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Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM | Next PageqqM qMMQmagsTHE WORLD’S NEWSSTAND ®LED production | TESTINGcontrol of the test sequence, with internalpass/fail criteria, calculations, and controlof digital I/O. The microprocessor stores auser-defined test sequence in memory andexecutes it on command to all SourceMeterinstruments in the test configuration,reducing set-up and configuration time.Communication between units takes placevia TSP-Link technology, a high-speed trigger-synchronization/inter-unitcommunicationbus, which connects multiple instrumentsin a master/slave configuration. Thiseliminates time-consuming GPIB (generalpurpose interface bus) traffic, and greatlyenhances system throughput.LED test system for a single deviceA system configuration for testing oneHB-LED at a time is shown in Fig. 3. Thecomponent handler transports the individualHB-LED to a test fixture, which isshielded from ambient light and housesa photodetector (PD) for light measurements.Two SMUs are used: SMU #1 suppliesthe test signal to the HB-LED andmeasures its electrical response, while SMU#2 monitors the photodiode during opticalmeasurements.The test sequence is programmed tobegin using a digital line from the componenthandler that serves as a start-of-test(SOT) signal. After the instrument detectsthis signal, the test sequence begins. Oncecompleted, a digital line signals measurement-completestatus to the componenthandler. In addition, the instrument’sbuilt-in intelligence performs all pass/failoperations and sends a digital commandthrough the instrument’s digital I/O port tothe component handler to bin the HB-LEDbased on the pass/fail criteria. Then, twoactions can be programmed to take placesimultaneously: data is transferred to thePCLANFIG. 2. Model 2651A High Power System SourceMeter instrument.PD26xxASMU #2LED2651ASMU #1PC for statistical process control, and a newDUT moves into the test fixture.Production testing of HB-LEDsTo achieve acceptable throughput, productiontest systems measure multiple partssimultaneously. Fig. 4 illustrates a devicetest system for three HB-LEDs that has onephotodiode (PD) channel.Junction self-heating can contributesignificant measurement error in HB-LED production testing. As the junctionheats over time, for a constant forwardbiascurrent, the forward voltage drops, soit’s crucial to manage device self-heatingto ensure accurate, repeatable measurements.Self-heating can be minimized byreducing the amount of time the test willtake, which in turn reduces the amount oftime necessary for the test current to beapplied to the device. Smart instrumentscan simplify configuration of the devicesoak time (which allows any circuit capacitanceto settle before the measurementbegins), as well as the integration time(which defines how long it takes the analog-to-digital(ADC) to acquire the inputsignal), because both factor into how longthe test will take. New SMU instruments,including Keithley’s 2651A, have digitizingComponenthandlerTest fixtureBinsDigital lines(DIO)FIG. 3. A test system for a single HB-LED, which uses twosource-measure units (SMU).50% duty cycle75% duty cycle25% duty cycleADCs, which can sampleat speeds up to onemicrosecond per pointor up to 50 times fasterthan high-performanceintegrated ADCs. Thesehigher measurementspeeds further improveoverall test times.The use of pulsed measurementsminimizestest times and junctionself-heating. ModernSMUs with high pulsewidthresolution ensure precise controlover how long power is applied to the device.Pulsed operation also allows these instrumentsto output current levels well beyondtheir DC capabilities.High-power LED module testingThe demand for a lot of light in a small packagehas led lighting manufacturers to developFIG. 5. In pulse-width modulation,the pulse level and frequency remainconstant, but the duty cycle is varied.high-power LED modules, which often consistof one or more large-die LEDs. When multipledie are present, they’re either wired inparallel or in series, depending on the applicationand the available power source. The die ofthese LEDs can be much larger than those oftypical HB-LEDs and can handle much highercurrents. In fact, it’s common for a single dieto be required to withstand current levels ashigh as 10A.Obviously, testing high powerHB-LED modules demands hardwarethat can deliver a lot of powerto the DUT. Although SMUs’ abilityto handle both sourcing and measurementnormally makes them the72 NOVEMBER/DECEMBER 2011 LEDsmagazine.comPrevious Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM qMM MQmags| Next Page q qTHE WORLD’S NEWSSTAND ®
Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM | Next PageqqM qMMQmagsTHE WORLD’S NEWSSTAND ®LED production | TESTINGbest solution for testing LEDs, most SMUson the market simply can’t deliver the levelof power that testing high power HB-LEDmodules requires. Most instrument-basedSMUs are capable of delivering only 20Wof power or less, but this application oftenrequires 100W or more. Keithley’s 2651Ainstrument is capable of delivering up to200W of continuous DC power and up to2000W of pulsed power.PD LED 1DIOPC LAN26xxA 2651ATSP-linkFIG. 4. Production setup for testing three HB-LEDs.LED 22651ALED 32651APulse-width modulation and HB-LEDsPulse-width modulation (PWM) offers a wayto control the brightness of LEDs. Althoughit’s also possible to control an LED’s brightnesssimply by lowering the forward drivecurrent, this method is undesirable becausethe color of the light produced will changeslightly with current level. PWM is the preferredtechnique because it uses a constantcurrent level for each pulse and thereforeoffers greater consistency in the color of thelight produced. PWM also offers more-linearcontrol over brightness, and greater powerconversionefficiency.In this technique, the current through theLED is pulsed at a constant frequency witha constant pulse level, but the width of thepulse is varied (Fig. 5), which changes theamount of time the LED is in the ON state, aswell as the perceived level of brightness. TheLED is actually flashing but at such a highfrequency that the human eye can’t distinguishit from a constant light level.Given that LEDs are often used withPWM, it’s only appropriate that they betested with PWM techniques. As part ofPWM testing, an LED is usually tested byrunning a series of pulses through it whileusing a spectrometer to take an integratedmeasurement of the light output over thecourse of many pulses. This measurementmay take tens or hundreds of millisecondsto complete. During the pulsed output, theforward voltage is measured on every pulseto look for changes as the temperature of theLED rises._____________LEDsmagazine.com NOVEMBER/DECEMBER 2011 73Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM qMM MQmags| Next Page q qTHE WORLD’S NEWSSTAND ®
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