|LEDs are low-voltage light sources, requiring a constant DC voltage or current to operate optimally. Operating on a low-voltage DC power supply enables LEDs to be easily adapted to different power supplies, permits longer stand-by power, and increases safety. Individual LEDs used for illumination require 2-4V of direct current (DC) power and several hundred mA of current. As LEDs are connected in series in an array, higher voltage is required.
In addition, during operation, the light source must be protected from line-voltage fluctuations. Changes in voltage can produce a disproportional change in current, which in turn can cause light output to vary, as LED light output is proportional to current and is rated for a current range. If current exceeds the manufacturer recommendations, the LEDs can become brighter, but their light output can degrade at a faster rate due to higher temperatures within the device which leads to a shorter useful life. One definition of useful life for LEDs is the point at which light output declines by 30 percent.
LEDs, therefore, require a device that can convert incoming AC power to the proper DC voltage, and regulate the current flowing through the LED during operation. The driver converts 120V (or other voltage) 60Hz AC power to low-voltage DC power required by the LEDs, and protects the LEDs from line-voltage fluctuations.
"An LED driver is the power supply for an LED system, much like a ballast is to a fluorescent or HID lighting system," says Al Marble, manager - sales & market development for Philips-Advance Transformer.
LED drivers may be constant voltage types (usually 10V, 12V and 24V) or constant current types (350mA, 700mA and 1A). Some drivers are manufactured to operate specific LED devices or arrays, while others can operate most commonly available LEDs. LED drivers are usually compact enough to fit inside a junction box, include isolated Class 2 output for safe handling of the load, operate at high system efficiency, and offer remote operation of the power supply.
Dimming and color-changing
Drivers can enable dimming and color-changing or sequencing of LEDs. LEDs are easily integrated with circuits to control dimming and color-changing so that these functions can respond to preset commands or occupant presence or commands. Most LED drivers are compatible with commercially available 0-10V control devices and systems such as occupancy sensors, photocells, wallbox dimmers, remote controls, architectural and theatrical controls, and building and lighting automation systems.
LEDs can also work with devices governed by the DMX and digital addressable lighting interface (DALI) protocols and, in the future, may include wireless (RF) as a control option.
"With the use of fully electronic drivers, the possibilities are endless," says Marble. "This area is only now being developed, but tighter integration of all electronic components is expected to reduce the use of discrete components in the field and simplify application."
Drivers with dimming capability can dim the LED light output over the full range from 100% to 0%. Dimming drivers can dim LEDs by reduction in the forward current, pulse width modulation (PWM) via digital control, or more sophisticated methods. Most dimming drivers operate using the PWM method. With this method, the frequency could range from a hundred modulations per second to as high as hundreds of thousands of modulations per second, so that the LED appears to be continuously lighted without flicker.
A benefit of the PWM method is that it enables dimming with minimal color shift in the LED output. According to the Lighting Research Center, dimming causes LEDs to experience a similar shift in spectral power distribution as an incandescent lamp. However, if colored LEDs in an array are used to produce white light, the amount of shift, particularly with red and yellow LEDs, may produce an undesirable effect on the white light that is produced by the system.
Dimming does not result in a loss of efficiency. During dimming, the LEDs are still operated at the same voltage and current as during full light output. In addition, lamp life is not affected by dimming, as is sometimes the case with frequently dimmed fluorescent lighting. Rather, dimming LEDs may lengthen the useful life of LEDs, because dimming can reduce operating temperatures inside the light source.
Drivers can also be used to enable color-changing or sequencing. This can be achieved by dimming a mix of colored LEDs in an array to change colors. Another option is that the driver can work with a color sequencer, which receives the 10V or 24V LED driver output and converts it into three-channel output - usually red, blue and green - that can be mixed to create a wide, dynamic range of colors.
When a sequencer is used, it generates a preset sequence, with color changes occurring at a speed determined by the specifier. A third option is for each LED to be individually controlled and programmed by interfacing with DMX digital controller, enabling thousands of LEDs to dynamically dim up or down to create a seemingly infinite spectrum of colors.
Sameer Sodhi, product marketing manager - LED power supplies & controls, OSRAM SYLVANIA, points out that a common problem with LED system operation involves overloading the driver. LED drivers are rated for a maximum load that must be paid proper attention.
"One of the most common mistakes is to connect too many LED strings in series," he says. "Putting too many strings in series may result in too low a voltage being available to the last string(s) in the chain."
Another common problem, he warns, is using the wrong voltage driver. "When a wrong voltage driver is used, the LEDs will either not light up or may operate at higher currents than intended," he says. "A prudent practice is to check the voltage rating of the LED load being used against the rated output voltage of the driver. For example, using a 12V driver on a 10V LED load could result in significantly shorter life of the module."
Sodhi also believes that one of the most important LED driver features to examine is the quality of the DC output voltage of the driver. "To maximize the light output from the LEDs without overstressing them requires a constant DC current to be maintained through them," he says.
In addition, he cautions that remote mounting of the driver results in voltage drops and power losses on the DC wiring that must be properly accounted for.
Finally, Sodhi advises specifiers to be aware of ambient temperatures at the application. While LEDs have the ability to start at temperatures as low as -40°C, operating them at cold ambient temperatures can cause operating problems. "LEDs draw higher power at cold ambient temperatures, the opposite of what happens with fluorescent lamps, and this can lead to system malfunction," he warns. "For outdoor applications where the power supply is mounted remotely, the maximum LED load on the driver should be de-rated by 10-20 percent to avoid system conflicts during cold temperatures."
Marble points out that special attention should be paid to the environmental rating of the driver: Most drivers are "dry location only" in type and must be installed in a weatherproof electrical enclosure if used outdoors. Damp location drivers should be used in signs or raceways where some moisture is expected, and wet location drivers are typically supplied in a pre-assembled, sealed enclosure for mounting outdoors.
"Make sure that the driver is rated for use in its environment," he says. "And make sure that the driver has been evaluated and rated for use within the particular LED system."
Marble also believes that UL Class 2 ratings, required for LEDs in sign applications, can benefit general lighting applications. "UL Class 2 mandates that the driver has voltage, current and power below certain levels on the secondary," he says. UL Class 2 rated LED drivers provide electrical isolation from the AC line voltage, which allows for safe handling of the LEDs being operated at low-level DC voltages.
He also recommends drivers that have short-circuit protection, that are designed specifically for the given application, and that can handle temperature extremes. "Off-the-shelf DC power supplies are typically designed for room temperature applications such as IT or telecom," he adds. "Such power supplies may operate erratically or not at all under the rigors of a lighting application."
Finally, Marble advises that there are heat issues with LEDs even during normal operation. "LEDs are occasionally and incorrectly believed to generate little or no heat," he says, pointing out that there can be substantial heat generated in higher-wattage LED fixtures. "Hopefully, the integrator/fixture manufacturer designed appropriate heat sinks for the system. Still, allowing ample heat dissipation in the installation is good practice, such as mounting to metal or allowing some ventilation if possible."