LED junction temperature tester

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.

1 - 18 . (canceled)  
   
   
       19 . A method for measuring a junction temperature of a test LED, comprising: 
 driving a test LED and a reference LED with a first drive current at a first temperature;    linking the first temperature to a first known voltage;    raising the junction temperature of the test LED to a second temperature;    linking the second temperature to a second known voltage;    driving the test LED with a drive current; and    measuring a forward drop of the test LED, wherein the forward drop of the test LED is responsive to the first drive current and indicates the junction temperature of the LED based on a linear characteristic obtained from the first and second known voltages corresponding to the first and second temperatures.    
   
   
       20 . The method of  claim 19 , wherein the step of raising the junction temperature of the test LED comprises placing the test LED in a temperature-controlled environment and raising the temperature of the environment to the second temperature.  
   
   
       21 . The method of  claim 19 , wherein the driving step comprises driving the test LED with a first drive current from a constant current source and a second drive current from an adjustable current source.  
   
   
       22 . The method of  claim 21 , further comprising: 
 driving the reference LED with the first drive current when the test LED is driven by the first drive current and the second drive current; and    comparing the forward drop of the test LED with a reference forward drop of the reference LED to determine the junction temperature of the test LED.    
   
   
       23 . The method of  claim 19 , wherein the measuring step comprises: 
 sampling and holding the forward drop of the test LED; and    comparing the forward drop of the test LED with a reference forward drop of the reference LED to determine the junction temperature of the test LED.