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Selecting power supplies for LED lighting apps

Posted: 21 Oct 2014     Print Version  Bookmark and Share

Keywords:LEDs  luminaire  power supply  series  parallel 

Next, define the overall power requirements. A single luminaire may only require a small power source but a complex system may need ones supplying hundreds of Watts. Also, will other features be required? For example, will the power supply be required to work in simple constant voltage mode or constant current mode, and will your application need to be dimmable?

Rules & regulations matter
Then it's time to think about regulations. Will the overall system need to operate within certain harmonic current limits? Will it need to conform to the safety standard for lighting or will an ITE power supply be adequate? And, in these energy-aware times, how efficienct does the power supply need to be to meet local or regional standards.

Equally important, will your product be sold in places where utilities offer rebates or other subsidies for products which meet a particular level of efficiency and power factor correction? It's also important to know whether the standards your design will need to comply with include any requirements for how much power the supply draws when the lamps are turned off.

Safety standards
There are various standards that apply to lighting systems. Internationally there is IEC61347 Part 1(1) of which covers the general safety requirements of lamp control gear and Part 2 Section 13(2) which is applicable to power sources for LED modules, the US have UL8750(3) and Europe has EN61347 following the IEC format of section naming.

Harmonic currents
A lighting application will generally require the harmonic current emissions to meet the requirements of EN61000-3-2(4) and the class of equipment which covers lighting is class C. Within this class there are one set of limits for above 25W active input power and another set for 25W and below. However, the standard specifically only mentions discharge lighting for 25W and below.

To meet the limits for above 25W will generally require power factor correction and, as the limits are calculated as a percentage of the fundamental rather than as an absolute value of Amps, it may be better to use a power source designed specifically for lighting applications rather than an ITE type power supply. However, an ITE power supply will probably meet the limits as long as the lighting load is above 40-50% of the power supply's full load rating.

An example of an power supply series specifically designed for LED lighting applications is the IP67-rated DLE series from XP Power. The range comprises 15, 25, 35 and 60 Watt models and complies with safety specifications EN61347 and UL8750.

LED configurations
Some lighting applications may use just a single LED. The power used by this will typically be around 1W as the forward voltage is in the range of 2-3V and forward current around 350mA. Although this will produce a bright source of light, it is more probable that LEDs will be used in an array of some kind within a luminaire or group of luminaires to produce a brighter and more even light source. The LEDs will be generally arranged in one of four types of configuration. Placing the LEDs in series, parallel or a matrix (combination of series and parallel) configurations enables them to be driven from a single power source. The fourth configuration utilises multiple channels which require multiple power sources.

Series configuration
In this configuration the individual LEDS are arranged in series. This gives the advantage that the same current flows through each of them resulting in the same brightness of light given off. Another advantage that if one LED fails in short circuit, the other LEDs are unaffected and hence still lit. A disadvantage is that if one LED fails in open circuit, then current flow is interrupted and all the other LEDs turn off. A further disadvantage is that if many LEDs are needed to produce the amount of light required then the total sum of the forward voltages can necessitate the use of a power source with quite a high output voltage.

Figure 1: LEDs connected in series.

Parallel configuration
When connected in parallel the LEDs may still be arranged in two or more strings of LEDs in series. The advantage is that for the same number of LEDs i.e. the same brightness, the power source could have a lower output voltage as the number of LEDs in each string can be reduced. Another advantage is that if one of the LEDs becomes open circuit in one string then the other strings are unaffected and the luminaire will still produce light albeit at a reduced brightness. The disadvantage is that the current in each string cannot be precisely controlled from a single power source due to the slightly different forward voltages present in each string and so a current balancing device in each string may be needed which could reduce the overall efficiency.

Figure 2: LEDs connected in parallel.


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