Datasheet

The Electrical Principle of LEDs
LED dies are semiconductors and hence different in their electrical characteristics from
conductors such as incandescent light bulbs. In the latter the electrical resistance R will increase
with rising temperature and thus with increasing electrical current, which gives a kind of
limitation of the electrical current flow. The direction of the current usually does not matter. With
a light bulb, by the heating of its filament, light is emitted as radiation in a large spectrum from
mostly IR to blue.
Semiconductors such as LED dies behave in a different way. As the name LED - Light Emitting
Diode - indicates, light is emitted from a diode, a semiconductor chip doped with impurities. Like
with a normal diode, the current direction and thus the polarity of its so-called forward voltage V
F
is given by the anode (positive doped side, p) and the cathode (negative doped side, n). The
corresponding forward current I
F
flows from p to n, leading to a recombination of the charge
carriers electrons (n) and holes (p) in the pn junction layer by emitting light. Depending on the
band gap energy of the pn junction materials used, the spectrum of this light is UV, VIS or IR
with a narrow bandwidth. Applying a voltage with wrong polarity does not light the LED but can
damage the chip if the voltage exceeds the reverse breakdown voltage. Therefore this
maximum reverse voltage V
R
that the chip can handle without damage is given in the specific
ACULED datasheets. Placing a diode in inverse polarity parallel to the LED can also protect the
LED against too high reverse voltage. With some of the ACULED VHLs and on demand with the
ACULED DYO, bipolar ESD protection diodes are used that protect the device according to this
principle.
Figure 2
Typical current vs. voltage
characteristic of an LED shown at
the example of the ACULED VHL
RGYB at 25°C board temperature.
The color of the curves responds to
the emitted color of the chips.
Figure 2 shows the typical curve of the forward current I
F
versus the forward voltage V
F
at high
power LED chips used with the ACULED VHL RGYB. We can see that even small changes of
the voltage result in huge changes of the according forward current, at least in the region where
the LED is typically driven (100 to 700 mA). Since most of the properties of interest like intensity
and color of an LED depend on the current I
F
, it should be kept stable and controlled. We will
learn in later chapters about the influence of I
F
on the LED parameters as well as methods to
keep these parameters constant. When using a power source without constant current like a
battery for example, we have to make sure that the current I
F
cannot exceed the maximum
forward current I
F max
to avoid damaging the LED. The easiest way to do this is using a current
limiting series resistor as shown in figure 3. Without a current limiting device, an LED product
www.excelitas.com Driving the ACULED® VHL™ 4