System and method for reducing wavelength variations between light emitting diodes

• US 6,392,359 B1
• Filed: 01/22/2001
• Issued: 05/21/2002
• Est. Priority Date: 01/22/2001
• Status: Expired due to Fees

• Current Assignee: Gelcore (General Electric Company)
• Original Assignee: Gelcore (General Electric Company)
• Inventors: Stalions, Drake C.
• Primary Examiner(s): VU, DAVID HUNG
• Application Number:
  US09/765,503
• Time in Patent Office: 484 Days
• Field of Search: 315/312, 315/185.R, 315/192, 315/193, 315/179, 315/189, 315/190, 362/800, 362/230, 362/231
• US Class Current: 315/192
• CPC Class Codes: H05B 45/24 using electrical feedback from LEDs or from LED modules

Abstract

A system and method for reducing wavelength variations between light emitting diodes (LEDs) is provided that determines an emission wavelength of each of a plurality of LEDs at a common drive current, and drives each of the plurality of LEDs with a respective operational drive current such that wavelength variations between the plurality of LEDs, when driven at the respective operational drive currents, is less than wavelength variations between the plurality of LEDs when driven at the common drive current. This system and method of the invention minimizes wavelength variations between LEDs, thereby allowing the use of more, and in some cases all, of the LEDs that are fabricated from a single semiconductor wafer, or multiple semiconductor wafers.

Claims

1. A method of driving a plurality of light emitting diodes (LEDs), comprising the steps of:

determining an emission wavelength of each of the plurality of LEDs at a common drive current;

choosing a respective operational drive current for each of the plurality of LEDs, wherein a wavelength variation between the plurality of LEDs, when driven with their respective operational drive currents, is less than a wavelength variation between the plurality of LEDs when driven with the common drive current; and

driving the plurality of LEDs with the respective operational drive currents.

2. The method of claim 1, wherein the respective operational drive currents are chosen to obtain a predetermined wavelength variation between the plurality of LEDs.

3. The method of claim 2, wherein the predetermined wavelength variation corresponds to a predetermined wavelength range.

4. The method of claim 1, wherein the step of choosing a respective operational drive current for each of the plurality of LEDs comprises the steps of:

determining a wavelength distribution of the plurality of LEDs at the common drive current; and

determining, based on the wavelength distribution of the plurality of LEDs at the common drive current, the respective operational drive currents that will result in wavelength variations between the plurality of LEDs that are less than the wavelength variations between the plurality of LEDs at the common drive current.

5. The method of claim 1, wherein variations in output light intensity between the plurality of LEDs, when driven at the operational drive currents, are less than 50 percent.

6. A method of generating light, comprising the steps of:

providing a plurality of light emitting diodes (LEDs), wherein at least one of the plurality of LEDs has an emission wavelength that falls outside of an acceptable wavelength range when the plurality of LEDs are driven at a common drive current; and

driving the plurality of LEDs with respective operational drive currents, wherein the operational drive current that drives the at least one of the plurality of LEDs is adjusted such that the emission wavelength of the at least one of the plurality of LEDs falls within the acceptable wavelength range.

7. The method of claim 6, wherein the step of driving each of the plurality of LEDs comprises the steps of:

identifying the at least one of the plurality of LEDs with an emission wavelength that falls outside of the acceptable wavelength range; and

determining, based on the emission wavelength of the at least one of the plurality of LEDs at the common drive current, an operational drive current for the at least one of the plurality of LEDs that will adjust the emission wavelength of the at least one of the plurality of LEDs to fall within the acceptable wavelength range.

8. The method of claim 7, wherein the step of identifying the at least one of the plurality of LEDs comprises the steps of:

driving each of the plurality of LEDs with the common drive current; and

determining an emission wavelength for each of the plurality of LEDs at the common drive current.

9. The method of claim 6, wherein variations in output light intensity between the plurality of LEDs, when driven at the operational drive currents, are less than 50 percent.

10. A method of generating light with a plurality of light emitting diodes (LEDs), comprising the steps of:

determining a target wavelength for the light;

determining an acceptable wavelength variation between the plurality of LEDs;

determining an available bandwidth based on a number of discrete drive currents available for driving the plurality of LEDs;

determining a wavelength distribution, wavelength range and wavelength span of the plurality of LEDs at a common drive current;

performing, if the wavelength distribution of the plurality of LEDs at the common drive current corresponds to a substantially normal distribution or if the wavelength span is less than the available bandwidth, the steps of;

determining a midpoint of the wavelength distribution,

designating, as a first reference LED, an LED with an output wavelength that is closest to the midpoint of the wavelength distribution, when driven at the common drive current, and

determining a first reference drive current that will adjust the output wavelength of the first reference LED to substantially coincide with the target wavelength;

performing, if the wavelength distribution of the plurality of LEDs at the common drive current does not correspond to a substantially normal distribution and the wavelength span is greater than or equal to the available bandwidth, the steps of;

determining a wavelength window having a bandwidth substantially equal to the available bandwidth, and having a wavelength range that encompasses output wavelengths of a maximum number of the plurality of LEDs, when the plurality of LEDs are driven at the common drive current,

designating, as a first reference LED, an LED with an output wavelength that is closest to a midpoint of the wavelength window, when driven at the common drive current, and

determining a first reference drive current that will adjust the output wavelength of the first reference LED to substantially coincide with the target wavelength;

establishing a first LED category having a wavelength span substantially equal to the acceptable wavelength variation;

grouping LEDs that fall within a wavelength range of the first LED category, when driven at a first operational drive current, into the first LED category;

establishing additional LED categories with respective wavelength spans substantially equal to the acceptable wavelength variation, wherein the number of additional LED categories established is equal to the number of discrete drive currents available, and wherein each additional LED category borders at least one other LED category;

grouping LEDs not grouped into the first LED category into the additional LED categories if their output wavelengths at the first reference drive current fall within a wavelength range of any of the additional categories;

for each additional LED category, determining a respective operational drive current that will adjust the output wavelength of the second reference LED to substantially coincide with the target wavelength;

driving the LEDs in the first LED category with the first operational drive current that is equal to the first reference drive current; and

driving the LEDs in the additional categories with their respective operational drive currents.

11. The method of claim 10, wherein the number of discrete drive currents available is multiplied by the acceptable wavelength variation to determine the available bandwidth.

12. The method of claim 10, wherein the number of discrete drive currents available for driving the plurality of LEDs is an odd number, and wherein the first LED category established has a wavelength range with minimum and maximum wavelengths defined by the following relationships:

13. The method of claim 10, wherein the number of discrete drive currents available for driving the plurality of LEDs is an even number, and wherein the first LED category established has a wavelength range with minimum and maximum wavelengths defined by the following relationships:

 

minimum wavelength=target wavelength;

and

14. The method of claim 10, wherein variations in output light intensity between the plurality of LEDs, when driven at the operational drive currents, are less than 50 percent.

15. A lighting system, comprising:

a plurality of light emitting diodes (LEDs) that, when driven with a common drive current, collectively emit light with an initial wavelength variation; and

a drive circuit for driving the plurality of LEDs, wherein the drive circuit drives each of the plurality of LEDs with a respective operational drive current such that the plurality of LEDs collectively emit light with an operational wavelength variation that is less than the initial wavelength variation.

16. The system of claim 15, wherein the operational wavelength variation corresponds to an operational wavelength band.

17. The system of claim 15, wherein variations in output light intensity between the plurality of LEDs, when driven at the operational drive currents, are less than 50 percent.

18. A lighting system, comprising:

a plurality of light emitting diodes (LEDs), wherein at least one of the plurality of LEDs has an emission wavelength that falls outside of an acceptable wavelength range when the plurality of LEDs are driven at a common drive current; and

a drive circuit for driving the plurality of LEDs with respective operational drive currents, wherein the operational drive current that drives the at least one of the plurality of LEDs is adjusted such that the emission wavelength of the at least one of the plurality of LEDs falls within the acceptable wavelength range.

19. The system of claim 18, wherein the acceptable wavelength range corresponds to an acceptable wavelength band.

20. The system of claim 18, wherein variations in output light intensity between the plurality of LEDs, when driven at the operational drive currents, are less than 50 percent.