Light Emitting Diodes are fast becoming the dominant technology in the South-African lighting market. LED’s require specific devices called LED drivers for proper operation. Most end-users are unaware of this as the driver is incorporated into the LED lamp or the LED luminaire.
Selecting the correct driver for an LED solution is common practice for designers of LED luminaires. However, for re-sellers looking to replace failed drivers in the field or trying to find the right driver for LED panels or downlights that were supplied without driver, finding the right driver for the job can be a challenge. This article, by Henk Rotman of Tridonic, gives practical guidelines.
Comparison with traditional technologies
Most people active in today’s lighting market grew up with traditional technologies and as a consequence are used to the fact that most light sources need additional components for proper operation, like ballasts and starters/ignitors for discharge lamps and transformers in the case of low-voltage halogen lamps.
Originally, ballasts and transformers were based on electro-magnetic technology. Choosing the right ballast was pretty straightforward as, in most cases, it was a matter of matching the wattage of the lamp with the wattage of the ballast. For example, a Mercury Vapour lamp 125 W needed a 125 W MV ballast.
The introduction of Electronic Control Gear (ECG) made choosing the right ballast a bit more complicated, as ECGs gave users the option to connect more lamps to the same ballast. Additionally ECGs offered the option of dimming, and choosing an ECG with the right dimming interface (1-10 V, DSI, DALI, etc) was important as it had to match the control system.
With the rise of LEDs the market has to learn how to choose the correct driver for the LED solution. With LEDs being fundamentally different from traditional light sources, choosing the right driver is also very different from finding the right components for a traditional light source. Fundamentally there are two main types of LED drivers: constant voltage drivers and constant current drivers, which are designed to operate LEDs with different sets of electrical requirements.
Constant voltage drivers supply a fixed voltage to the LED solution, usually 12 V or 24 V, but the current may vary up to its maximum rating. Constant voltage drivers are common in signage applications and are also used for LED strips.
Constant current drivers offer a fixed current to the LED solution, while the output voltage will vary within a specified voltage range. Constant current drivers are used mainly for general lighting applications, like LED downlights and LED panels. This article focuses on constant current drivers.
Matching LED driver and LED solution
How does one find the correct (Constant Current) LED driver for an LED solution? Unlike selecting the correct control gear for traditional light sources, the first parameter to look at is not wattage but the operating current required for the LED solution.
Common values are 350 mA and 700 mA, but it usually is between 200 and 1400 mA. Pay attention to the fact that the forward current can be temperature dependent. There are cases where the LED solution requires a specific current, one not offered by an LED driver. In those cases the advice is to choose a driver offering a lower current. A lower current will lead to a lower light output. Do not go for a driver offering a higher current as this might lead to overheating the LED solution.
The next parameter to look at (and one which is sometimes ignored) is the required forward voltage (Vf) of the LED solution. The forward voltage of a single LED is the voltage that must be applied across the leads of the LED, from anode to cathode, in order for a current to flow through the LED and hence for the LED to generate light. The forward voltage of an LED module is the sum of the forward voltages of the single LEDs that make up that module or panel (this is in case the LEDs are connected in series). As there are tolerances in the forward voltage of single LEDs (partly because the Vf of an LED slightly increases over lifetime), the forward voltage of an LED module/ panel is usually given as a voltage range, not a single value. As with the forward current, the forward voltage is temperature dependent.
Besides offering a fixed operating current any constant current driver offers an output voltage range. When selecting an LED driver, the output voltage range of the driver must cover the full forward voltage range of the LED module (see Figure 1). If the forward voltage of an LED module falls outside the output voltage range of a driver, the following will happen:
- Forward voltage LED solution > output voltage range driver: LED module will not work/no light.
- Forward voltage LED solution < output voltage range driver: Risk of unstable behaviour of the LED module/flashing.
In cases where the forward voltage of the LED module is just outside the output voltage range of the driver, the LED module may turn on and off a few times before remaining off. At this stage one might ask, “What about the wattage ... surely the wattage of the driver has to match the wattage of the LED module?” The answer is that this is not necessarily the case. If one has selected a driver that offers the correct operating current and output voltage range one will notice that the wattage of the LED solution is lower than or equal to the wattage of the driver. If the wattage of the LED solution is higher than the wattage of the driver the matching of operating current and output voltage has not been done correctly and one needs to review the choice of the driver.
If the above steps have been followed, hopefully one has found several suitable drivers. Now it is time to look at other (mostly practical) requirements to narrow the selection.
Additional requirements can be:
- Size/form factor: LED drivers are available in many sizes and form factors, in linear, rectangular and round shapes.
- Mounting options: Most drivers are designed to be built into a luminaire, but there are also drivers with a so-called strain relief, which can be placed on top of a ceiling and then connected to an LED solution.
- Application: As outdoor lighting is a particularly demanding application, there are drivers specifically developed for use in outdoor lighting. These drivers are often potted, have a high immunity against voltage peaks/transients and higher lifetime specifications. Indoor drivers
have specifications aligned with indoor use.
There are also specific drivers for industrial lighting, these have some of the higher specifications in common with outdoor drivers, like higher immunity, and better lifetime in relation to ambient temperature.
- Dimming: Many drivers are for fixed-output use. However, there is also a wide choice when it comes to dimmable drivers. Important is the communication between driver and control devices. Communication can be based on protocols like 1-10 V, DALI or DSI, or it can be based on using concepts like phase-cutting or switch dimming (using a bell press). Advice is to spend the necessary time matching the control device/control system with the right driver.
- Ingress Protection (IP) rating: Most drivers are rated IP20 and are designed to be built into a luminaire. There are however more and more drivers with a higher IP rating, mainly for outdoor applications.
- Power factor: Most drivers have a high power factor (usually 0.95 and higher). However, some low wattage drivers might have a low power factor.
- Ripple current: The output ripple current of an LED driver determines the ‘flicker’ (better described as ‘unacceptable light variation that is directly perceived by an average observer’) of an LED solution. The lower the ripple current of the driver the lower the risk of flicker. In this instance, knowing the application is important, for example, a driver used in a floodlight installed in a stadium where TV images are recorded should have an extremely low ripple current, while a driver used in a floodlight to light up your yard can have a higher ripple current.
Any person struggling to find a suitable driver not need despair: reputable driver suppliers can be contacted to assist in making the right selection.
Let’s go into a bit more detail about the operating current offered by constant current LED drivers. Many of these drivers offer a single operating current (350 mA or 700 mA) but we see more and more programmable drivers being used. Programmable drivers offer not a single operating current but an operating current range (for example, from 200 mA to 700 mA). This greatly increases the versatility of the driver, as the same driver can be used for different LED solutions. Another advantage of programmable drivers is that it is possible to optimise the LED solution for energy-efficiency or lowest initial investment, since a low operating current increases the efficacy of the solution and a high operating current reduces the component costs.
To set the correct operating current for LED drivers, different programming methods are available: using dipswitches, resistors or software and programming the driver using Near Field Communication. Programmable drivers are also called ‘window drivers’ as they not only offer an output voltage range, but also an operating current range. When visualising both the output voltage range and the operating current range in a graph it looks like a ‘window’. Important is to realise that the driver is able to offer any combination of output voltage and operating current as long as it is within the window.