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LED energy saving, long service life, durability and flexible design, LED is rapidly replacing incandescent and fluorescent lamps in indoor and outdoor lighting equipment. But choosing the right LED is only part of the design equation. For your solid-state lighting design to achieve full efficiency, durability, and service life, you need to select the appropriate power source to match your application requirements with the leds used. This article will give you some useful advice on what to consider in the power selection process.

Background information

Once the voltage of the LED's power supply is equal to or greater than the forward voltage drop of the diode (usually in the region of 2-3v), the LED begins to produce light. The current required for full brightness varies from device to device, but for 1W leds (usually the smallest size in lighting applications) it is usually 350mA. Unlike incandescent lamps, leds are non-linear. This means that once the supply voltage exceeds the forward voltage of the diode, the current passing through increases exponentially with the supply voltage. Without some form of current regulation, the LED chip will become an expensive, monostable flash bulb.

To prevent this, the power supply must provide the right voltage at the right current. The simplest method is to select a power supply whose output voltage is greater than the selected LED forward voltage, and use the current-limiting resistor to limit the current to the maximum specified by the LED manufacturer. The disadvantage of this approach is that one of the main advantages of LED lighting is that high efficiency is affected by the dissipative power of the current limiting device.

Another problem with this approach is that the LED junction temperature affects the forward voltage. Since the output voltage of the power supply is fixed, this in turn means that the current will change if the voltage at both ends of the current limiter changes. The changing current will affect the amount of light emitted, reducing the reliability of the LED. The best way is to drive the LED with a constant current source. In this way, the current can be set to the maximum specified by the LED manufacturer for maximum efficiency and reliability, or the required precise brightness, and the effect of junction temperature can be eliminated when the LED or ambient temperature changes.

One advantage of using LED in lighting applications is that it is easy to change the brightness. This can be achieved by changing the current flowing through the LED, but running the LED at less than its maximum current reduces efficiency and can cause slight changes in color. A better approach, therefore, would be to pulse the current between zero and the maximum to change the average light emitted. As long as this is done at a high enough frequency to avoid the pulse being seen by the human eye as a flash, this is the best way to achieve dimming. The pulse of current is usually carried out at a fixed frequency, and the ratio of zero to total current changes. This is the PWM method.

Select power

The type of power selected for lighting applications will be based on several factors. First, consider the environment in which the application runs. Is it used indoors or outdoors? Does the power require waterproofing or special IP grade? Is the power supply capable of using conduction cooling or only using convection cooling?

Next, define the overall power requirements. A single light fixture may only require a small power supply, but a complex system may require a power supply of hundreds of watts. What else do you need? For example, does the power supply need to work in simple constant voltage mode or constant current mode, and does the application require dimming?

regulations and regulations are important

So it's time to think about regulations. Does the whole system need to operate in a certain range of harmonic currents? Does it need to meet lighting safety standards, or is ITE enough? And in this energy-sensitive age, how can power supplies effectively meet local or regional standards?

It is also important to note that some local government agencies offer discounts or other subsidies to products that meet certain efficiency and power factor correction levels. It is also important to know if your design standards meet the requirements, including any requirements for power consumption when power is turned off.

security standard

There are various standards applicable to lighting systems. Internationally, part 1 of IEC61347 covers general safety requirements for lamp control devices, and part 2, section 13 (2) applies to LED module power supplies. The United States has UL8750, and Europe has EN61347, all in the IEC format named after the chapter.

Harmonic current

Lighting applications generally require harmonic current emission to meet the requirements of en 61000-3-2, and lighting category is class C. In this category, there is one set of limitations for active input power above 25W and another set for active input power below 25W. However, this standard only mentions 25W and below discharge lighting.

Power factor correction is usually required to meet the limit of more than 25W, and since the limit is calculated as a percentage of amperes rather than absolute values, it is best to use a power source specifically for lighting applications, not ITE. However, as long as the lighting load is 40-50% higher than the full load rating of the power source, ITE power may reach the limit.

Here's an example: XP Power's ip67level DLE series is a Power line designed for LED lighting applications. The range includes models of 15,25,35 and 60 watts in accordance with the EN61347 and UL8750 safety specifications.

How to choose LED lighting power? Reading this will help you

An example LED Power source from XP Power's 15-60 watt DLE series.

LED configuration

Some lighting applications may use only one LED. The power used is usually about 1W, the forward voltage is in the range of 2-3v, and the forward current is about 350mA. Although this produces a bright light source, it is more likely that the LED will be used for some sort of array in a single lamp or set of lamps to produce a brighter and more uniform light source. Leds are typically used in one of four configurations. The LED is configured in series, parallel, or matrix (series and parallel combinations) to be driven from a single power source. The fourth configuration USES multiple channels and requires multiple power supplies.

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