Illumination device and method for avoiding flicker

• US 9,578,724 B1
• Filed: 08/20/2013
• Issued: 02/21/2017
• Est. Priority Date: 08/20/2013
• Status: Expired

• 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): Tra, Quan
• Application Number:
  US13/970,990
• Time in Patent Office: 1,281 Days
• Field of Search: 315/186, 315/151, 315/152
• US Class Current: 1/1
• CPC Class Codes: H04B 10/116 Visible light communication
• 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

Abstract

An illumination device comprising a plurality of light emitting diodes (LEDs) and a method for controlling the illumination device while avoiding flicker in the LED output is provided herein. According to one embodiment, the method may include driving the plurality of LEDs substantially continuously with drive currents configured to produce illumination, periodically turning the plurality of LEDs off for short durations of time during a first period to take measurements or communicate optical data, and increasing the drive currents supplied to the plurality of LEDs by a small amount when the LEDs are on during the first period to compensate for lack of illumination when the LEDs are periodically turned off during the first period.

Claims

1. An illumination device, comprising:

a plurality of light emitting diode (LED) chains;

a driver circuit configured for driving the plurality of LED chains with drive currents substantially continuously to produce illumination, periodically turning the plurality of LED chains off during a first period to take measurements or communicate optical data, and supplying a non-operative drive current to each LED chain, one chain at a time, during 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, a table of calibration values and one or more interpolation techniques, wherein during the first period, the control circuit instructs the driver circuit to increase the respective drive currents supplied to the plurality of LED chains by a small amount when the plurality of LED chains are on to compensate for a decrease in or lack of illumination when the LED chains are periodically turned off during the first period.

2. The illumination device as recited in claim 1, wherein each LED chain is configured for producing illumination at a different peak wavelength.

3. The illumination device as recited in claim 1, wherein the small amount comprises approximately 1% to approximately 10% of the drive currents supplied to the plurality of LED chains to produce illumination substantially continuously.

4. The illumination device as recited in claim 1, 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 first period.

5. The illumination device as recited in claim 1, further comprising a storage medium configured for storing the table of calibration values, and wherein the table of calibration values correlate forward voltage and drive current to luminous flux at a plurality of temperatures for each of the plurality of LED chains.

6. The illumination device as recited in claim 5, wherein for each LED chain, the table of calibration values comprises:

a first forward voltage value measured across the LED chain using a non-operative 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 non-operative 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.

7. The illumination device as recited in claim 1, wherein the non-operative drive current ranges between approximately 0.1 mA and approximately 10 mA.

8. The illumination device as recited in claim 6, wherein the control circuit is further configured to:

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

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; and

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

9. The illumination device as recited in claim 8, 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 given LED chain.

10. The illumination device as recited in claim 8, 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 given LED chain.

11. The illumination device as recited in claim 8, 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 for the given LED chain.

12. The illumination device as recited in claim 8, wherein the control circuit is configured to generate the relationship by assuming a typical curvature from data sheets provided by a manufacturer of the given LED chain.

13. The illumination device as recited in claim 1, further comprising a phase locked loop (PLL), which is coupled to an AC mains and configured to lock onto a frequency and phase of the AC mains.

14. The illumination device as recited in claim 13, wherein the control circuit is coupled to the PLL and configured to produce a timing signal in synchronization with the AC mains, wherein the timing signal is supplied to the driver circuit for periodically turning the LED chains off in synchronization with the AC mains to produce a plurality of time slots in a first communication channel for communicating optical data.

15. A method for controlling an illumination device comprising a plurality of light emitting diodes (LEDs), the method comprising:

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

periodically turning the plurality of LEDs off during a first period to take measurements or communicate optical data;

supplying a non-operative drive current to each LED, one LED at a time, during the first period, in which the LEDs are periodically turned off, to measure an operating forward voltage developed across each LED;

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

increasing the respective drive currents supplied to the plurality of LEDs by a small amount when the LEDs are on during the first period to compensate for a decrease in or lack of illumination when the LEDs are periodically turned off during the first period.

16. The method as recited in claim 15, wherein the small amount comprises approximately 1% to approximately 10% of the drive currents configured to produce illumination.

17. The method as recited in claim 15, wherein the step of periodically turning the plurality of LEDs off for the short durations of time comprises periodically turning the plurality of LEDs off in synchronization with an AC mains frequency to generate a plurality of time slots in the first period.

18. The method as recited in claim 17, further comprising measuring an output characteristic of each LED, one LED at a time, during the time slots.

19. The method as recited in claim 18, wherein the output characteristic is selected from a group consisting of forward voltage, luminous flux or chromacity.

20. The method as recited in claim 17, further comprising configuring at least one of the plurality of LEDs for communicating optical data during the time slots.

21. The method as recited in claim 17, further comprising configuring at least one of the plurality of LEDs for measuring ambient light during the time slots.