Due to LED energy saving, long service life, durability and flexible design, LEDs are currently rapidly replacing incandescent and fluorescent lamps in indoor and outdoor lighting equipment. But choosing the right LED is only part of the design equation. In order for your solid-state lighting design to achieve full efficiency, durability, and service life, you need to choose the right power source to match your application requirements with the LEDs used. This article will provide you with some useful suggestions and considerations when choosing a power supply.

Related background introduction

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

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

Another problem with this method 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 if the voltage across the current limiting device changes, the current will also change. The changing current will affect the amount of light emitted and reduce 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 value specified by the LED manufacturer to achieve maximum efficiency and reliability, or to achieve the required precise brightness, and it can also eliminate the effect of junction temperature when the LED or ambient temperature changes.

In lighting applications, one advantage of using LEDs is that it is easy to change the brightness. This can be achieved by changing the current through the LED, but running the LED at less than its maximum current will reduce efficiency and may cause slight changes in color. Therefore, a better method is to pulse the current between zero and the maximum value to change the average light emitted. As long as this is done at a high enough frequency to avoid the pulse being seen as a flash by the human eye, this is the best way to achieve dimming. The current pulse is usually performed at a fixed frequency, and the ratio of zero to full current changes. This is the pulse width modulation (PWM) method.

Choose power

The type of power supply selected for lighting applications will be based on several factors. First, consider the environment in which the application runs. Is the application indoor or outdoor? Does the power supply need to be waterproof or need a special IP rating? Can the power supply use conduction cooling or only convection cooling?

Next, define the overall power requirements. A single lamp may only need a small power source, but a complex system may need to provide hundreds of watts of power. In addition, do you need other functions? For example, does the power supply need to work in a simple constant voltage mode or a constant current mode, and does the application require dimming?

Laws and regulations are important

Well, now is the time to consider regulations. Does the entire system need to operate within a certain harmonic current range? Does it need to comply with lighting safety standards, or is ITE power sufficient? And in this energy-sensitive era, how can power supplies effectively meet local or regional standards?

It is also important that some local government agencies provide discounts or other subsidies to products that meet specific efficiency and power factor correction levels. Are the products sold in these places? Similarly, it is important to understand whether your design standards meet the requirements, including any requirements for power consumption when the power is turned off.

safety standard

There are various standards applicable to lighting systems. Internationally, Part 1 of IEC61347 covers general safety requirements for lighting control devices, and Part 2 and Section 13(2) are applicable to LED module power supplies. The United States has UL8750 and Europe has EN61347, both of which are IEC formats named after chapters.

Harmonic current

Lighting applications usually require harmonic current emission to meet the requirements of EN61000-3-2, and the lighting category is Class C. In this category, there is one set of restrictions for active input power above 25W, and another set for those with 25W and below. However, the standard only mentions discharge lighting of 25W and below.

In order to meet the limit above 25W, power factor correction is usually required, and since the limit is calculated as a percentage of amperage rather than an absolute value, it is better to use a power supply specifically for lighting applications instead of an ITE-type power supply. However, as long as the lighting load is higher than 40-50% of the power supply's full load rating, the ITE power supply may reach its limit.

Here is an example. XP Power's IP67 DLE series is a power supply series designed for LED lighting applications. This series of products includes 15, 25, 35 and 60 watt models, which comply with EN61347 and UL8750 safety regulations.

How to choose LED lighting drive power? Reading this can help you

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An example LED power supply, from XP Power's 15-60 watt DLE series.

LED configuration

Some lighting applications may use only one LED. This kind of used power is usually about 1W, the forward voltage is in the range of 2-3V, and the forward current is about 350mA. Although this will produce a bright light source, it is more likely that LEDs will be used in a single lighting fixture or some kind of array in a group of fixtures to produce a brighter and more uniform light source. LEDs are usually used in one of four configurations. Configure LEDs in series, parallel or matrix (combination of series and parallel) to drive them from a single power supply. The fourth configuration uses multiple channels and requires multiple power supplies.