POWER
Howtobuy tHe rigHt LeD power suppLy
There are some application-specific factors that require careful consideration before buying an LED power supply By ConorQuinn, Emerson Network Power
Inmanyways, the factors influencing the choice of a LED power supply (or in lighting parlance, LED driver) are similar to buying a power supply formost other applications. However, there are also some application-specific factors that require careful consideration.
Purchasing professionals should first review some of the choices
that are available and how theymight fit in various lighting applications. Then buyers should consider the critical topic of reliability and life, where it is important to examine and understand the design capabilities of potential suppliers along with that supplier’s ability to provide support over the long life of this type of product.
Current driver or constant voltage supply Take a step back and considerwhat is required of a LED driver, power supply or power system. (Emerson intentionally broadens the scope to power systembecause the drive electronics sometimes consist ofmultiplemoduleswhich can be packaged and located separately. Together these separate elements forma power system.)
Whether a single supply/driver ormultiple-element systemis
being considered, the primary goal is to deliver a reliable, predictable and regulated supply of current to the LEDs or LEDmodules.
A singlemodule solution provides an outputwhose current is
regulated to directly drive the LEDs. This type is often referred to as constant current (CC) driver. At higher power levels, the CC function may be implemented separately and a constant-voltage (CV) power supply feeds the CC driver (see Figure 1). Some drivers/supplies can support both CC and CVmodes, providing the userwith a single solution that can be re-used acrossmultiple lighting platforms. A typical characteristic, as implemented in Emerson’s LDS70 family of drivers (inset photo), is illustrated in Figure 2.
power levels
Power levels associated with LED lighting can vary fromless than 1 watt, e.g. flashlights, to several hundred watts, e.g. warehouse or street lights. Across this power range, package size and shape will
44 | September 2010
Figure 1: A single supply/driver
vary significantly as will features and functions. For example, a driver to power a recessed “can” lightmight be approximately 25W and require triac- compatible dimming whereas a parking lot lightmight be 60-100Wand require only a simple two-level dimming circuit.
Figure 2: Amulti-element system
Efficiency improvements in LEDs, optics and power supplieswill result in a downward trend in power for a given light level.
environmental considerations Another important consideration is the environment in which the power supply will be used. The supply will be required to operate over a defined temperature range, sometimes as low as - 20C or -40C, and sometimes as high as 60C or even 80C. These extreme temperature ranges can even apply to indoor applications – high end because of enclosed spaces which trap heat, and at the low end in refrigeration applications. The applicationmay also require a sealed power supply to protect against dust,moisture and water. This will be specified in the context of ingress protection (IP), e.g. IP64 which protects against dust and splashing water.
Whether a product is being applied indoors or outdoors can
also affect its exposure to electrical surges due to power line disturbances or natural phenomena such as lightning strikes. Power supplies are specified for operation in various surge environments. Higher levels of protectionmay require the addition of external surge components.
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