Illumination device and temperature compensation method

• US 9,237,620 B1
• Filed: 08/20/2013
• Issued: 01/12/2016
• Est. Priority Date: 08/20/2013
• Status: Active Grant

• Current Assignee: Lutron Technology Company LLC
• Original Assignee: Ketra, Inc. (Lutron Electronics)
• Inventors: Knapp, David J., Ho, Horace C., Savage, Joseph A.
• Primary Examiner(s): Cox, Cassandra
• Application Number:
  US13/970,944
• Time in Patent Office: 875 Days
• Field of Search: None
• US Class Current: 1/1
• CPC Class Codes: H05B 45/10 Controlling the intensity of the light
• CPC Class Codes: H05B 45/12 using optical feedback
• CPC Class Codes: H05B 45/18 using temperature feedback
• CPC Class Codes: H05B 45/20 Controlling the colour of the light
• CPC Class Codes: H05B 45/22 using optical feedback
• CPC Class Codes: H05B 45/28 using temperature feedback
• CPC Class Codes: H05B 45/375 using buck topology
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines
• CPC Class Codes: H05B 47/19 via wireless transmission
• CPC Class Codes: H05B 47/195 the transmission using visible or infrared light

Abstract

An illumination device comprising a plurality of light emitting diodes (LEDs) and a method for controlling the illumination device, so as to maintain a desired luminous flux and/or a desired color point of the device over variations in temperature and process, is provided herein. According to one embodiment, the method may include measuring a forward voltage developed across a first LED of the illumination device upon applying a first drive current to the first LED, determining a drive current needed to achieve a desired luminous flux from the first LED using the measured forward voltage, a table of stored calibration values correlating forward voltage and drive current to luminous flux at a plurality of different temperatures, and one or more interpolation techniques, and driving the first LED with the determined drive current to produce illumination having the desired luminous flux. The steps of measuring, determining and driving may be performed for each of the plurality of LEDs.

Claims

1. A method for controlling an illumination device, wherein the method comprises:

measuring a forward voltage developed across a first light emitting diode (LED) of the illumination device upon applying a first drive current to the first LED;

determining a drive current needed to achieve a desired luminous flux from the first LED using the measured forward voltage, a table of stored calibration values correlating forward voltage and drive current to luminous flux at a plurality of different temperatures, and one or more interpolation techniques; and

driving the first LED with the determined drive current to produce illumination having the desired luminous flux.

2. The method as recited in claim 1, wherein the first drive current ranges between approximately 0.1 mA and approximately 10 mA.

3. The method as recited in claim 1, further comprising detecting an ambient temperature surrounding the first LED and performing the steps of measuring and determining only when the ambient temperature changes by a specified amount.

4. The method as recited in claim 1, wherein the illumination device comprises a plurality of LEDs including the first LED, and wherein the steps of measuring, determining and driving are performed for each of the plurality of LEDs.

5. The method as recited in claim 4, wherein the method further comprises:

driving the plurality of LEDs with the determined drive currents substantially continuously to produce illumination; and

periodically turning the plurality of LEDs off for short durations of time during a compensation period, wherein the step of measuring a forward voltage is performed for each LED, one at a time, during the short durations of time the plurality of LEDs are periodically turned off.

6. The method as recited in claim 5, wherein during the compensation period, the step of driving the plurality of LEDs comprises increasing the determined drive currents supplied to the plurality of LEDs by a small amount when the LEDs are on to compensate for the lack of illumination when the LEDs are periodically turned off.

7. The method as recited in claim 6, wherein the small amount comprises approximately 1-10% of the determined drive currents.

8. The method as recited in claim 1, wherein the table of stored calibration values comprises:

a first forward voltage value measured across the first LED using the first drive current when the first LED was previously subjected to a first temperature;

a second forward voltage value measured across the first LED using the first drive current when the first LED was previously subjected to a second temperature;

a first plurality of luminous flux values detected from the first LED using a plurality of different drive currents when the first LED was previously subjected to the first temperature; and

a second plurality of luminous flux values detected from the first LED using the plurality of different drive currents when the first LED was previously subjected to the second temperature.

9. The method as recited in claim 8, wherein the illumination device comprises a plurality of LEDs including the first LED, and wherein the table of stored calibration values further comprises a respective first forward voltage value, a respective second forward voltage value, a respective first plurality of luminous flux values, and a respective second plurality of luminous flux values for each of the plurality of LEDs.

10. The method as recited in claim 8, wherein the step of determining a drive current comprises:

calculating a third plurality of luminous flux values corresponding to the forward voltage measured across the first LED by interpolating between the first plurality of luminous flux values and the second plurality of luminous flux values;

generating a relationship between the third plurality of luminous flux values, if the desired luminous flux differs from one of the third plurality of luminous flux values; and

determining the drive current needed to achieve the desired luminous flux by selecting, from the generated relationship, a drive current corresponding to the desired luminous flux.

11. The method as recited in claim 10, wherein the step of calculating a third plurality of luminous flux values comprises using a linear interpolation technique or a non-linear interpolation technique to interpolate between the first and second plurality of luminous flux values, and wherein selection between the linear interpolation technique and the non-linear interpolation technique is made based on a color of the first LED.

12. The method as recited in claim 10, wherein the step of generating a relationship comprises applying a higher-order interpolation to the third plurality of luminous flux values to generate a non-linear relationship between luminous flux and drive current.

13. The method as recited in claim 10, wherein the step of generating a relationship comprises applying a piece-wise linear interpolation to the third plurality of luminous flux values to approximate a non-linear relationship between luminous flux and drive current.

14. The method as recited in claim 10, wherein the step of generating a relationship comprises assuming a typical curvature from data sheets provided by an LED manufacturer.

15. An illumination device, comprising:

a plurality of light emitting diode (LED) chains;

a storage medium configured for storing a table of calibration values correlating forward voltage and drive current to luminous flux at a plurality of temperatures for each of the plurality of LED chains;

a driver circuit configured for driving the plurality of LED chains substantially continuously to produce illumination, periodically turning the plurality of LED chains off for short durations of time during a compensation period, and supplying a small drive current to each LED chain, one chain at a time, during the short durations of time to measure an operating forward voltage developed across each LED chain; and

a control circuit configured for determining respective drive currents needed to achieve a desired luminous flux from each LED chain using the operating forward voltages measured across each LED chain, the table of calibration values and one or more interpolation techniques.

16. The illumination device as recited in claim 15, wherein each LED chain is configured for producing illumination at a different peak wavelength.

17. The illumination device as recited in claim 15, wherein the small drive current ranges between approximately 0.1 mA and approximately 10 mA.

18. The illumination device as recited in claim 15, wherein for each LED chain, the table of calibration values comprises:

a first forward voltage value measured across the LED chain using the first drive current when the LED chain was previously subjected to a first temperature;

a second forward voltage value measured across the LED chain using the first drive current when the LED chain was previously subjected to a second temperature;

a first plurality of luminous flux values detected from the LED chain using a plurality of different drive currents when the LED chain was previously subjected to the first temperature; and

a second plurality of luminous flux values detected from the LED chain using the plurality of different drive currents when the LED chain was previously subjected to the second temperature.

19. The illumination device as recited in claim 18, wherein the control circuit is configured to:

calculate a third plurality of luminous flux values corresponding to an operating forward voltage measured across the LED chain by interpolating between the first plurality of luminous flux values and the second plurality of luminous flux values;

generate a relationship between the third plurality of luminous flux values, if the desired luminous flux differs from one of the third plurality of luminous flux values;

determine a drive current needed to achieve a desired luminous flux from the LED chain by selecting, from the generated relationship, a drive current corresponding to the desired luminous flux.

20. The illumination device as recited in claim 19, wherein the control circuit is configured to calculate the third plurality of luminous flux values by using a linear interpolation technique or a non-linear interpolation technique to interpolate between the first and second plurality of luminous flux values, and wherein selection between the linear interpolation technique and the non-linear interpolation technique is made based on a color of the LED chain.

21. The illumination device as recited in claim 19, wherein the control circuit is configured to generate the relationship by applying a higher-order interpolation to the third plurality of luminous flux values to generate a non-linear relationship between luminous flux and drive current for the LED chain.

22. The illumination device as recited in claim 19, wherein the control circuit is configured to generate the relationship by applying a piece-wise linear interpolation to the third plurality of luminous flux values to approximate a non-linear relationship between luminous flux and drive current.

23. The illumination device as recited in claim 19, wherein the control circuit is configured to generate the relationship by assuming a typical curvature from data sheets provided by a manufacturer of the LED chain.

24. The illumination device as recited in claim 15, further comprising a phase locked loop (PLL) coupled to an AC mains and configured for producing a timing signal in synchronization with a frequency of the AC mains, wherein the timing signal is supplied to the driver circuit for periodically turning the plurality of LED chains off for the short durations of time during the compensation period.

25. The illumination device as recited in claim 15, wherein during the compensation period, the control circuit instructs the driver circuit to increase the drive currents supplied to the plurality of LED chains by a small amount when the LED chains are on to compensate for the lack of illumination when the LED chains are periodically turned off.

26. The illumination device as recited in claim 15, further comprising a temperature sensor configured for detecting an ambient temperature surrounding the plurality of LED chains, and wherein the control circuit is further configured for determining the respective drive currents needed to achieve the desired luminous flux from each LED chain only when the ambient temperature changes by a specified amount.