Method of detecting a LED failure, a controller therefor, a lighting unit and lighting system

• US 9,161,414 B2
• Filed: 09/13/2013
• Issued: 10/13/2015
• Est. Priority Date: 10/04/2012
• Status: Active Grant

• Current Assignee: NXP B.V. (NXP Semiconductors NV)
• Original Assignee: NXP B.V. (NXP Semiconductors NV)
• Inventors: Brancken, Pascal, Nguyen, Viet Hoang, Surdeanu, Radu
• Primary Examiner(s): He, Amy
• Application Number:
  US14/026,156
• Publication Number:
  US 20140097849A1
• Time in Patent Office: 760 Days
• Field of Search: 324/414, 324/522
• US Class Current: 1/1
• CPC Class Codes: G01R 31/2635 Testing light-emitting diodes, laser diodes or photodiodes
• CPC Class Codes: H05B 45/50 responsive to malfunctions or undesirable behaviour of LEDs; responsive to LED life; Protective circuits
• CPC Class Codes: H05B 45/56 involving measures to prevent abnormal temperature of the LEDs
• CPC Class Codes: H05B 45/58 involving end of life detection of LEDs

Abstract

Disclosed herein is a method of detecting an LED failure in a series-connected string of LEDs using a parameter indicative of a voltage difference between a voltage across the string at a predetermined relatively high current and a voltage across the string at a predetermined relatively low current. The disclosure extends to controllers configured to detect an LED failure in a string of LEDs, to LED lighting units comprising such controllers, and to lighting subsystems, for instance automobile lighting subsystems.

Claims

1. A method of detecting an LED failure in a series-connected string of LEDs using a parameter indicative of a voltage difference between a voltage across the string at a predetermined relatively high current and a voltage across the string at a predetermined relatively low current,

wherein at both the relatively low current and the relatively high current levels the current through an un-failed LED is dominated by minority-carrier diffusion current, the method comprising:

determining a calibration value of the parameter during a first start-up phase;

storing the calibration value;

determining a subsequent value of the parameter during a subsequent measurement phase; and

,

in response to a difference between the calibration value and the subsequent value exceeding a threshold, determining that an LED failure has occurred.

2. The method of claim 1, wherein the relatively high current is such that the effect of a series resistance of an un-failed LED operating at the relatively high current is to reduce the current by no more that 1%, compared with an idealised un-failed LED having zero series resistance.

3. The method of claim 1, wherein each of the relatively high current and the relatively low current is in the range of 1 μ

A to 100 mA, and the relatively high current is at least 5 times greater than the relatively low current.

4. The method of claim 1 wherein the calibration value is determined when the string of LEDs is first operated in an application.

5. The method of claim 1, wherein, during normal operation, the string of LEDs is operated in a pulsed mode between an operating current, which is higher than the relatively high current, and a quiescent current, which is either the relatively low current or zero current.

6. The method of claim 5, further comprising, in response to determining that an LED failure has occurred, adjusting at least one of the operating current and a mark-space ratio of the pulsed mode to compensate an optical output from the string of LEDs for the LED failure.

7. A controller configured to detect an LED failure in a series-connected string of LEDs using a parameter indicative of a voltage difference between a voltage across the string at a predetermined relatively high current and a voltage across the string at a predetermined relatively low current, wherein at both the relatively low current and the relatively high current levels the current through an un-failed LED is dominated by minority-carrier diffusion current

the controller comprising:

a measuring unit configured to determine a calibration value of the parameter during a first start-up phase, and to determine a subsequent value of the parameter during a subsequent measurement phase;

a storage means configured to store the calibration value;

and a comparison unit configured to indicate an LED failure in response to a difference between the calibration value and the subsequent value exceeding a predetermined threshold.

8. A controller according to claim 7, wherein the high current is such that the effect of a series resistance of an un-failed LED operating at the high current is to reduce the current by no more that 1%, compared with an idealised un-failed LED having zero series resistance.

9. A controller according to claim 7, wherein each of the relatively high current and the relatively low current is in the range of 1 μ

A to 100 mA, and the relatively high current is at least 5 times greater than the relatively low current.

10. A controller according to claim 7, further configured to, during normal operation, operate the string of LEDs in a pulsed mode between an operating current, which is higher than the relatively high current, and a quiescent current, which is either the relatively low current or zero current.

11. A controller according to claim 10, further configured to, in response to determining that an LED failure has occurred, adjust at least one of the operating current and a mark-space ratio of the pulsed mode to compensate an optical output from the string of LEDs for the LED failure.

12. An LED lighting unit comprising a controller as claimed in claim 7, and a series-connected string of LEDs.

13. An automobile lighting system comprising at least one LED lighting unit as claimed in claim 12, and further comprising a microcontroller, the microcontroller comprising at least the controller as claimed in claim 12 and being configured to store LED failure information.