Visible light-emitting diodes ( LEDs ) combine high efficiency with long life. Currently, they are widely used. Manufacturers have developed new devices with higher luminous flux, longer lifetime, more color and higher lumens per watt through in-depth research on LED devices. Accurate and cost-effective testing is critical to ensuring device reliability and quality.
LED testing has different types of test sequences at different stages of production, such as design development and testing, wafer level testing during production, and final testing after packaging. Although LED testing typically involves electrical and optical measurements, this article focuses on electrical characterization and introduces some optical measurement techniques only at the appropriate locations. Figure 1 shows the electrical IV curve of a typical diode. A complete test should contain a large number of voltage values ​​and corresponding current operating points, but in general a limited sample point is sufficient to test the quality factor of the device.
Figure 1. DC IV curve and test points for a typical LED
Many tests need to provide a known current and then measure the voltage, while others require a voltage and then measure the resulting current. Therefore, high-speed test instruments with integrated, synchronized source and measurement capabilities are ideal for this type of testing.
Forward voltage test
In the LED test sequence, the forward voltage (VF) test verifies the forward operating voltage on the visible LED. When a forward current is applied to the diode, it begins to conduct. At the beginning of the low current, the voltage drop across the diode rises rapidly, but as the drive current increases, the voltage slope begins to flatten. Diodes typically operate in a region where this voltage is relatively constant. It is also useful to test diodes under these operating conditions. The VF test needs to provide a known current and then measure the voltage drop across the diode. Typical test currents range from tens of milliamps to a few amps, and the resulting voltage is typically in the range of a few volts. Some manufacturers use the results of this test for device sorting because the forward voltage is related to the chromaticity of the LED (the color quality that is characterized by the primary or complementary color of the color and its purity).
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