Because of its 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 just part of designing the equation. In order for your solid-state lighting design to achieve overall efficiency, durability and life, you need to choose the right power supply to match your application requirements with the LED used. This article will provide some useful suggestions for you to consider in the selection of power supply.

 

Background introduction

 

Once the power supply voltage of the LED equals or exceeds the forward voltage drop of the diode (usually in the 2-3 V region), the LED begins 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 350 mA. Unlike incandescent lamps, LED is a nonlinear device. 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 appropriate voltage at suitable current. The easiest way is to select a power supply whose output voltage is greater than the forward voltage of the selected LED, and to limit the current to a maximum specified by the LED manufacturer using a current limiting resistance. The disadvantage of this method is that one of the main advantages of LED lighting is that efficiency is affected by the power dissipated by the current limiting device.

 

Another problem with this method is that the junction temperature of LED affects the forward voltage. Since the output voltage of the power supply is fixed, this in turn means that if the voltage at both ends of the current limiter changes, the current will also change. The changing current will affect the amount of light emitted and reduce the reliability of LED. The best way is to drive LED with a constant current source. In this way, the current can be set to the maximum specified by the LED manufacturer to achieve maximum efficiency and reliability, or to achieve the required precise brightness, and the 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 brightness. This can be achieved by changing the current through the LED, but running the LED at less than its maximum current can reduce efficiency and may cause slight color changes. Therefore, a better approach is to pulse the current between zero and maximum 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, this is the best way to achieve dimming. The current pulse is usually fixed at a fixed frequency, and the ratio of zero to full current varies. This is the pulse width modulation (PWM) method.

 

Selection of power supply

 

Choosing the type of power supply for lighting applications will be based on several factors. First, consider the application running environment. Is the application indoors or outdoors? Does the power need to be waterproof or require a special IP rating? Can the power be conduction cooled or convective cooled?

 

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

 

Laws and regulations are very important.

 

Well, it's time to consider legislation. Does the whole system need to operate within a certain harmonic current range? Does it need to meet lighting safety standards, or is ITE power enough? In this era of energy sensitive, how can the power supply effectively meet local or regional standards?

 

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

 

safety standards

 

There are various standards applicable to lighting systems.  Internationally, Part 1 of IEC 61347 covers the general safety requirements for luminaire control devices and Section 13 (2) of Part 2 applies to LED module power supplies. The United States has UL8750, Europe has EN61347, are named in the IEC format.

 

harmonic current

 

Lighting applications usually require the harmonic current emission to meet the requirements of EN61000-3-2, while the illumination category is C class. 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 refers only to discharge lighting under 25W and below.

 

In order to meet the above 25W limit, power factor correction (PFC) is usually required, and since the limit is calculated as a percentage of amperes rather than absolute values, it is preferable to use a power supply dedicated to lighting applications rather than an ITE power supply. However, as long as the illumination load is 40-50% higher than the full load rating of the power supply, the ITE power supply may reach the limit.

 

For example, XP Power's IP67 DLE series is designed for LED lighting applications. The products include 15,25,35 and 60 watts models, which comply with EN61347 and UL8750 Safety specifications.