Evaluation and quality analysis of lighting products
There are clouds in the ancient world, and the quality is seen by the eyes, but in most cases the quality is difficult to describe. When you want to describe the performance of a semiconductor lighting product, the design elements that affect the quality of the product are generally invisible. The LED chip is only a small part of the overall system, and the quality of the entire system is not only determined by it. Only a systematic approach to evaluating semiconductor lighting products can accurately predict the performance of the product during installation and service life. Let's consider a few design elements that will help semiconductor lighting developers succeed.
LED technology is evolving rapidly, but many LED light source manufacturers on the market have unverified their product performance metrics, including color quality, luminous flux, performance, durability, and general quality characteristics of semiconductor lighting products. Low-quality semiconductor lighting products can cause electrical over-stress due to poor thermal design or poor circuit design. These conditions, combined with chemical compatibility issues in the manufacturing process, can cause the quality of the LED to gradually decrease, even if the light source is completely ineffective. Figure 1 shows design or process issues that can cause degradation or even failure of semiconductor illumination sources.
Systematic assessment
To ensure the quality of semiconductor lighting products, extensive testing of the light source system is performed to accurately determine the design margin and predict the long-term reliability of the product. More than just LED chips, the entire semiconductor lighting system needs to ensure quality. CREE has implemented a systematic testing process to test the thermal, electrical, mechanical, photometric, and optical (TEMPO) parameters of a light source to evaluate the quality of semiconductor lighting products. The TEMPO project, led by the Career Services team, focuses on helping LED customers solve their technical challenges. The program helps R&D personnel quickly overcome system design challenges, save on research and development costs, and speed time-to-market.
The correct steps for lighting product system evaluation include multi-point testing and analysis. Engineers must conduct a series of tests for thermal, electrical, mechanical, photometric, and optical properties and provide a comprehensive report containing all relevant data to confirm the performance of LED lighting products. Specific test items for the TEMPO project are detailed in the attached table. In addition, the results of this test project can be used by the Lighting Engineers Association IES approved TM-21 standard to predict LED lifetime.
Measuring and evaluating LED light sources is challenging, especially for lighting manufacturers who are new to LED design. Solid-state light sources include many different components, including LED chips, boards, optics, diffusers, current drivers, power supplies, heat sinks, and mechanical enclosures. Any of these components can affect the performance, quality, or longevity of an SSL product.
The system performance of the light source also needs to be evaluated in terms of mechanical structure and long-term reliability. In addition, the results measured for the LED light source also need to comply with the LM-79-08 standard. The assessment needs to provide a comparison of the measured results with relevant regulations or safety standards, including Energy Star, DLC (Design Lights Consortium) and UL standards.
Thermal and mechanical testing
Most LED failures are temperature dependent. Thermal management and the resulting junction temperature of the LED semiconductor chip are closely related to the performance and lifetime of the LED. Excessive junction temperatures reduce light output and accelerate the decay of LED life. Proper thermal management of the LED source and mechanical structure are critical to performance. A variety of techniques are needed to evaluate mechanical structures, such as X-ray imaging of LED pads. The actual thermal performance must be measured to verify that the thermal design assumption is to ensure the quality and reliability of the semiconductor lighting product.
However, thermal measurements of semiconductor illumination are not simple. Incorrect placement of the thermocouple or exposure to large amounts of photons can result in incorrect temperature measurements. The design problems caused by the above errors may affect the life of the LED final product.
The correct approach should be to accurately measure the solder joint temperature of the LED to confirm the junction temperature of the LED in the semiconductor lighting fixture. Taking the TEMPO test as an example, it includes the measured solder joint temperature, the infrared light of the light source in the steady state, and the junction temperature calculated from the above measurement results. Temperature measurement and infrared imaging as shown in Figure 2 can help to measure and clarify the thermal performance based on LED sources.
The difference in infrared imaging of the two identical PCBs on the right side in Figure 2 shows the difference in thermal performance between the two. The yellow PCB on the left is cooler, the PCB on the right is bright red, and the LEDs are white and hot, indicating that there is a thermal interface problem with the PCB on the right.
The quality of the physical interface between the LED and the PCB will have a major impact on the thermal performance of the semiconductor lighting system, which is primarily evaluated by X-ray imaging. Analysis of the PCB by X-rays can effectively verify the quality of the soldering process and detect the presence of voids or excess solder. Place the X-ray camera on the LED hemisphere, and the focal plane and the pad coincide to clearly show the quality of the soldering interface. If you see a lot of solder voids or excessive solder particles between the pads, it means that the LED soldering quality is not good.
Electrical test
Important electrical parameters to consider when evaluating semiconductor lighting designs include efficiency, power factor, drive current transient analysis, brightness adjuster compatibility, and overall light source efficiency. Figure 3 shows an example of a partial electrical test. Driver efficiency can be calculated by dividing the power output to the LED by the measured input power of the luminaire, which is a suitable indicator for evaluating the performance of a semiconductor driver's current driver. In this test, the output power of the current driver to the LED is the sum of the product of the forward voltage and current of each LED.
Luminaire Efficacy, sometimes referred to as Wall-plug Efficacy, is an indicator of how much electrical energy a luminaire can convert into photons. When in a static state, this indicator can be obtained by dividing the total luminous flux by the total input power in lm/W. Efficacy is a number that is a good indicator of the performance of a system because it is determined by a combination of electrical, photometric, optical, and thermal properties. A high-efficiency light engine does not necessarily achieve high overall efficiency because the light loss of the external lens or the inefficiency of the driver circuit that provides the drive current to the LED can have a negative impact on the latter.
The power factor is another electrical indicator related to the performance of the LED driver and is often a key parameter for streetlights because the number of street lights connected to the grid is too large. The DLC clearly requires a power factor greater than 0.9 for streetlights. In general, the closer the power factor is to 1, the better the performance.
A power factor of 1 indicates that the applied voltage has the same phase as the current consumed. For wide input voltage systems (eg 120-277 VAC), the power factor should be measured for all possible nominal input voltages (120, 220 and 277 VAC for this example). And the worst power factor among them is recorded in the report submitted to the DLC, which usually occurs at the highest nominal voltage.
Brightness adjuster compatibility
The compatibility of the brightness adjuster is also a very critical quality factor, and the pain-prone phase-controlled brightness adjuster designed for traditional incandescent lamp loads is not well suited for semiconductor lighting systems and their drives. The brightness regulator compatibility test generally connects a semiconductor lighting product to a series of commonly used brightness adjusters, and qualitatively observes the following features: flicker, smoothness of brightness adjustment, adjustment of the control panel but no brightness After the change (deadtravel), the brightness is adjusted to a lower level or the western medicine is adjusted to a higher brightness to be re-lighted (pop-on), the human ear can be discerned (audiblenoise) and unadjusted to the lowest brightness, it is dropped.
NEMA (American Electrical Manufacturers Association) recently released a standard called "Cut-phase Brightness Adjustment for Semiconductor Solid-State Lighting: Basic Compatibility" (SSL7A-201X), which specifies maximum and minimum conduction of the brightness regulator circuit. Corner test. The NEMA dispatch regulator test includes a maximum light output (MLO) and a reference minimum light output (RMLO) compared to a NEMA given brightness adjuster circuit. The test results obtained using the NEMA specified brightness adjustment circuit are exemplified in the attached table.
The occurrence of transient over-current is also one of the important mechanisms for LED performance degradation. That is, the current delivered to the LED in a short period of time exceeds the maximum current specified in the data sheet. The instantaneous overcurrent current is usually only generated in milliseconds, usually caused by hot plugging and the open transient response of the current driver. It is also often a source of potential electrical over-stress for LED drivers. The high-speed current is checked for digital storage oscillation characteristics and the transient response is recorded. These electrical characteristics require testing to help ensure the system quality of semiconductor lighting products.
Photometric and optical testing
Photometric tests include total radiant flux (Radiant Flux), luminous flux (Luminous Flux), chromaticity, relative color temperature (CCT), and color rendering index (CRI). The unit of radiant flux is watts, which is obtained by calculating the total power of electromagnetic radiation (light) emitted from the light source. The luminous flux is the measurement result weighted according to the human visual perception. Mathematically, the measured radiant flux is convoluted with the filtered response of the human eye in units of lumens. Photometric measurements should be made during a one-minute interval that is repeated for a sufficient period of time to allow the luminaire to be in thermal equilibrium.
For TEMPO luminosity and optical testing, Cree uses a Labsphere integrating sphere of 2 m diameter, model CSLMS-7660, and the MC-9801 spectrometer of Labsphere-Otsuka Electronics. The photometric test follows the IESLM-79-081 specification and uses NIST traceable lamps for absorption correction procedures, and also ensures that the emission plane of the device under test is collinear with the sensor baffle of the integrating sphere. In Cree's lab, Chroma's Model61503 AC/DC power supply was used to power the unit under test, and power measurements were performed by Xitron's Model 2801 power analyzer.
In addition, Cree uses the C-type goniophotometer introduced by UL's Lighting Science Department to measure the light intensity of the light source. The instrument scans the source beam at a vertical angle on a series of horizontal planes, that is, the three-dimensional beam pattern of the source at several specific angles. The IESLM-79-08 requires a C-type goniophotometer because the LED source being measured is measured in actual operating conditions and there is no significant air flow to artificially cool the source. Figure 4 shows a picture of the photometric device.
We mentioned in the above that the unit of radiant flux is watt, which is obtained by calculating the total power of the electromagnetic radiation CCy (light) emitted from the light source, and the luminous flux is the measurement result weighted according to human visual perception. . To properly characterize system characteristics, the Luminous Flux test requires multiple measurements at one minute each time for a sufficiently long period of time to allow the test sample to achieve the steady state required by LM-79-08. The sample test results shown in Figure 5 show that the source takes approximately 1.6 hours to stabilize.
Materials and chemicals
The final step in the system evaluation must focus on the materials and chemicals used in the product. Chemical reactions that occur after prolonged use and are at relatively high temperatures can affect the performance of LED and semiconductor lighting products.
In order to ensure that the appropriate components are selected and that no chemical reactions that degrade performance are introduced in the manufacturing process, the LEDs should be removed from the finished source and fully characterized. Cree usually takes out 5 LEDs from the light source for inspection and individually measures the photometric performance of each LED at 700 mA. Figure 6 plots the sample test results into a binning chart (BinningChart). Their flux is attributed to T3 (minimum 220 lm ± 7%) and chromaticity is 6C1, 6D2, 7B4 and 7A3. The above grading data can be compared to the factory data for LEDs used in semiconductor lighting products to verify if the LED has experienced degradation in performance.
In summary, the speed of market introduction is crucial in the current competitively stimulating semiconductor lighting industry, but the quality of products is also the same. Cree is committed to overcoming technical difficulties and quickly providing thermal, electrical, mechanical, photometric and optical (TEMPO) test data for the system as needed. This will help the semiconductor lighting industry to provide high quality products to the market in the shortest possible time. Proper system testing, report generation, and measurement results telephone consultations can clarify the quality of semiconductor lighting products and help end users get high quality semiconductor lighting products.
At Cree's headquarters in Durham, North Carolina, the Cree Duham Technology Center is accredited by the National Voluntary Laboratory Accreditation Program (NVLAP), NVLAP Lab No. 500070-0, which meets ISOAEC17025:2005, IESLM-58 -94 and IESLM-79-08 standard requirements.
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