Error Analysis in Long-range Testing of LED Street Light Characteristics

1 Introduction

In recent years, LED light sources

The light source product has the characteristics of LED display, small size, light weight, easy to carry, battery power supply, high performance-price ratio, etc. It is intuitive and fast, and is an extremely simple and convenient test tool. The product has been treated with shock and moisture proofing and can work for a long time in harsh outdoor environments.

The characteristics of high efficiency, long life, and green environmental protection are recognized and valued by people. At the same time, with the rapid development of LED technology in the past 10 years, the relevant LED secondary optical design technology has become more mature, and the use of semiconductor light sources to replace existing urban road lighting sources has become a major trend in the field of semiconductor lighting, which has attracted increasing attention. However, in order to promote LED street lights in large quantities, it is necessary to have a unified technical standard and measurement standard for LED street light products, and stipulate some mandatory performance indicators to ensure the performance and quality of the products. From the current status of LED street light product technology development, the most important key indicators are the whole lamp life, the whole lamp light effect and the light decay curve.

As a system, LED street lights have many factors that affect the above performance, such as the characteristics of the power driver, the weather resistance of the optical components, the waterproof and dustproof performance of the entire lamp structure, etc., which will seriously affect the luminous efficiency and life of the system. In other words, the optical characteristics of the LED chip or module alone cannot fully describe the performance of the lamp. Therefore, an aging test of the entire lamp must be carried out to obtain data that meets the actual performance of the system.

As an outdoor lighting product, LED street lights must undergo long-distance on-site testing to verify their reliability, which often takes thousands of hours. The accuracy of long-distance on-site measurements is affected by factors such as the environment and test methods, and will be significantly different from the test results in indoor laboratory environments.

Figure 1 is a comparison of the light decay results of LED street lamps provided by different companies organized by the testing department in a laboratory environment and in an outdoor field.

Figure 1 Comparison of light attenuation of LED street lamps in laboratory environment and outdoor environment tests

Figure 1 shows the 2000-hour light decay test data of 22 types of LED street lamp products. From the figure, we can see that:

①The light decay values ​​of LED street lamps tested in outdoor venues are greater than those tested in laboratory environments;

② If the laboratory test is used as the basis, most of the light decay is below 5%, but if the outdoor test is used as the basis, the light decay of these street lamps is obviously too large and is not suitable for mass production;

③ The test results for different street lamp products are also different, ranging from 5% to 15%.

This makes it difficult for end users to choose LED street light products, that is, what method should be used for testing and which set of test data should be trusted?

At present, strict requirements are imposed on the light decay of LED street lamps in the standards and specifications related to LED street lamps at all levels. Basically, it is required that the light decay of street lamps should not exceed 2% after 3000 hours of continuous lighting. To meet such high light decay test requirements, various key factors that cause measurement errors must be analyzed and appropriately compensated.

2 Main factors affecting the measurement accuracy of LED street light efficiency

The relevant national standards provide a method for measuring the expected life of LED street lamps. The method stipulates that the LED street lamps should be aged under certain technical specifications and the temperature rise of the LED street lamp shell should be monitored at the specified temperature measurement point. The illumination of the point under the LED street lamp at a certain distance should be recorded every 100 to 300 hours until the measurement reaches 6000 hours. If the LED street lamp life is not reached after 6000 hours, the life of the LED street lamp can be extrapolated based on the law of the decrease in illumination at the point under the lamp, that is, the expected life. In the expected life measurement and extrapolation calculation, the luminous characteristics of the LED street lamp at 1000 hours are taken as the initial value, and the time when the extrapolated LED street lamp flux drops to 70% of the initial value is the expected life.

It can be seen that the time span for the light decay and life test of LED street lamps is more than 3 months. Due to the influence of the test site, it is often necessary to test at the street lamp installation site, as shown in Figure 2.

Figure 2 Schematic diagram of street light test

This will inevitably lead to the problem of determining the test accuracy and eliminating interference factors. Here we first analyze the main factors that affect the measurement accuracy of LED street light efficiency.

(1) Power supply stability

The power supply voltage requirements in the "Measurement Method for Integral LED Street Lights" are: During the stabilization period, the power supply voltage should be stable within ± 0.5% of the rated value; during measurement, the power supply voltage should be stable within ± 0.2% of the rated value, the fundamental frequency deviation should not be greater than 0.1%, and the harmonic distortion should be less than 3%; the power supply voltage for the life test should be stable within ± 2%.

In terms of LED driving, constant current driving is usually used. The fluctuation of current directly affects the luminous flux of the LED. In order to verify the performance of the actual product, the LED drivers of two power supply manufacturers that have specially designed and have extensive experience in the use of LED street lamp power supplies were investigated. The current control accuracy of the two manufacturers is ± 2% and 3% respectively.

However, this only indicates the steady-state condition. Here we should pay more attention to several parameters: the fluctuation of the stable current with the change of the external voltage, the chronic drift with the running time, and the stability with the change of the external temperature.

The laboratory tested a company's 75W LED switch

The switch is the most common electronic component, and its function is to connect and disconnect the circuit. When it is connected, the current can pass through, otherwise the current cannot pass through. Switches can be seen in various electronic devices and household appliances.

Power supply testing shows that the switching power supply switching power supply

A switching power supply 1 is a power supply that uses modern power electronics technology to control the time ratio of the switch tube to be turned on and off to maintain a stable output voltage. A switching power supply is generally composed of a pulse width modulation (PWM) control IC and a MOSFET. Compared with linear power supplies, the cost of both switching power supplies increases with the increase of output power, but the growth rates of the two are different. At a certain output power point, the cost of a linear power supply is higher than that of a switching power supply. This point is called the cost reversal point. With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating. This cost reversal point is increasingly moving towards the low output power end, which provides a broad development space for switching power supplies.