Basically, the light emitted from a led arises from the release of energy produced during the recombination process between electrons (-) and holes (+) of the PN junction of the diode directly polarized.
The quantity of these combinations (-+), and therefore, the amount of light emitted, depends on the movement of these electrons, or what is the same, on the current that circulates though the semiconductor.
Therefore, we can say that the light emitted from a LED will depend on the current that circulates through it. And this dependence will be quasi linear although its efficiency will decrease when the current increases.
Understandably, to get a uniform light emitted by various diodes that make up a module or luminaire, the current by each of them should be as similar as possible. Here is where constant current drivers emerge as the most suitable option wherever it is possible to use them. Let’s look at the reasons.
Constant voltage systems are mainly used when one or more strips of LEDs with different lengths (number of LEDs) must be parallel connected. These drivers are characterized by a low voltage and high output current.
Fig. 2 shows how we can supply with the same voltage two strips with different led number. If they are well chosen and equalized, the current through each led is the same.
It is important to note that each string will normally have several LEDs in series (a typical value is 3 LEDs to be powered at 12VDC).
In this case
Iout = I1+I2+….+In
where “n” is the number of connected strips.
I1 = Id11 + Id12 + …. + Id1N ≈ N1 x Id
In = Idn1 + Idn2 + …. + IdnN ≈ Nn x Id
being “Nn” the number of connections in parallel (strings) of the Strip “n”. The second equality is given to a equalized circuit where:
Id1 ≈ Id2 ≈ …. ≈ IdN ≈ Id
The driver must supply an output constant voltage suitable to the number of diodes in each parallel string. Typical values are 12VDC, 24VDC and 48VDC.
But let’s see why the equalization of the LEDs is necessary.
A typical graph of led current as a function of its direct polarization voltage would be that of Fig. 3.
Fig.3.-Output current for a typical led as a function of the forward voltage.
Supposing that we are working with diodes with a voltage tolerance (binning), of 0,2V (say between 2,9 and 3,1V). This would imply that by applying an identical voltage to the diodes, their current differences would be up to 900 mA!. It is possible to check it in Fig. 3 for three diodes with Vf maximum differences of 0,2V (for the same current) if we suppose we connect them to 3VDC.
The difference would be approximately half (450mA), for a binning of 0,1V. But there will also be voltage differences in the successive strings due to voltage drops.
These differences are clearly unacceptable, as it can be seen in the relative luminous flux (RFL) as a function of the output current graph (fig. 4), where for 400mA of current difference the flux can change by 50% what leads to the need to incorporate some circuitry to match the currents by diode (typically a series resistance), which will incorporate some losses by heat, decreasing the efficiency of the system.
In addition, there will be differences of temperature (dissipated power) in each diode dependent of each diode voltage and its current because both factors are subject to variability.
Fig.5.- Diagrama de conexión de leds en paralelo con resistencias de ecualización.
The best alternative whenever possible, will be powering the LEDs with a constant current Driver, since the source (driver) will tend at all times to set the current trough the LEDs, which therefore will match their luminous flux and shall adjust its characteristic voltage.
In this circuit:
Iout = Id1 = Id2 = …. = IdN = Id
Vout = Vf1 + Vf2 + …. + VfN » N x Vf
Where Vf will vary according to the binning of voltage and where “N” is the number of diodes in series.
As the RLF shown in Fig. 4 depend on the Id, that in this case it is identical for all LEDs, there will be the same luminous flux for each of them, and a lighting of the highest quality and balance in brightness and color is achieved.
Temperature differences in each diode (dissipated power) will only depend on the differences of their forward voltages.
In those cases where several strings are connected in parallel, the behavior is not ideal and will depend on the selection of the LEDs. Even so, we will see how the differences are smaller than those typical in the case of applying a constant voltage.
As in the previous case, we assume a binning between Vf1 and Vf2. That in a configuration like the figure 6 it would give:
If Vf1 = 2, 9V and Vf2 = 3, 1V (0.2V binning) we will have:
I2 = Iout x 0,483
I1 = Iout x 0,517
I1 = 1.07 x I2 (7% max variation)
If Vf1 = 2, 95V and Vf2 = 3, 05V (binning 0, 1V) will have:
I2 = Iout x 0,492
I1 = Iout x 0,508
I1 = 1.03 x I2 (3% max variation)
Which indicates that, although there is a deviation, the appropriate selection of LEDs may limit it considerably, allowing systems with a greater amount of LEDs with lower current (increased efficiency).
Rafael del Águila / Ingeneer – Export Area Manager / firstname.lastname@example.org