METHODS, APPARATUSES, AND SYSTEMS FOR OPERATING LIGHT EMITTING DIODES AT LOW TEMPERATURE

• US 20160050725A1
• Filed: 10/29/2015
• Published: 02/18/2016
• Est. Priority Date: 04/30/2013
• Status: Active Grant

• Current Assignee: Digital Lumens, Inc. (Ams-Osram AG)
• Original Assignee: Digital Lumens, Inc. (Ams-Osram AG)
• Inventors: Johnston, Scott D., Elledge, Christopher, Morgan, Frederick M., Egan, John F., Medal, Hugh
• Granted Patent: US 9,924,576 B2
• Time in Patent Office: Days
• 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 45/395 Linear regulators
• 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: H05B 45/54 in a series array of LEDs
• CPC Class Codes: H05B 45/56 involving measures to prevent abnormal temperature of the LEDs

Abstract

Light-emitting diodes (LEDs) generate light more efficiently than high-intensity discharge lamps or high-intensity fluorescent lamps. Driving a series of LEDs with a constant-voltage primary supply and a low-voltage LED driver keeps efficiency high. Unfortunately, LED forward voltage varies as a function of temperature: at low temperature, the forward voltage rises. Placing the LEDs in series magnifies the forward voltage increases. This makes it difficult to drive a series of LEDs at low temperature with a constant-voltage supply because the forward voltage can exceed the power supply voltage. To account for this behavior, an exemplary LED lighting fixture includes a “bypass” circuit that, when engaged, effectively removes at least one LED from each series string of LEDs to bring the total forward voltage below the power supply voltage. The low-voltage driver circuit monitors temperature, and engages the “bypass” circuit when necessary to ensure that DC voltage is not exceeded.

Claims

1. A lighting fixture comprising:

a plurality of light emitting diodes arranged in series, the plurality of light emitting diodes comprising at least one first light emitting diode;

a constant-voltage power supply, operably coupled to the plurality of light emitting diodes, to provide a constant voltage across the plurality of light emitting diodes;

a sensor, in electrical communication with the plurality of light emitting diodes, to measure a decrease in temperature of the plurality of light emitting diodes, the decrease in temperature of the plurality of light emitting diodes causing an increase in series voltage across the plurality of light emitting diodes; and

a bypass circuit, operably coupled to the sensor, to short-circuit the at least one first light emitting diode in response to the increase in the series voltage so as to reduce the series voltage below the constant voltage provided by the constant-voltage power supply.

2. The apparatus of claim 1, wherein:

the bypass circuit is configured to enable the at least one first light emitting diode for a predetermined period after disabling the at least one first light emitting diode in response to the increase in the series voltage; and

the sensor is configured to measure a change in the temperature of the plurality of light emitting diodes while the at least one first light emitting diode is enabled.

3. The apparatus of claim 2, wherein the predetermined period is less than about 20 milliseconds.

4. The apparatus of claim 3, wherein the bypass circuit is configured to short-circuit the at least one first light emitting diode after the sensor has measured the change in temperature of the plurality of light emitting diodes.

5. The apparatus of claim 1, wherein the bypass circuit is configured to short-circuit the at least one first light emitting diode if the series voltage exceeds a threshold voltage.

6. A method of operating a plurality of light emitting diodes arranged in series at low temperature, the method comprising:

(A) providing, via a constant-voltage power supply operably coupled to the plurality of light emitting diodes, a constant voltage across the plurality of light emitting diodes;

(B) measuring, with a sensor in electrical communication with the plurality of light emitting diodes, a decrease in the temperature of the plurality of light emitting diodes, the decrease in temperature of the plurality of light emitting diodes corresponding to an increase in series voltage across the plurality of light emitting diodes; and

(C) short-circuiting, with a bypass circuit operably coupled to the sensor, at least one first light emitting diode in the plurality of light emitting diodes in response to the increase in the series voltage so as to reduce the series voltage below the constant voltage provided by the constant-voltage power supply.

7. The method of claim 6, further comprising, after (C):

(D) enabling, with the bypass circuit, the at least one first light emitting diode; and

(E) measuring, with the sensor, a change in the temperature of the plurality of light emitting diodes while the at least one first light emitting diode is enabled.

8. The method of claim 7, wherein (D) comprises enabling the at least one first light emitting diode for a period less than about 20 milliseconds.

9. The method of claim 7, further comprising:

(F) short-circuiting, with the bypass circuit, the at least one first light emitting diode after measuring the change in the temperature of the plurality of light emitting diodes.

10. The method of claim 6, comprising:

disabling the at least one first light emitting diode if the series voltage exceeds the constant voltage provided by the constant-voltage power supply.

11. An apparatus for illuminating an environment at cold temperature, the apparatus comprising:

at least one light emitting diode;

a linear driver circuit, operably coupled to the at least one light emitting diode, to provide a drive current to the at least one light emitting diode;

a sensor, in electrical and/or thermal communication with the at least one light emitting diode, to sense a variation in the drive current from a predetermined drive current caused by a decrease in temperature of the at least one light emitting diode;

a processor, operably coupled to the to the sensor, to generate a drive current control signal based on at least in part on the variation measured by the sensor; and

a switch, operably coupled to the processor and to the linear driver circuit, to control the drive current provided to the at least one light emitting diode by the linear drive circuit in response to the drive current control signal from the processor.

12. The apparatus of claim 11, wherein the sensor is configured to sense the variation in the drive current based on a temperature of the at least one light emitting diode.

13. The apparatus of claim 11, wherein the drive current control signal comprises a pulse-width modulated digital signal.

14. The apparatus of claim 11, wherein the processor is configured to dim the at least one light emitting diode by varying the drive current control signal.

15. The apparatus of claim 11, wherein the switch comprises a transistor.

16. A method for illuminating an environment at cold temperature, comprising:

(A) providing a drive current to at least one light emitting diode with a linear driver circuit;

(B) sensing a variation in the drive current provided in (A) from a predetermined drive current caused by a decrease in temperature of the at least one light emitting diode;

(C) generating, with a processor, a drive current control signal based on at least in part on the variation measured in (B); and

(D) controlling the drive current provided to the at least one light emitting diode by the linear drive circuit in response to the drive current control signal generated in (C).

17. The method of claim 16, wherein (B) comprises:

sensing the variation in the drive current based on a temperature of the at least one light emitting diode.

18. The method of claim 16, wherein (C) comprises:

generating a pulse-width modulated digital signal representative of a desired change in the drive current.

19. The method of claim 16, wherein (D) comprises:

actuating a transistor in electrical communication with the linear driver circuit.

20. The method of claim 16, further comprising:

dimming the at least one light emitting diode by the processor by varying the drive current control signal.