How to select high brightness LED and make it work properly

The current LED market is very hot. With the increase of energy costs and the concern about climate change, governments and industries are beginning to promote efficient lighting solutions. LEDs are an excellent solution due to their high efficiency and long service life. At the same time, LED technology is undergoing a period of rapid change and innovation, and will continue to launch brighter and more efficient products, but this will make it difficult for most engineers to keep up with the latest products. Fortunately, the tool program to assist in the selection of LED components and product planning and design has been very complete, which will make it easier for engineers to select the appropriate LEDs and LED drivers . Even with powerful design tools, engineers still need to understand the parameters that affect LED selection in order to truly maximize the effectiveness of the tools.

How to choose an LED

The first step is to choose the color of the LED. The various colors of LEDs are mainly due to different dominant wavelengths and wavelengths available in the ultraviolet to infrared range. White light LEDs are set for their color temperature and warm white light LEDs, which are usually in the range of 2800K to 3500K, suitable for indoor lighting. Compared to white LEDs, conventional tungsten filament bulbs have a color temperature range of about 3000K. Cool white LEDs have a color temperature range of 6300K to 7500K, while white LEDs fall into the middle range of 3600K to 6200K. How bright should

an LED be?

Luminous flux (in lumens ) is generally used to measure the brightness of an LED. This is the total amount of light emitted within the spectrum that the human eye can detect. Table 1 shows the luminous flux values ​​of some typical light sources.

Individual high-brightness LEDs typically have a luminous flux value of less than 100 lumens (although this value is rising rapidly). Therefore, in most applications, LEDs are combined into arrays to achieve higher brightness. For multiple LED lighting, most use parallel strings and use a single current control driver to drive them. However, the parallel method may cause the light strings to produce different brightness due to different forward voltages of the LEDs and the current flowing through each light string. Therefore, it is best to use LEDs in series to keep the brightness and color consistent. However, the series voltage will increase as more LEDs are used, which will affect the driver topology, buck and boost that can be used.

Because LED light has a directional nature, it will show different brightness when viewed at different angles. Under certain viewing angles, the brightness we observe may be reduced to 50%. In directional applications, the actual brightness will be higher than that of a point source using a spherical light emission method. In other words, if some applications require a spherical light emission method, the corresponding array must be designed for the application, and optics can be used to diffuse or focus the light.

The efficiency of lighting components is measured in terms of luminous efficacy , which is measured in lumens per watt. Due to the rapid increase in the number of LED developments and designs, components with luminous efficacy values ​​of 75 lumens per watt have increased rapidly, and recently LEDs with 115 lumens per watt have begun to appear on the market. Using the same standard to compare the luminous efficacy of different light sources, tungsten filament bulbs are about 17 lumens per watt, CFL bulbs are about 60 lumens per watt, and low-pressure sodium street lamps are 100 to 200 lumens per watt.

Voltage and current

The forward voltage of an LED is a characteristic of the LED process. Generally speaking, the forward voltage range of yellow/orange/red LEDs is 2-3V, while the forward voltage range of blue /green/white LEDs is 3-4V. The current flowing through the LED controls the LED brightness and also affects the color presented, so LEDs are operated in a stable current mode. High brightness LEDs usually use currents above 0.35A, 0.7A, 1.0A, 1.4A. Another thing to consider is the size and height of the LED. The product provided must have heat dissipation function, which will be the key to high current applications. Of course, cost is another key parameter that needs to be taken into consideration.

Choosing LEDs is easier

Many LED suppliers have launched online selection tools to make LED selection easier. These tools can compare multiple solutions one by one and provide graphical analysis, which allows LED design engineers to quickly obtain the best solution.

LED Temperature Control

If LEDs are so efficient, why do we need to temperature control and monitor them? Don't LEDs operate at lower temperatures than incandescent light sources? It turns out that while LEDs are more efficient than tungsten bulbs, they still generate a lot of heat. Incandescent bulbs generate heat primarily through infrared radiation, but LEDs generate photons in addition to heat in the structure of semiconductor diodes . This heat is not part of the radiation spectrum and must be removed through conduction and convection. If LEDs are operated in hot conditions, some problems will occur. The brightness of the LED will dim significantly as the temperature rises. In addition, the color of the LED will change with temperature, which may cause problems in applications that need to maintain consistent color, such as white light generated by RGB. When designing driver circuits, you should consider the electronic characteristics of LEDs that change in forward voltage due to temperature changes. If the LEDs are configured to share current in parallel, the change in LED forward voltage may also be a problem. Long-term exposure to high junction temperatures will accelerate the aging of LEDs and reduce their service life and reliability. Therefore, the system must be designed to operate within the temperature specification of the LED. This is usually achieved by using heat sinks, such as large copper areas on the printed circuit board, additional heat sinks, or using heat-reinforced/metal printed circuit boards ( PCBs ) to mount the LEDs; forced airflow can also be used. However, a fail-safe mechanism can be implemented to deal with unexpected conditions such as high heat generated by abnormal climate changes or heat sink failure. Most buck-topology LED drivers have a thermal shutdown function, so if the driver exceeds a specified temperature (usually 125 to 150°C), the driver and LED will be turned off. Boost-topology drivers will protect themselves and will not load when turned off, so a crowbar circuit or other protection device for the LED is required. In either case, the temperature of the LED may be higher than that of the driver, so the LED itself needs its own temperature sensor and monitoring circuit fail-safe protection. This temperature sensor can reduce or shut down the current to the LED, turn on the cooling fan, and provide an alarm mechanism for the user or maintenance personnel. LED temperature sensor type Generally speaking, the accuracy of the temperature sensor must provide enough flexibility to detect overheating problems while not triggering fault warnings at normal operating temperatures. For example, if the system's normal operating temperature is 80°C and it needs to detect fault conditions not exceeding 100°C, if a temperature sensor system with a +/- 2°C accuracy is used, the allowable value can be set to 98°C, but if a temperature sensor system with a +/-10°C accuracy is used, the allowable value can be set to 90°C as the minimum value.