Digital Systems
 

Technical application guide

Matching of LED module and LED driver in constant-current systems

Matching the driver and the LED module is not as easy as it may seem. Therefore, OSRAM offers not only components but also released systems of module/driver combinations, which solve all the issues described on the following pages. You can learn more about these systems in the OSRAM Matchmaker.

Safety first

Drivers providing an internal SELV isolation barrier are the most frequently used drivers. They allow for a simpler construction of the luminaire. Drivers without mains isolation, however, are gaining in market share because they excel in costs, efficacy and size. When a non-isolated driver is used, the LEDs show live voltages against earth. Consequently, a safety isolation has to be established between the LEDs and touchable parts of the luminaire. In order to support realistic constructions of the luminaire, safety issues already have to be considered during the construction of the module. This requires detailed know ledge of safety standards, drivers and LEDs.

Immunity/surge

Transient voltages from the mains can travel through the driver to the module. Common mode transients can cause high stress to the isolation between LED and PE. If a driver offers an earth terminal, it is always advisable to implement the earth connection. The earth connection of the driver greatly reduces the common mode transients that are transferred to the module. This recommendation is valid also for luminaire constructions that would meet safety regulations even without connecting the driver to earth.

Operating window of an LED driver

Operating points

The operating point of the LED module must be within the operating range of the LED driver. Due to natural variations of modules and drivers, multiple pitfalls have to be avoided to meet this simple requirement.

Voltage-current characteristic of a single LED

The characteristic of the single LED can be visualized in a V-I graph.

  • VF depends on binning
  • VF depends on temperature
  • VF depends on current
  • VF depends on aging

Already in pure DC operation, the prediction of the operating point is not a point but a line.

Module meets LED driver

By drawing the parameters of the LED module and the LED driver in a common V-I graph, the matching situation can be visualize.

The above V-I graph shows a correct system match. The predicted line of the operating points is completely within the operating range of the LED driver.

Voltage-current characteristic of LEDs

V-I graph

Note: In case a module exceeds the limits of the LED driver operating range, shutdown or blinking may occur with some drivers.

Influence of the LED module on VF

The forward voltage can differ from the rated values and has the potential to cause an undesired shutdown or blinking. It is influenced by the following effects.

Forward voltage influencing factors

  • LED driver does not deliver pure DC (ripple current)
  • LED driver output current deviates from selected value
  • Voltage binning of module
  • Aging of module
  • Temperature of module

In addition to the correct selection of the forward voltage, it is important to avoid an overload of the LED module. Check the module current load by taking into account the current accuracy of the LED driver and system ripple current.

Additional requirements on driver/module matching for dimming

The real number of supported LEDs needs to be checked according to the minimum and maximum forward voltage in the worst case conditions. They have to match the minimum and maximum output voltage of the used LED drivers. The forward voltage of the connected LED module in dimming condition is lower than the forward voltage in nominal condition but still has to be above the minimum output voltage of the LED driver.

Influence of dimming and temperature on the forward voltage

Influence of binning, aging and driver tolerances on the forward voltage