LED of the first color comprises a plurality of LEDs of the first color connected in series, and wherein each current controller comprises:

a constant current source connected to the plurality of LEDs of the first color connected in series;

a transistor switch connected in parallel across at least one LED in the plurality of LEDs of the first color connected in series; and

a pulse width modulated (PWM) controller connected to the transistor switch,

wherein the step of controlling the plurality of controllers to vary brightness levels comprises setting a duty cycle of the switch to cause an average current magnitude, determined by the duty cycle, to pass through the at least one LED in the plurality of LEDs of the first color connected in series.

30. The method of claim 29 wherein the transistor switch is connected in parallel across a group of LEDs in the plurality of LEDs of the first color connected in series.

31. The method of claim 29 further comprising one or more additional transistor switches, each switch connected in parallel across at least one LED in the plurality of LEDs of the first color connected in series, and each switch being controlled by an associated PWM controller, wherein the step of controlling the plurality of controllers to vary brightness levels comprises setting duty cycles of the switches to cause an average current magnitude, determined by each duty cycle, to pass through the associated at least one LED in the plurality of LEDs of the first color connected in series.