Detailed explanation of the influence of power supply configuration on the test in LED integrating sphere test

There are many types of energy-saving lamps on the market, and the most advanced and environmentally friendly ones are LED energy-saving lamps. Data shows that LED energy-saving lamps save 80% more electricity than incandescent lamps and 50% more electricity than fluorescent energy-saving lamps. With the development of high-power LED driving technology, LED lamps are increasingly becoming the main lighting system in the future, and their application scope also covers the fields of mobile phone backlights, notebook backlights, LCD TVs, street lights, garden lights, etc. Because the application occasions have more stringent requirements on the quality of LED lighting, LED light sources need to be tested for luminous efficiency, color rendering and color temperature consistency before leaving the factory, and provide data-based test reports.

　　introduction

　　As we are all familiar with, the laptop backlight is composed of many LED lamp beads, and the color difference problem is becoming more and more obvious. If the LED lamp beads used are not from the same batch, it is easy to cause problems such as different wavelengths and color differences. This problem is inevitable even if the LED lamp beads from the same batch are used. For the testing of LED chips, an integrating sphere test system is generally used.

　　Using an integrating sphere to measure parameters such as luminous flux can make the measurement results more reliable. The integrating sphere can reduce and eliminate errors caused by differences in light shape, divergence angle, etc. Such high-precision and cautious measurements also drive LED integrating sphere manufacturers to increasingly choose higher-performance power supplies to be configured in the system to power the LEDs to be tested.

　　LED Integrating Sphere Test System

　　LED integrating sphere is mainly used to measure the parameters of LED light source performance indicators such as luminous efficiency, color rendering and color difference. In the large-scale testing of the production line every day, there are always two problems: one is how to improve the test efficiency and the other is how to better protect the LED under test during the measurement.

　　It is understood that in the current power supply industry, the total time from the power supply turning on to supply power to the LED lamp to the software control system collecting stable voltage and current values ​​is about 600ms. Is there a possibility of a breakthrough on this basis? The answer is yes. So how can it be achieved? In fact, the power supply configured in the system to supply power to the LED to be tested plays a very important role.

LED integrating sphere test system schematic diagram

　　1. Configuring a professional power supply can suppress overshoot and improve test efficiency

　　When a general power supply is turned on, due to the loop response time, the power supply output does not stop when it reaches the set output voltage value. Therefore, during this period of time, the voltage will overshoot. Excessive overshoot voltage will affect the life of the LED, and in severe cases, it may even break down the LED.

　　For low-power LED lamps, the constant current working current is usually between tens and hundreds of milliamperes. When working at a low current, the current and voltage climbing speeds decrease accordingly. Sometimes the time it takes to light up the LED is even longer than the time it takes to power the LED lamp normally and collect stable voltage and current values ​​from the software control system.

　　In the face of the above two problems, if the improvement measures of increasing the loop speed are taken to speed up the ramp-up speed and improve the test efficiency, it usually leads to more serious overshoot. However, if the overshoot problem is taken care of, the overall test efficiency will decrease. Such a problem can be solved by configuring a power supply for professional LED testing . For example, the IT6874A power supply of Itech can solve this problem. The following two figures are the waveforms of the voltage rise stage when the IT6874A DC power supply supplies power to 80V/20mA and 80V/400mA LEDs.

Figure 1: LED (80V/20mA) IT6874A ramp-up time: about 80ms

Figure 2: LED (80V/400mA) IT6874A ramp-up time: about 50ms

　　As can be seen from the above figure, the ITECH IT6874A DC power supply can basically meet the high-speed and no-overshoot test requirements within 100ms, while the general power supply requires about 600ms. The ITECH IT6874A DC power supply increases this number by nearly 6 times, which is of great significance to the improvement of test efficiency.

　　1. Configure a professional power supply to make the integrating sphere test system collect more stable values

　　When using an integrating sphere for testing, the stability of the power supply in the system is also very important. Stability is mainly reflected in:

　　(1) Power supply error range

　　When performing an integrating sphere test, if the power supply used has a higher accuracy, the measured data will be closer to the real value during batch testing, and the deviation between the LEDs to be tested will also be more accurate. For example, when testing LED lamps, assuming that the allowable error range of the power supply readback value is 20mV, when measuring two LED lamps (theoretical rated indicators are both 350mA), the measured value of #1-LED may be 330mA, and the measured value of #2-LED may be 370mA. Assuming that the real values ​​of these two LED lamps are both 352mA, it can be seen that the values ​​of 330mA and 370mA are far from the real values, and also cause a large deviation in the performance indicators of the two LED lamps. If a power supply with higher accuracy is used, assuming that the allowable error range of the voltage readback value is 10mV, the value obtained by testing the same two LED lamps may be between 342mV and 362mV, which is closer in value and can more truly reflect the performance indicators of a single LED lamp and the deviation range of consistency between multiple LEDs.

　　The voltage accuracy of ITECH IT6874A series power supply is as high as 0.5% and the current accuracy is as high as 0.1%. Such high accuracy can bring more accurate test results.

　　(2) Stability of collected values

　　What does the stability of the collected values ​​mean? Let's take an example. For example, the power supply can light up the LED lamp within 400ms (the parameters of the LED lamp to be tested are 80V/20mA). At this time, the voltage value collected by the integrating sphere test system software is 80.099V. When the LED lamp is turned off and turned on again, the value read by the software may be 82.152V. Therefore, using an oscilloscope alone, we can only see that the waveform rises to the rated current and voltage values, but it cannot guarantee that the values ​​collected by the software also reach a stable range. This will affect the test of LED lamp performance indicators during LED mass production testing due to the instability of the power supply value. In general, software engineering developers will delay to 500-600ms to ensure the stability of the values ​​during continuous reading operations. Therefore, the time required for the integrating sphere test is about 600ms, not the 400ms measured by the oscilloscope.

　　The ITECH IT6874A DC power supply can ensure that the time from lighting the LED lamp to reading the stable voltage and current values ​​is less than 150ms.

IT6874A measures LED lamp parameters: 60V/20mA, takes about 150ms

　　It can be seen that it is of great significance to configure a power supply for LED testing in the LED integrating sphere test system. The IT6874A DC power supply of ITECH has overcome the defects of general power supplies in the LED integrating sphere test system, can perfectly suppress the overshoot at the start-up moment, and greatly improve the test efficiency.