LED junction temperature tester

• US 7,052,180 B2
• Filed: 01/06/2003
• Issued: 05/30/2006
• Est. Priority Date: 01/04/2002
• Status: Expired due to Fees

• Current Assignee: Kelvin Shih
• Original Assignee: Kelvin Shih
• Inventors: Shih, Kelvin
• Primary Examiner(s): VERBITSKY, GAIL KAPLAN
• Application Number:
  US10/337,246
• Publication Number:
  US 20030133491A1
• Time in Patent Office: 1,240 Days
• Field of Search: 374/163, 374/1, 374/178, 374/2, 374/133, 374/121, 374/183, 257/48, 327/108, 327/109, 315/291, 315/224, 315/185.R, 315/200.A
• US Class Current: 374/178
• CPC Class Codes: G01K 7/01 using semiconducting elements having PN junctions G01K7/02, G01K7/16, G01K7/30 take precedence
• CPC Class Codes: G01R 31/2635 Testing light-emitting diodes, laser diodes or photodiodes
• CPC Class Codes: H01L 2924/00 Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
• CPC Class Codes: H01L 2924/0002 Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00

Abstract

An instrument measures the LED junction temperature directly by taking advantage of the linear relationship between the forward current driven through the LED, the forward drop of the LED, and the junction temperature to determine the LED junction temperature.

Calibration is conducted by placing two LEDs from the same family in ambient temperature and passing a small test current through each of the LEDs to obtain the forward drop of the LED at ambient temperature. The LED under test is then placed in an environmentally-controlled chamber where the temperature is raised a known amount above ambient temperature. Known low and high voltage values are associated with the ambient temperature and the environmental chamber temperature, causing the LED under test becomes a calibrated thermometer that can measure its own junction temperature due to the linear relationship between the forward drop and the junction temperature.

Claims

1. A test instrument for measuring a junction temperature of an LED, comprising:

an interface that holds a test LED and a reference LED;

a first current source that sends a fixed current through the test LED and the reference LED during a calibration mode;

a second current source that sends an adjustable drive current through the test LED during a run mode; and

a differential meter that measures a difference between a forward drop of the test LED when the adjustable drive current goes through the test LED and a forward drop of the test LED at ambient temperature to cause the test LED to serve as its own temperature sensor.

2. The test instrument of claim 1, further comprising a switch that bypasses the second current source during the calibration mode and connects the second current source to the test LED during the run mode.

3. The test instrument of claim 2, wherein the switch operates such that the fixed current goes through the test LED during the calibration mode and the fixed current and the adjustable drive current go through the test LED during the run mode.

4. The test instrument of claim 2, further comprising a timing circuit that generates a duty cycle pulse to control the switch.

5. The test instrument of claim 4, wherein the timing circuit generates a sample and hold pulse based on the duty cycle pulse, and wherein the test instrument further comprises a sample and hold circuit driven by the sample and hold pulse to operate the sample and hold circuit during the run mode, wherein the sample and hold circuit samples and holds the forward drop of the test LED during the run mode.

6. The test instrument of claim 5, wherein the sample and hold pulse is shorter than the duty cycle pulse.

7. The test instrument of claim 4, wherein to timing circuit generates a drive pulse that is longer than to duty cycle pulse to drive the switch.

8. The test instrument of claim 1, wherein the fixed current is sent through the reference LED as the fixed current and the adjustable drive current are sent through the test LED during the run mode.

9. The test instrument of claim 1, further comprising a potentiometer to eliminate a forward drop difference between the test LED and the reference LED during the calibration mode.

10. The test instrument of claim 1, further comprising:

an inverting summing amplifier to convert an output of the differential meter to a temperature reading reflecting to junction temperature of the tested LED; and

a display coupled to the inverting summing amplifier to display the temperature reading.

11. A test instrument for measuring a junction temperature of an LED, comprising:

an interface that holds a test LED and a reference LED;

a first current source that sends a fixed current through the test LED and the reference LED during a calibration mode;

a second current source that sends an adjustable drive current through the test LED during a run mode;

a switch that bypasses the second current source during the calibration mode such that the first drive current goes through the test LED during the calibration mode and the fixed current and the adjustable current go through the test LED during the run mode;

a sample and hold circuit that samples and holds the junction temperature of the test LED during the run mode;

a differential meter that measures a difference between a forward drop of the test LED when the adjustable drive current goes through the test LED and a forward drop of the test LED at ambient temperature to cause the test LED to serve as its own temperature sensor; and

a display coupled to an inverting summing amplifier to display at least one of difference measured by the differential meter and a temperature reading.

12. The test instrument of claim 11, further comprising a timing circuit that generates a duty cycle pulse to control the switch.

13. The test instrument of claim 12, wherein the timing circuit generates a sample and hold pulse based on the duty cycle pulse, wherein the sample and hold pulse drives the sample and hold circuit.

14. The test instrument of claim 13, wherein the sample and hold pulse is shorter than the duty cycle pulse.

15. The test instrument of claim 12, wherein the timing circuit generates a drive pulse that is longer than the duty cycle pulse to drive the switch.

16. The test instrument of claim 11, wherein the fixed current is sent through the reference LED as the fixed current and the adjustable drive current are sent through the test LED during the run mode.

17. The test instrument of claim 11, further comprising a potentiometer to eliminate a forward drop difference between the test LED and the reference LED during the calibration mode.