Given recent LED technology improvements and the drive toward greater energy efficiency, the LED lighting industry—including general illumination from commercial refrigeration to outdoor area lighting—is experiencing explosive growth. As manufacturers expand their offerings or enter the market, it is important they choose reliable, long-life LED power supplies to achieve optimal lighting performance.
In many ways the factors influencing the choice of an LED power supply (LED driver) are similar to those for most other applications. However, some application-specific factors require careful consideration.
Purchasing professionals should first review available choices and how they might fit in various lighting applications. Next, buyers should consider reliability and life, as it is critical to understand the design capabilities of potential suppliers along with their ability to provide support over the product’s long life. To help with this process, let’s look at six factors to consider when selecting an LED power supply.
Factor 1: Mode Of Operation
LED drivers deliver a reliable, predictable, and regulated supply of current to the power supply or power system. A single-module solution, often called a constant-current (CC) driver, provides an output whose current is regulated to directly drive the LEDs.
At higher power levels, the CC function may be implemented separately, and a constant-voltage (CV) power supply feeds the CC driver (Fig. 1). Some drivers/supplies can support both CC and CV modes, providing a single solution that can be reused across multiple lighting platforms (Fig. 2).
Factor 2: Application Power Requirements
Over the past 10 years, the power-handling capabilities of white, single-chip LEDs have increased from tens of milliwatts to watts. Also, their light output efficiency, usually measured in lumens per watt, has grown tenfold from around 10 lm/W to more than 100 lm/W.
Power levels associated with modern LED lighting can vary from less than 1 W for flashlights to several hundred watts for warehouse lights or streetlights. Across this power range, package size and shape will vary significantly, as will features and functions.
For example, a driver to power a recessed “can” light might be approximately 25 W and require TRIAC-compatible dimming, whereas a parking lot light might be 60 to 100 W and require only a simple two-level dimming circuit.
Efficiency improvements in LEDs, optics, and power supplies will result in a downward trend in power consumption for a given light level.
Factor 3: Lighting Environment
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, as low as –20°C or –40°C and as high as 60°C or 80°C. These extreme ranges even can apply to indoor applications at the high end, because of enclosed spaces that trap heat, and at the low end in refrigeration applications.
For indoor lighting fixtures, the power supply may only require minimal protection and require a rating of IP20 or similar. Alternatively, it may need to be dust-tight and protected against vertically dripping water, demanding a rating of IP62 or higher. But for industrial applications, environmental conditions have a greater influence on the choice of LED power supply.
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Commercial refrigeration systems, for example, which are inherently clean environments, may require only an LED power supply with an IP54 rating, whereas outdoor applications such as car-park illumination and street lighting might demand much higher levels of protection against water ingress.
It all depends on whether the power supply will be exposed to the full force of the weather, when an IP66 or IP67 rating is almost certainly a requirement, or contained within the lighting fixture and therefore shielded to some degree.
Whether a product is being used indoors or outdoors also can affect its exposure to electrical surges from power-line disturbances or natural phenomena such as lightning strikes. Power supplies are specified for operation in various surge environments. Higher levels of protection may require adding external surge components.
Factor 4: Custom Versus Standard Solutions
Detailing the many factors involved in selecting a custom versus standard solution requires an article of its own. Briefly, then, the number of standard (off-the-shelf) products continues to increase and provide solutions for a wide range of lighting applications.
Custom solutions still are required in applications with unique requirements, including form-factor constraints, or special-performance or functionality. Custom solutions may also be considered in high-volume applications, where functions and parameters can be optimized to minimize cost.
Factor 5: Reliability And Useful Life
Beyond energy efficiency, another primary factor driving LED penetration is the suitability of LEDs to reliably perform over a long period of time. But LED reliability and long life are not sufficient by themselves, as the other components in the system must also have matching reliability and life characteristics.
For example, if the optical systems discolor over time, light output is reduced and performance will disappoint. Similarly, the power supply/driver must continue to perform as specified for the life of the lighting fixture.
Many of the leading LED lighting systems on the market claim a typical operating life of 50,000 hours. Assuming an “average use” of 12 hours per day, which is probably on the high side for residential lighting, the projected lifetime is more than 11 years. For optimal life to be achieved, everything must operate reliably. To cite an over-used expression, the chain is only as strong as the weakest link.
It should be noted that reliability and life are not the same. Reliability is a measure of ongoing failure rates attributable to random component failures, batch problems, and/or manufacturing issues. This is reflected on the flat part of the well-known “bathtub” curve (Fig. 3). The ideal situation of no failures would be indicated by this part of the curve lying on the x-axis.
Product life, on the other hand, is a function of predictable wear-out mechanisms. In the case of LED drivers, wear-out is usually driven by electrolyte loss in certain capacitors. Good design practices can ensure that this does not occur within the specified lifetime of the LED driver. On the bathtub curve, the rising part indicates the end of useful life.
It is important to evaluate your power supplier’s capabilities in reliability and useful life. Everything in the design and manufacturing process must be geared toward high reliability and long life. It is not effective to start with a poor design and then screen or test for reliability.
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Design must start with how the power-supply designer chooses components, how these components are derated, how the design is qualified and proven, and ultimately how the product is manufactured. High-quality suppliers will be able to address these topics readily. Their goal is constant improvement in power-supply reliability, as characterized by the bathtub curve.
Factor 6: Warranties And Their Support
A supplier’s ability to offer and support a warranty depends on its ability to control failure rates and design for an adequate useful life. But these warranties may also vary significantly depending on application. Some applications are relatively benign, others extremely harsh; some applications may be predictable, others highly unpredictable.
You can expect a good supplier-partner to ask questions in that regard to better understand the application and how it affects the details of the warranty. Suppliers who don’t ask these questions and don’t understand these issues may not be in a position to support the offered warranties.
Although there are many power-supply choices for LED lighting applications, they can be narrowed quickly by matching the power supply or driver specifications and functionality to the application. Multiple suppliers can be narrowed further by critical evaluation of their design and manufacturing processes. This will ensure that the power supply not only works as required, but will also provide a reliable solution for the life of the lighting fixture.