Lighting in Design Q2 2018

Selecting the correct LED driver

L ight Emitting Diodes are fast becoming the dominant technology in the South-African lighting market. LED’s require specific de- vices called LED drivers for proper operation. Most end-users are unaware of this as the driver is incor- porated 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 conse- quence 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 125W needed a 125 W MV ballast. The introduction of Electronic Control Gear (ECG) made choosing the right ballast a bit more compli- cated, 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. LED drivers 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 tradi-

tional 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 cur- rent 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 12V or 24V, but the cur- rent 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 tempera- ture 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 volt- age that must be applied across the leads of the LED, from anode to cathode, in order for a current to fow through the LED and hence for the LED to

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LiD Q2 - 2018