Average light sensing for PWM control of RGB LED based white light luminaries

• US 6,596,977 B2
• Filed: 10/05/2001
• Issued: 07/22/2003
• Est. Priority Date: 10/05/2001
• Status: Expired due to Term

• Current Assignee: Signify Holding B.V. (Signify N.V.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Muthu, Subramanian, Schuurmans, Frank
• Primary Examiner(s): Porta, David
• Assistant Examiner(s): Meyer, David C
• Application Number:
  US09/972,111
• Publication Number:
  US 20030066945A1
• Time in Patent Office: 655 Days
• Field of Search: 250/205, 250/214.R, 250/214.AG, 250/552, 327/514, 356/221-223, 315/291, 315/307-309, 315/158, 315/159, 345/82, 345/83
• US Class Current: 250/205
• CPC Class Codes: H05B 45/22 using optical feedback
• CPC Class Codes: H05B 45/28 using temperature feedback

Abstract

An LED array is controlled by determining a constant relating the peak light output of an LED to the peak driving current of a PWM pulse driving the LED, and multiplying the average current of the PWM pulse by the constant to obtain a value of average light output for the LED. The constant may be determined by simultaneously measuring peak light output of the LED and peak current of a PWM pulse driving the LED. The constant is then calculated by dividing the peak light output by the peak current of the PWM pulse. By making the simultaneous measurements at a time during the duration of the PWM pulse where the pulse has reached its full magnitude, rise and fall times of the pulse do not affect the measurements. The average current of the PWM pulse may be determined by a variety of methods including integrating current in the PWM pulse over time, or passing the PWM current through a low pass filter configured for providing an average value of PWM current. Determining average current in this manner further reduces the effect of rise and fall time on determining the average light output of the LED.

Claims

1. A method for determining the average light output of an LED having a peak light output, with the LED being driven by a PWM pulse having a peak current and an average current, the method comprising:

determining a constant relating the peak light output of the LED and the peak current of the PWM pulse; and

multiplying the average current of the PWM pulse by the constant.

2. The method of claim 1 wherein determining the constant comprises simultaneously measuring the peak light output of the LED and the peak current of the PWM pulse and calculating the constant by dividing the peak light output by the peak current.

3. The method of claim 2 further comprising:

determining the average value of current in the PWM pulse.

4. The method of claim 2 further comprising:

determining the average value of current in the PWM pulse by integrating the current in the PWM pulse over time.

5. The method of claim 2 further comprising:

determining the average value of current in the PWM pulse by passing the current in the PWM pulse through a low pass filter configured for producing an average value of current in the PWM pulse.

6. The method of claim 3 further comprising:

multiplying the constant by the average value of the current in the PWM pulse.

7. The method of claim 1 further comprising:

determining the average value of current in the PWM pulse by integrating the current value of PWM pulse over time.

8. A method for determining the average light output of a first LED of a first and a second LED, each having a peak light output, when the first and second LED are driven respectively by a first and a second PWM pulse, with the first and second PWM pulses each having a peak current and an average current, the method comprising:

determining a first LED constant relating the peak light output of the first LED with the peak current of the first PWM pulse; and

multiplying the average current of the first PWM pulse by the first LED constant.

9. The method of claim 8 wherein determining the first LED constant comprises simultaneously measuring the peak light output of the first LED and the peak current of the first PWM pulse and calculating the first LED constant by dividing the peak light output of the first LED by the peak current of the first PWM pulse.

10. The method of claim 9 further comprising:

determining the average value of current in the first PWM pulse.

11. The method of claim 9 wherein the first and second PWM pulses partially overlap as a function of time and the peak light output of the first and second LED are measured with a single light sensor, the method further comprising:

simultaneously measuring the peak light output and peak current of one of the first and second LEDs at a point in time when the first and second PWM pulses do not overlap as a function of time;

simultaneously measuring the peak light output from both of the first and second LEDs and the peak current of the other of the first and second PWM pulses at a point in time when the first and second PWM pulses overlap as a function of time; and

determining the peak light output of the other of the first and second LEDs by subtracting the peak light output measured for the one of the first and second LEDs at the point in time when the first and second PWM pulses do not overlap from the combined peak light output of the first and second LED'"'"'s measured at the point in time when the first and second PWM pulses do overlap.

12. The method of claim 11 further comprising:

determining the second LED constant by measuring the peak current of the second PWM pulse simultaneously with measuring the combined peak light output of the first and second LEDs; and

dividing the peak light output of the second LED by the peak current of the second PWM pulse.

13. The method of claim 12 further comprising:

determining the average value of current in the second PWM pulse.

14. The method of claim 13 for further determining the average light output of a third LED having a peak light output, when the first, second, and third LED are driven respectively by a first, a second, and a third PWM pulse, with the first, second, and third PWM pulses each having a peak current and an average current, and wherein the first, second, and third PWM pulses partially overlap as a function of time and the peak light output of the first, second, and third LED are measured with the single light sensor, the method further comprising:

determining a third LED constant relating the peak light output of the third LED with the peak current of the third PWM pulse, and multiplying the average current of the third PWM pulse by the third LED constant.

15. The method of claim 14 further comprising:

determining the third LED constant by simultaneously measuring the peak light output and peak current of the third LED at a point in time when the first, second, and third PWM pulses do not overlap as a function of time; and

dividing the peak light output of the third LED by the peak current of the third LED.

16. The method of claim 15 further comprising:

determining the average value of current in the third PWM pulse.

17. The method of claim 16 further comprising:

multiplying the third LED constant by the average value of the current in the third PWM pulse.

18. An apparatus for determining the average light output of an LED having a peak light output, with the LED being driven by a PWM pulse having a peak current and an average current, the apparatus comprising:

means for determining a constant relating the peak light output of the LED and the peak current of the PWM pulse; and

means for multiplying the average current of the PWM pulse by the constant.

19. The apparatus of claim 18 wherein the means for determining the constant comprises:

means for simultaneously measuring the peak light output of the LED and the peak current of the PWM pulse; and

means for calculating the constant by dividing the peak light output by the peak current.

20. Code on a computer readable medium for determining the average light output of an LED having a peak light output, with the LED being driven by a PWM pulse having a peak current and an average current, the code comprising instructions for determining a constant relating the peak light output of the LED and the peak current of the PWM pulse, and instructions for multiplying the average current of the PWM pulse by the constant.

21. The code of claim 20 wherein the instructions for determining the constant comprises instructions for simultaneously measuring the peak light output of the LED and the peak current of the PWM pulse and instructions for calculating the constant by dividing the peak light output by the peak current.