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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 

Light emitting diodes (LEDs) are rapidly replacing incandescent and fluorescent technologies in both indoor and outdoor luminaires. This is due to the energy savings, long service life, durability and design flexibility that they offer. But choosing the right LED is only part of the design equation. In order for your solid-state lighting design to realise its full efficiency, durability, and longevity, you'll need to choose a power supply with characteristics which closely match the requirements of your application and the LEDs you're using. This brief tutorial offers some helpful pointers on what you'll need to consider during the selection process.

Start with the basics
LEDs begin to produce light once their supply voltage is equal to or greater than the diode's forward voltage drop (typically in the region of 2-3V). The current required for full brightness varies from device to device but is typically 350mA for a 1W LED (usually the smallest size used in lighting applications). But unlike incandescent bulbs, LEDs are non-linear devices. This means that once the supply voltage exceeds the diode's forward voltage, the current they pass increases exponentially as a function of supply voltage. Without some sort of current regulation, the LED chip will become an expensive, one-shot solid-state flash bulb.

To prevent this inconvenient behaviour, the power source therefore must provide a suitable voltage at the appropriate current. The simplest way to achieve this would be to select a power supply with an output voltage above the forward voltage of the chosen LED and to limit the current to the maximum specified by the LED manufacturer using a current limit resistor. The down-side of this approach is that one of the main benefits of LED lighting – that of high efficiency, is compromised by the power dissipated by the current limiting device.

A further problem with this approach is that the LED junction temperature affects its forward voltage. As a power supply's output voltage is fixed, this in-turn means that the voltage across the current limiting device changes and hence the current will change too. The changing current will affect the amount of light being emitted and decrease the reliability of the LED. The best approach is to drive the LED from a constant current source. This allows the current to be set to the maximum specified by the LED manufacture to achieve greatest efficiency and reliability, or to achieve the exact brightness required and also to remove the effects of junction temperature as the LED or ambient temperature changes.

One of the benefits of using LEDs in lighting applications is the ease of varying the brightness of the light. This can be achieved by varying the current through the LED which proportionally varies the amount of light emitted, however, running the LED with less than its maximum current reduces the efficiency and may result in slight changes in colour. A better way is therefore to pulse the current between zero and maximum to vary the average light emitted. As long as this is done at a high enough frequency to avoid the pulsing being seen as flicker by the human eye this is the optimum way to achieve dimming. Pulsing of the current will usually be done at a fixed frequency with the ratio of zero to full current being changed. This is the pulse width modulation (PWM) method.

Selecting a power supply
The type of power supply selected for a lighting application will be based on several factors. First, consider the environment your application will be operating in. Is the application for indoor or outdoor use? Does the power supply need to be water-proof or have any special IP rating? Will the power supply be able to use conduction cooling or only convection cooling?

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