Current distribution across LED modules connected in parallel

Technology: Electronic Control Gears
Type: Technical information

In the past, the wiring of conventional lamps used to be specified in advance: Lamps were either directly connected to the mains supply or there was a specific terminal pair available on the driver for each lamp. Today, state-of-the art LED technology not only enables a larger variety of possible applications, but also provides various options for the connection of modules and drivers in order to enable the ideal combination of constant current drivers and LED modules. However, new problems arise at the same time and perfect circuitry is more important than ever before.

Figure 1:
Top: Measuring setup with driver and modules without diffuser
Center: Real photo
Bottom: Luminance image in false color representation with single-side power supply
Right: Scale of the luminance image

Figure 1 shows a sample setup with 12 PrevaLED Slim3 units (PL-LIN S-Z3 1100-840-280) connected in parallel to an OTI DALI 80/220…240/2A1 LT2 L. All the modules together have an overall length of 3.36 meters. The false color representation clearly shows that the first module directly connected to the driver looks significantly brighter than the last module. This can lead to disturbing brightness jumps, for example when using a light strip where the luminaires installed are flush with each other. In this example, power is supplied from one side and is looped through the individual modules. In this context, single-side power supply means that the plus and minus terminals on the same module side are used. This is illustrated once again in the following schematic representation:

Figure 2:
Schematic representation -> “single-side power supply”

Figure 3:
Equivalent circuit diagram – single-side power supply Source 1

Figure 3 shows an equivalent circuit diagram of the setup. The parasitic resistances shown in the equivalent circuit diagram correspond to the unavoidable resistances of the lines, conductors and transition resistances on the terminals. These resistances add up depending on the number of modules and due to the voltage drop on each module, the voltage decreases from one module to the next. For this reason, the current supplied by the driver cannot be distributed homogeneously across all LED modules – and the brightness level differs from module to module.

Figure 4:
Top: Real photo;
Bottom: Luminance image in false color representation with crossover power supply
Right: Scale of the luminance image

Figure 4 shows the same setup, but with different wiring. All the modules achieve almost the same brightness level. This is due to the fact that the positive terminal is connected to one side of the modules connected in parallel and the negative terminal is connected to the other side. The following schematic representation illustrates this connection principle.

Figure 5:
Schematic representation -> “crossover power supply”

Figure 6:
Equivalent circuit diagram – crossover power supply. Source 1

Figure 6 shows the equivalent circuit diagram of this connection principle. When looking at the current flow through the modules in the equivalent circuit diagram (orange and bright orange lines) and adding up the parasitic resistors, the total resistance per module is 3x Rseg. This ensures that the current supplied by the driver is evenly distributed across all the LED modules.

The problem of uneven current distribution across LED modules connected in parallel especially concerns luminaires with Safety Extra-Low Voltage (SELV), because the currents are higher and the voltages are lower. If the LED modules are consistently connected in series, each module is supplied with the same current and the problem described cannot occur.

Last but not least, it should be noted that a difference in brightness within a ready-manufactured luminaire can also be caused by other conditions such as incorrect module installation, inequalities or the contamination of reflectors and diffusers. Furthermore, other chemical impacts on the LEDs through the use of improper materials or caused by the environment, etc. are also possible.

Source 1: Luminous Flux and Current Uniformity Analysis in Linear LED Modules
Ming Lia, Keng Chena, Arnulf Ruppb, Daniel Chua & Divya Vangaria
a Osram Sylvania, Inc., Danvers, Massachusetts, USA
b OSRAM Licht AG, Garching, Germany
Published online: Nov 21, 2014.