Datasheet

forward voltages V
F
of the chips on the one hand and the dependency of the temperature and other
parameters of the dies from the current I
F
on the other hand. Please note the basic rules for parallel
and serial connections:
• In parallel circuits, the voltage is the same in each track, whereas the current
flow is divided according to the power consumption of the components in the
tracks.
• In serial circuits, the current is the same in the line, whereas the voltage drop at
each component depends on the power consumption of the component.
Remember that LED chips are driven and thus controlled by the current flow. Driving chips
serially as shown in figure 5 leads to the same current I
F
in each chip and therefore to a
homogeneity in brightness and color. Small differences between the chips are represented in
the different forward voltages of the chips. The overall voltage needed to drive a number of
chips serially at a given current I
F
can easily be calculated by the following equation:
n
V F sum
= V
F1
+ V
F2
+ V
F3
+ V
F n
=
∑
V Fi (5)
i = 1
Please note that the forward voltage changes with the current and the temperature of the chip. When
designing or looking for a power supply unit (PSU) to drive serially connected chips, be sure the
voltage of the PSU is high enough, i.e. V
F sum
or higher, otherwise some LED dies may not emit light.
Especially when connecting only small chains of LEDs serially, as for example the four chips of a
monochromatic ACULED VHL, it is safer to calculate rather with the maximum as with the typical V
F
.
Both values can be found in the ACULED datasheets.
Figure 5
Circuit of serial connection of
the ACULED VHL chips.
In a parallel connection as shown in figure 6, the current will not be the same through all chips, but
will depend on the forward voltage of each LED die in the specific track:
n
I
F sum
= I
F1
+ I
F2
+ I
F3
+ I
F n
=
∑
I
Fi
(6)
i = 1
As we can see in figure 2, even small variations in the forward voltage, which is quite regular
with LED chips even on one wafer, result in large changes in the forward current. Therefore, in a
parallel circuit the currents I
F i
flowing in track i can be quite different, leading to inhomogeneous
intensity and color and even damaging a chip once the forward current exceeds the maximum
allowable value. But even if no damage occurs, the unwanted effects will increase over time:
The chips with the higher current will get hotter than the others, resulting in lower forward
voltages due to the negative temperature coefficient TC
VF
. This will again increase the current
and therefore the temperature, etc. Even if not damaged, the LEDs will at least drift away in
brightness and wavelength. Figure 7 shows this effect at a line of LED chips driven in parallel for
a longer time. The higher the variation in the forward voltage, the faster the effects occur.
www.excelitas.com Driving the ACULED® VHL™ 7