The majority of LED lamps and luminaires feature an electronic driver that performs the same basic function as a fluorescent ballast. The driver converts AC supply voltage to DC and drives current to the LED source.
In the traditional lighting aftermarket, electrical distributors select a compatible ballast based on the type of lamp in the luminaire. Form factors and other attributes are standardized. In relatively rate cases, the ballast is dimmable, requiring a ballast compatible with the selected dimming method.
In the LED market, as with fluorescent luminaires, distributors typically select a luminaire with its manufacturer choosing the driver. After installation, the driver may be replaced in the field when it fails if the luminaire is serviceable. This can be more challenging than replacing a ballast for several reasons, including non-standardization of form factors, wide range of driver-LED module pairings, and short product cycles.
“While LED drivers perform a similar function to electronic ballasts, they are not the same,” says Kevin Boyce, Director of Product Management, Universal Lighting Technologies. “More information is needed to select the proper LED driver replacement than was required in electronic fluorescent ballast replacement.”
Drivers may vary in regards to output, safety rating, input voltage, programmability, temperature rating, electromagnetic interference (EMI) and form factor.
The industry typically categorizes drivers as constant-voltage or constant-current. Constant-voltage drivers operate LEDs requiring a fixed voltage (typically 12 or 24VDC), popular for applications where the LED load is unknown, such as sign and track lighting. Constant-current drivers (e.g., 350mA, 700mA, 1A) operate LEDs requiring a constant current, and are used in the large majority of LED general lighting. Good performance depends on matching output current and voltage to the LEDs.
Besides output characteristics, the industry also evaluates drivers based on input characteristics. The larger majority of drivers are universal (120-277V, 50-60Hz), but some are fixed or single voltage. The driver must be compatible with the supply voltage.
Drivers may be rated as Class 1 or Class 2. The majority of indoor LED products use Class 2 drivers, which simplifies luminaire construction. The majority of outdoor LED products use Class 1 drivers, which operate more efficiently when high light output is required.
The majority of LED products is dimmable and designed to operate with 0-10V and/or digital (e.g., DALI) controls.
LED drivers are technically advancing as the LED lighting market matures and the technology continues to evolve.
“There are many trends that driver manufacturers are looking into,” says Ethan Biery, LED Engineering Leader, Lutron Electronics. “Incremental improvements in efficiency, variations in size and wattage, increased connectivity options, multichannel drivers for support of color-tuning applications, and drivers integrated directly into light engines.”
Programmability: Drivers may be programmed or tuned (typically at the factory) to set the maximum output for the LED load. This allows precise pairing of the driver and LED module and resulting light output and wattage. A majority of drivers used in indoor luminaires are tunable. Most in the outdoor market are not, though Boyce says demand is growing.
“The industry is moving towards tunable drivers, which allows for luminaire manufacturers to reduce the number of LED drivers they use,” he says. “This also allows distributors to stock fewer LED drivers, though it does require the distributor to tune or set up the output of the LED driver to the requirement of each luminaire. Conversely, some LED driver manufacturers offer quick-ship programs of factory-tuned drivers to alleviate the distributor of the responsibility to tune the drivers.”
In addition to the programming maximum output, some drivers offer programming of dim levels and dimming curves.
Color tuning: As demand for color-tunable lighting such as tunable white products increases, driver manufacturers are investing resources in products to support that market. Says Biery, “New technologies are beginning to deliver multichannel tunable drivers for LED lighting that allows the color to be adjusted almost infinitely to deliver the perfect color temperature for any application.”
Flicker concerns: Flicker concerns have focused industry attention on the driver, its interactions with dimming controls, and metrics that can be used to evaluate and compare drivers. These metrics are likely to be released this year.
“The LED driver plays a very significant role in delivering flicker-free, high-quality dimming performance,” Biery notes. “Recommend digital drivers whenever possible, as they generally use more robust filtering components and are less prone to noise and external interference that can cause flicker. Simpler, less expensive, less complex drivers generally have fewer filtering components and use analog instead of digital circuitry, making them more prone to undesirable modulation in their output and to external electrical noise sources, which can manifest as flicker.”
Replacing the driver
The goal of driver replacement is that the new driver provides the same functionality as one it is replacing. The new driver’s characteristics must match the replaced driver. This is critical as mismatching can cause operating failure or performance and/or safety issues. Further, with the majority of luminaires, the driver is installed in the luminaire, requiring a form factor allowing installation. Many commercial luminaires permit field replacement, with some featuring quick disconnects for easy servicing. However, a lack of standardization can make finding the right replacement driver challenging.
“There is little effort being put toward standardization of LED driver form factors or technical capabilities,” Biery says. “The number of options demanded by fixture makers is too great. The best advice for distributors who want to stock drivers is to work with a quality manufacturer who demonstrates a commitment to having short lead times as well as supplying products with a long product lifecycle.”
In 2016, the National Electrical Manufacturers Association (NEMA) published LSD 74, Considerations of Field LED Driver Replacement. This whitepaper provides recommendations for driver replacement.
If using the same manufacturer’s driver, replacement can be simple. Send the model number and programmed current level (on the label or on a second label) to the luminaire or driver manufacturer. If using a different manufacturer, determine the LED module’s rated current; the manufacturer may provide a list of suitable replacement drivers. If the type of LED module is unknown, take a photograph of it and submit it to the driver manufacturer. If using a tunable driver, note the programmed current typically does not transfer from one manufacturer to another, so tuning can be difficult to replicate.
Finally, be sure that the replacement driver is a quality product made by a reputable manufacturer. “Characteristics of high-quality LED drivers are high efficiency, high reliability and stable output,” Boyce says. “Due to the long life of LED modules, all replacement drivers should be high-quality.”