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In the Driver's Seat: LED Drivers

By Craig DiLouie | Apr 15, 2015
Drivers Seat Image.jpg

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The LED driver acts as a power supply for LED modules, regulating output voltage or current for the light source. It transforms and conditions incoming power (typically AC, but it may be DC) and drives the current to the LEDs. In doing so, the LED driver performs a similar function as a fluorescent ballast.


The driver is a critical component of the LED lighting system. Its design affects operation, presence of flicker, service life, controllability and ability to withstand power surges. The overall trend is toward greater functionality.


Constant voltage versus constant current


Drivers are categorized as constant voltage or constant current, depending on whether they regulate output to provide a constant voltage or current to the LEDs. (See chart on page 70.)


Constant-voltage drivers are designed to operate with LED modules requiring a fixed voltage, typically 12 or 24V DC. Modules are connected in parallel across the driver’s output power. In the past, this type of device was called an LED power supply, but today, the terms “driver” and “power supply” are used interchangeably.


However, all LEDs need to be controlled by their current. For the constant-voltage driver to maintain a constant voltage, an internal regulator that limits current to the LED is required. This regulating element dissipates power, making constant-voltage LED systems less efficient. Constant-voltage drivers are best suited to applications where the LED load is not known and the designer needs flexibility regarding the number of modules to be connected to the power supply. As constant-voltage LED modules are added to the load, driver current will increase to the maximum level. Typical applications include sign and track lighting.


Constant-current drivers (e.g., 350 mA, 700 mA) are matched with constant-current LED modules, which are then connected in series or parallel. They provide higher efficiency, relatively easy dimming, longer expected life and greater control of brightness consistency across different LEDs in the same application. As a result, constant-current drivers are used in the large majority of LED products manufactured for general lighting applications. Many configurations are available to satisfy various applications.


Trends


“Top trends in LED driver design include programmable drivers, gaining popularity due to a plethora of benefits their flexibility offer to a luminaire manufacturer; flicker-free design, especially with low levels of dimming; and high levels of surge protection for harsh applications such as outdoor,” said Naveen Tumula, product group manager—Optotronic LED power supplies, Osram Sylvania.


“LED drivers are now available with a variety of control options,” said Greg Bennorth, director, Systems Products, Universal Lighting Technologies. “Bilevel switching, 0­–10V control, phase control and DALI digital dimming are the most well-known, but there are other propriety-protocol systems now available with LED drivers, similar to those that are used with fluorescent ballasts.”


Wireless and DMX-512 are also available.


LED drivers may use pulse-width modulation (PWM) or pulse-amplitude modulation (PAM) to achieve dimming. PWM varies the width of the electrical charge sent to the LEDs, flashing each LED on and off over time to dim the source. With PAM, the amount of current is pulsed over time using a fixed or variable frequency, dimming the source.


“The design of an LED driver has a huge impact on its integration with controls,” said Bruce Roberts, L.C., senior electrical engineer, New Technology Introduction, GE Lighting. “For example, a lot of people in the United States use the 0–10V input, which decides what system you’re going to hook it up to. But the 0–10V control is very simple and just for dimming. With something like DALI, you now have two-way communication with the fixture. The fixture can tell you how long it’s been running, how much power it’s using and more, so now your control system can be much more sophisticated in determining not only dimming level but even color and many other lighting parameters.”


Dimmable LED drivers feature two control wires used to acquire the 0–10V signal—a purple positive wire and a gray negative wire. Ten volts trigger full output, while zero volts dims to the low end of brightness, typically 5–10 percent of full light output. High-quality drivers provide dimming to this level without flicker. The relationship between the dimming signal and light output should be close to linear except for a “dead zone” at each end, typically 0–1V and 9–10V, though it may vary by manufacturer.


With LEDs, control has gone beyond the typical idea of switching and dimming into additional specifiable parameters.


“Tunable drivers allow for the fixture manufacturer to set the specific output current of the driver to achieve fixture output targets,” Bennorth said. “Since there is not a standard for LED modules, this flexibility allows for a wide range of application flexibility from a single driver.”


Besides tuning output and wattage to satisfy precise application needs, programming permits other capabilities. One notable capability is constant light output, a form of lumen maintenance dimming. Luminaires are dimmed to produce a constant target light output, which depends on required maintained light levels, over the life of the product. As the LEDs age and undergo lumen depreciation, the dimming eases off to maintain constant light output. This approach can generate additional energy savings while potentially extending the service life of the luminaire. If the LED modules are serviceable and replaced, the programming must be reset to restart the cycle.


Meanwhile, LED drivers are being designed for greater reliability in rugged applications, such as outdoor lighting.


“LED drivers used in outdoor applications are now available to handle tough transient environments to which they are exposed,” Bennorth said. “Not only do the drivers need to survive high transients, they need to suppress the transients so they do not pass the driver and damage the LED load. This was never an issue with magnetic ballasts and HID lamps but has been experienced with early-generation outdoor LED fixtures.”


Need to know


“When specifying an LED luminaire, electrical contractors should focus on efficacy, maximum operating temperature, power factor, THD, IP rating and surge-protection rating,” Tumula said. 


Paying attention to these features ensures the driver uses power efficiently, has a longer life, operates safely and poses a minimal impact on the electrical system.


Roberts advises contractors to ensure the driver matches the needs of the LEDs.


“There are three key specifications that need to be considered, including the current, the voltage range and the power available,” he said. “When buying a light fixture or package from a supplier, the driver and LEDs would already be matched. However, if you have a retrofit application where you might be choosing individual parts, then you will need to match: 1) the current that’s going to drive the LEDs in the fixture, 2) the voltage range that the LEDs will operate across, and 3) the power available.”


The LED driver may be connected to the load in series or parallel, offering design flexibility. When connected in series, the forward-voltage drop in each LED (typically about 3V) is additive. Therefore, a constant-current driver needs an output voltage range that can handle the applicable voltage drop. Multiple series-connected LED strings can be connected for parallel operation by a multi-output constant-current driver.


The wattage is the product of the drive current and the voltage. The drive current must match the needs of the LED; if the LEDs are subjected to higher levels of current than the design maximum, light output may increase, but more watts dissipate in the LED. If the heat sinking cannot get rid of this heat, lumen depreciation may accelerate, resulting in shorter product life.


As LED service life continues to improve, driver failure (rather than lumen depreciation) may be seen in the field as a lighting product end-of-life failure. LED drivers are designed with service-life ratings ranging from 50,000 to 100,000 hours, on par with the life expectancy of a majority of LEDs. These ratings are highly dependent on ambient temperature; the luminaire design and application characteristics can significantly affect actual life. When selecting a product, the temperature range of the driver should be evaluated and appropriately matched to the application.


“The hotter you make the driver and the LED, the lower the service life of each,” Roberts said. “The life of a driver is based on maintaining a reasonable temperature, which is typically 40°C. Ideally, you’ll want to stay at an ambient temperature around 25°C. A good way to look at it is, if the people in your room are happy and comfortable with the temperature, your LED drivers will likely be happy too.”


If the LED driver fails prematurely, it may need to be replaced in the field. LED drivers are typically mounted within luminaires. A high-quality luminaire offers good heat sinking; the metal enclosure offers additional EMI shielding; and proximity enhances efficiency. However, the driver may be mounted remotely for aesthetic or mechanical reasons. Examples include some decorative pendant luminaires, clean rooms where electronics need to be remote to minimize hazard, where a single driver operates multiple luminaires, or where there is very limited space.


Many commercial LED luminaires allow the driver to be replaced in the field; some drivers feature quick disconnects for easy servicing. However, unlike fluorescent ballasts, there are no industry standards for LED drivers at present. Each driver is customized to its LED load. Therefore, a driver should be replaced with one that is either provided by, or approved by, the LED luminaire manufacturer—offering appropriate current and voltage and power range for the given LED load, at a minimum. Additionally, the replacement driver must have a form factor that allows it to fit in the same space within the luminaire.


“The LED driver market is rapidly improving with new technologies and product offerings,” Bennorth said. “Selection of replacement drivers can be confusing. When you come across a luminaire where the driver needs to be replaced, get as much information as possible to help get the right replacement; take pictures of the driver label and, if possible, the module labels, and note how they are connected. Technical support is available to assist you as the industry changes; take advantage of this.”


LED drivers power it all


The driver is a critical component of the LED system, acting as the power supply and providing controllability, ability to withstand transient voltages, and, in programmable drivers, tuning and intelligence. Specifiers should pay attention to the manufacturer and features of the driver to ensure the lighting product provides reliable performance in its intended application.


A s low-voltage sources, light-emitting diodes (LEDs) require a constant source of direct current (DC) during operation. Individual LEDs might require 2–4 volts (V) and several hundred milliamperes (mA) of current. When connected in series in an array, higher voltage is required. To ensure consistent light output, the light source must be protected from line-voltage fluctuations that can cause disproportional changes in current. Driving the LEDs at current higher than manufacturer design limits can result in a brighter LED, but its light output would degrade faster due to higher internal temperatures. For these reasons, the majority of general lighting products that use LEDs need an electronic device that can convert incoming alternating current (AC) to DC and protect the LED from line-voltage fluctuations. This device is the LED driver.

About The Author

DiLouie, L.C. is a journalist and educator specializing in the lighting industry. Learn more at ZINGinc.com and LightNOWblog.com.

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