LED junction temperature tester
First Claim
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.
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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.
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Citations
17 Claims
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1. A test instrument for measuring a junction temperature of an LED, comprising:
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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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A test instrument for measuring a junction temperature of an LED, comprising:
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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. - View Dependent Claims (12, 13, 14, 15, 16, 17)
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Specification