Abstract:
A system and method of testing High Brightness LED (HBLED) is provided, and more particularly, a system and method of Controlled Energy Testing of HBLED with improved accuracy and repeatability is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present utility patent application claims priority of U.S. Provisional Patent Application Ser. No. 61/230,968, filed Aug. 3, 2009; subject matter of which is incorporated herewith by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a system and method of testing solid state light emitting devices such as High Brightness Light Emitting Diodes (HBLED), and more particularly, to a system and method of controlled energy testing of HBLED with improved accuracy and repeatability of measurement. 
     BACKGROUND OF THE INVENTION 
     History of Light Emitting Diodes (LED) 
     The earliest records of scientific observation of the luminescence of forward-biased diodes began in 1907 with the discovery of the LED. This invention was widely credited to the Russian technologist Oleg Vladimirovich Losev, who observed that zinc oxide and silicon carbide diodes used in the receiver circuits of radio sets he was working with emitted a small amount of light when forward biased. Losev&#39;s research on the cause and nature of diode luminescence became the foundation for a series of 16 papers published from 1924 to 1930 which described the results of his research on light-emitting diodes (Light Emitting Diodes or LEDs). Tragically, Losev was trapped in Leningrad during Hitler&#39;s Siege of Leningrad and died of starvation in 1942. With his untimely death and the confusion in the aftermath of WWII, his work fell into obscurity. 
     The history of the LED resurfaces again in 1962 when American researchers demonstrated a functioning LED-based laser. Between 1962 and the mid-1990s, the LED became a staple electronic component and was used in a variety of signage and signaling applications. The next major breakthrough in LED occurred in 1995 when Isamu Akasaki and H. Amano demonstrated an operational GaN-based High Brightness Light Emitting Diode (HBLED). The HBLED is much brighter, more efficient, and can be constructed to generate more colors than the traditional LED. 
     The electrical efficiency and compact size of the HBLED has opened up a vast array of new applications for these solid state lighting devices. In recent years, HBLEDs have found their way into a variety of scientific and illumination applications, fueling the expansion of the market for HBLEDs to over 11 billion dollars in the year 2006. 
     While the name “Light Emitting Diode” accurately describes the device from the perspective of a physicist, the name does not accurately describe how the device is used. From an application perspective, the HBLED is generally not used as a diode. A diode is used to pass current flow when forward-biased and to block current flow when reverse-biased. If the water analogy is invoked, the diode acts as a one-way flow valve. The primary function of an HBLED is to emit light. The HBLED converts electrical power to optical power. In the water analogy model, the diode is a one-way flow valve and the HBLED is a generator. 
     LED Testing 
     The test methodologies used to evaluate HBLED devices in the manufacturing process are based on traditional diode tests, broadened to include the measurement of the power and color (i.e. wavelength) of the HBLEDs optical output. The typical HBLED test starts with a simplified diode test sequence in which the diode is reverse-biased to determine the amount of leakage current and then forward-biased using a forcing current while the forward voltage is measured. To measure the optical properties of the HBLED, a forcing current is applied while a photodetector and/or spectrometer are/is used to measure the power and color of the light emitted by the HBLED Device Under Test (DUT). 
     Testing HBLEDs using the traditional test methodology in the production environment is especially challenging as these tests must be performed in the shortest amount of time possible—often 50 milliseconds or less. In most cases, the production HBLED test sequence is completed before the device being tested has reached thermal equilibrium, which significantly reduces the accuracy and repeatability of the forward voltage measurement as well as the optical power and color measurements. 
     From a test and measurement perspective, since the HBLED is fundamentally an energy conversion device, one would expect the test methodology or strategy used to evaluate a HBLED would be very different than a diode test methodology or strategy. 
     SUMMARY OF THE INVENTION 
     The traditional approach to HBLED testing starts with the idea that the HBLED is a diode that emits light. Instead, the Controlled Energy Testing approach treats the HBLED as an energy transducer and assumes that the HBLED converts electrical power to optical power. The goal of the test, then, would be to evaluate the efficiency of that conversion in terms of energy-in vs. energy-out by measuring the optical power that is produced by the HBLED as a function of time while controlling the electrical power input to the HBLED DUT. 
     While both traditional and Controlled Energy Testing methods or strategies will allow measurement of the same properties of the devices, Controlled Energy Testing methods or strategies are better aligned to fit the true nature of the devices as well as the type of application in which the devices will be used. The Controlled Energy Testing methods or strategies improve the accuracy and, more importantly, the repeatability of the measurements without sacrificing the speed of the test. 
     Controlled Energy Testing methods or strategies overcome two fundamental weaknesses associated with the traditional method of HBLED testing and test systems. First, because traditional HBLED test systems are optimized to force current and measure voltage, the actual amount of power being sent to a HBLED DUT is not monitored, and is therefore unknown. Further, because traditional HBLED test system implementations do not provide precise control over the timing of all aspects of a test sequence, the precise amount of energy delivered to the DUT (power*time) cannot be determined either. Because of the uncertainty in the amount of power delivered to the DUT and the uncertainty in the timing of test and measurement sequences, the accuracy and repeatability of measured results are compromised and measured data will vary from test to test. 
     The Controlled Energy Testing strategy addresses these above issues by 1) forcing a programmable (given or predetermined) amount of power (not simply voltage or current but the product of both) during forward-bias and optical tests, 2) precisely controlling junction temperature of a DUT, and 3) precisely coordinating the timing of all test and measurement sequences including reverse voltage, forward voltage and output spectral power distribution. A Controlled Energy Testing sequence forces a given amount of power (time variant or invariant) to the DUT, bringing the junction temperature of the DUT to thermal equilibrium. After a pre-determined amount of energy has been sent to the DUT and the junction temperature of the DUT is stable, the color and power of the HBLED&#39;s optical output is measured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of one embodiment of a Controlled Energy Testing system in accordance with the principles of the present invention. 
         FIG. 2  shows a Controlled Energy Testing method with precise timing of input energy to a DUT and coordinated measurement of output energy and spectral power distribution after thermal equilibrium of the DUT is attained. 
         FIG. 3  shows one embodiment of an implementation of the Controlled Energy Testing system in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In one embodiment of the present invention, Controlled Energy Testing is implemented whereby the Controlled Energy Testing is a method for improving the accuracy and repeatability of measurements taken during the course of performing parametric and/or functional testing of electronic or optoelectronic devices. 
     The Controlled Energy Testing technique uses a programmable constant power source in conjunction with precise timing circuits to deliver a given or predetermined amount of power to a Device Under Test (DUT) for a predetermined amount of time, while coordinating timing of acquisition of various parametric data. 
       FIGS. 1 and 2  illustrate a system and method of a Controlled Energy Testing system  100  and method  200  for a DUT HBLED. The implementation of the Controlled Energy Testing system and method, e.g. a Controlled Energy Testing Parametric Measurement Unit (PMU)  300  (diagrammed in  FIG. 3 ) is based on an analog parametric measurement unit controlled by a network of processors. The design of the processor network is optimized to implement a control system required to provide the DUT with a predetermined amount of power, thus giving precise control over the timing of the HBLED test sequence. The ‘force power’ or ‘constant power source’ functionality of the Controlled Energy Testing PMU 300 is analogous to a constant voltage source or a constant current source. The term ‘constant power source’ means that the source adjusts both the output voltage and the current so as to ensure that a given amount of power is supplied to the HBLED DUT. The amount of power sent may vary with time or be constant over time. 
     In  FIG. 1 , the Controlled Energy Testing system  100  provides and inputs controlled energy test sequence (i.e. controlled electrical power) into a DUT  102  (e.g. LED or HBLED, etc.) which converts electrical power to optical power which is then acquired by a test data acquisition system  104 . The test data acquisition system  104  may include, in one embodiment, a spectrometer  106  for acquiring test data related to both power and color output, and in another embodiment, both a spectrometer  106  and a photodetector  108  whereby the spectrometer  106  acquires test data related to power and color, and the photodetector  108  acquires test data related to power output. It will be appreciated that the test data acquisition system  104  may include other suitable devices or configurations for acquiring other test data from the DUT  102  without departing from the scope or spirit of the present invention. 
       FIG. 2  shows a Controlled Energy Testing process  200  with precise timing of input energy to a DUT (in an exemplary case, a HBLED) and coordinated measurement of output energy and spectral power distribution after thermal equilibrium of the DUT HBLED is attained. 
     Initially the DUT HBLED is in a so-called “Cold-Junction” state, i.e. an unheated state of equilibrium at ambient temperature, typically at 25C. The method or process  200  starts with a step  202  of applying a series of short forced current pulses to a DUT HBLED. Then, a forward voltage is measured to monitor the junction temperature of the DUT HBLED in step  204 . Then, the controlled energy is applied in step  205 . If the junction temperature of DUT HBLED has reached thermal equilibrium in step  206 , then functional tests of DUT HBLED are performed in step  208 ; otherwise, if the junction temperature of DUT HBLED has not reached thermal equilibrium in step  206 , the process  200  goes back to step  205  by continuously applying controlled energy, and the junction temperature of the DUT HBLED is continuously monitored in step  206 , until the junction temperature reaches equilibrium. Finally, the functional test data of the DUT HBLED, such as power, color, etc., of the output energy are acquired in step  210 . 
       FIG. 3  shows one embodiment of an implementation of a Controlled Energy Testing system  300  in accordance with the principles of the present invention. The Controlled Energy Testing system  300  includes a plurality of processors. The processors in this implementation use a shared system clock in order to maintain a shared time reference, flat memory maps, and a reduced instruction set architecture. All of these functions allow for the group of processors to be independently programmed yet maintain tight coordination with each other. Each of the processors is assigned to a specific function within the circuits to ensure integrity in the timing of execution for each task. The tasks are synchronized by use of the shared system clock. 
     There are two control loops  301 ,  302  associated with the circuits&#39; constant power output. The inner control loop  301  is an analog feedback loop used to implement a constant voltage or constant current mode amplifier (selectable)  304 . The inner control loop  301  is comprised of a Voltage Sense Processor  306 , a Current Sense Processor  308 , an A/D converter  320 , a Current Sensor  321 , and an A/D converter  318 . 
     The outer control loop  302  is a digital feedback loop that implements the constant power output and is comprised of the Voltage Sense Processor  306  and the Current Sense Processor  308  (and associated analog circuits), a Math Processor  310 , a Control Processor  312  and a Force Processor  314 . 
     These processors  306 ,  308 ,  310 ,  312  constantly monitor the DUT voltage and current, detect any irregular power conditions, and if necessary, signal the Force Processor  314  to increase or decrease an output current or voltage as appropriate to ensure a constant amount of power is being delivered to the DUT HBLED  102 . 
     The Control Processor  312  manages the execution of the HBLED test sequence. This processor  312  is dedicated to the task of managing the execution of a test sequence. 
     Control over the constant power circuit is managed through the communications links to the Math and Force Processors  310 ,  314 . The Control Processor  312  also synchronizes the start and end of acquisition of output energy of the DUTHBLED  102  by sending a precisely timed signals to the spectrometer  106  and, if configured, photodetector  108 . 
     The Communications Processor  316  is dedicated to managing communications with a host computer system  317 , including configuration of the test sequence and acquisition timing. The function of the Communications Processor  316  is to ensure that host communications can be handled and executed without degrading the accuracy of the timing of any test sequence that may be underway. The Voltage Sense Processor  306  monitors the voltage across the DUT HBLED  102 . An analog buffer circuit samples the low side DUT voltages and presents a scaled DUT voltage to the high-speed, high-accuracy A/D converter  318 . The Voltage Sense Processor  306  monitors and logs an output of the voltage sense A/D converter  318 . 
     The Current Sense Processor  308  monitors the amount of current flowing into the DUT HBLED  102 . An analog buffer circuit samples a voltage across a Current Sensor  321  and presents a scaled DUT current to the high-speed, high-accuracy A/D converter  320 . The Current Sense Processor  308  monitors and logs an output of the current sense A/D converter  320 . 
     The Math Processor  310  takes the DUT HBLED voltage and current information supplied by the Current Sense and Voltage Sense Processors  308 ,  306  and calculates the total power sourced to the DUT HBLED  102 . The Math Processor  310  then compares the results of the calculation against the desired power levels and instructs the Force Processor  314  to modify the amount of power delivered to the DUT HBLED  102  if necessary. 
     The Force Processor  314  manages an analog circuitry associated with forcing a current or voltage to the DUT HBLED  102 . The analog circuit features a current and/or voltage mode force amplifier  304  driven by a high-speed and high-accuracy D/A converter  322 . The Force Processor  314  executes commands from the Math Processor  310  to adjust the amount of voltage or current being supplied to the DUT HBLED  102 . 
     It will be appreciated to a person skilled in the art that the above described Controlled Energy Testing system  100  and method  200  can be used for testing other suitable devices and/or configurations, including but not limited to, any other suitable energy conversion devices which convert electrical power to optical power. 
     It is appreciated that the drawings and the detailed descriptions thereof are to be regarded as illustrative in nature and not restrictive. 
     These and other features of the present invention will become apparent to those skilled in the art from the above description. As it will be realized, the invention is capable of modifications in various obvious aspects, all without departing from the spirit and scope of the present invention. Also, it is appreciated that the configuration and circuitry of the above-mentioned processors, converters, sensors, spectrometers, etc., can be implemented in various ways without departing from the scope of the present invention.