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How to Choose the Right Power Supply for Your LED Lighting Project
April 28, 2016
Published by Administrator

Understand your LED lighting application first 

In order to determine the type of power supply best suited for a particular lighting project, it is first necessary to understand the nature of the application, i.e. what form the LED lighting will take and where and how it will be deployed. There is no unique segmentation, but the ‘where’ usually relates to the broad market, while the ‘what’ or ‘how’ is more about the function or characteristic of the lighting. Typical market segments for lighting are retail, industrial, and architectural along with the terms ‘indoor’, covering commercial and residential use, and ‘outdoor’ for street lighting and building illumination. Already there is an overlap in these descriptions, so it is probably more helpful to explore a few examples in a little more detail to appreciate what they may require from a power supply. 

Retail applications can range from space lighting to window, shelf, or display cabinet lighting including point-of-sale displays and signage. The lighting needs to be of high quality with a consistent color for showing products to best effect. Ambient lighting will provide unobtrusive background illumination, while accent lighting will be more obvious in highlighting a particular product or display area. Cabinet lighting usually needs to be concealed, so as not to detract from the product, whereas ambient lighting is less likely to be in the customer’s line of sight, e.g. illumination from ceiling fixtures. Lighting in retail outlets is often operating for long periods of time, so high reliability and good efficiency, to reduce running costs and temperatures, are important requirements for the power supply. Because of its reduced heat output, low-power LED lighting is proving popular in refrigerated display cabinets; clearly moisture-sealed supplies with low heat dissipation are likely to be equally important for powering such lights. 

Outdoor applications embrace street lighting, including related traffic signals and signage, and building illumination, which covers all forms of external lighting such as safety/security lights, floodlights, and other decorative lighting such as up/down and color-wash effects. A key specification for any power supply unit (PSU) used for outdoor lighting will be its IP rating, necessary to provide an appropriate level of protection against moisture, water spray, and even immersion as well as against dust and solid objects. One of the main drivers for the adoption of LEDs for outdoor lighting is their long lifetime, which enables a ‘fit and forget’ approach to installation and maintenance. Consequently, it is important that the power supply chosen for outdoor LED lighting also offer a high-reliability solution for long lifetime and low maintenance.

Commercial and residential uses for indoor LED lighting are myriad. The starting point for LEDs is the replacement of more established light sources such as incandescent or fluorescent lamps. This can include conventional ceiling and wall lights and similar lighting fixtures, as well as table/desk lamps and the illumination of pictures and ornaments. However, LED technology offers more than just a means of energy saving, with the opportunity to readily embed LED lights in floors, staircases, and walkways for both safety and decorative effect, and to implement display lighting similar to the retail lighting applications described above. Dimming is a common requirement for domestic lighting, and power supplies that provide this capability are clearly important. Many LED lights also offer mood-enhancing or color-changing effects, and these require power supplies with multiple outputs that can be individually controlled to provide the desired ambience or color balance. Other requirements for power supplies for indoor applications will vary widely according the type of lighting, and whether it is built-in or a standalone unit. The challenge may be physically accommodating the supply, especially where LED lighting is being retrofitted into the existing structure of a building, so its size and form factor may be important. Issues of accessibility once installed may also dictate a unit designed for high-reliability and long life. 

Understand what to look for when selecting your LED power supply 

Understanding your LED lighting application is a good start, but determining the ideal power supply also requires a little more insight into the operation of LEDs before we can narrow down the choices available. A light emitting diode (LED) is a semiconductor device that emits light when a current flows across the P-N junction in the device. The forward voltage (i.e. positive bias across the junction) for optimum performance is typically around 3 V, but the relationship of voltage to current is non-linear. Therefore, as can be seen in Figure 2, the LED starts to turn on at a lower voltage but rapidly draws a much higher current as the voltage increases above its nominal rated value. The relationship between relative luminous flux (a measure of the light output) and forward current is also non-linear so, while driving an LED with a higher current will increase the light output, this is generally less efficient than using multiple LEDs to achieve the same result. However, analog dimming of LEDs over a limited brightness range is possible by varying the drive current up to and sometimes even beyond its nominal rating. However, as we will see later, there are several other methods for implementing dimming. Another characteristic to be aware of is the effect of temperature, which, as it increases, results in a decrease in the forward voltage (for a given forward current) and less efficient operation.

Given the above characteristics, the conventional approach to driving LEDs is by controlling the current through the device. The simplest method employs a series resistor to limit current such that 

where IF = forward current
VCC = supply voltage
VF = forward voltage (across one or more LEDs in series)
R = series resistor