Light Emitting Diodes (LEDs),
from an electrical as opposed to an optical perspective,
can be operated using the same drive considerations
as other diodes. Similar limiting properties must
be taken into account as with standard electrical
diodes including maximum reverse voltage and maximum
LEDs are preferably used in a circuit configuration
that controls the current through the device rather
than the voltage across the device since this
will yield the most stable light output, which
is critical in most applications. As with any
diode, there are small variations in the junction
voltage at a specific current due to unavoidable
variations in the manufacturing process. The
equations that describe the electrical behavior
of an LED are the same as for any other diode
and any simple electronics book will serve as
a suitable reference.
The temperature dependent behavior of the diode
forward voltage (Vf) at a specific drive current
(I) implies that at higher drive conditions the
electrical power dissipated in the device increases.
Basically, power is dissipated in three areas
in the device: the p-contact region, the pn junction
region, and the n-contact region.
For properly designed devices the
voltage drop across the resistances in the p and
n regions is much smaller than the junction voltage
and hence most power is dissipated in a very narrow
volume of semiconductor material near the junction.
This volume heats up very quickly, typically on
the order of microseconds, and then heat flows
out of the device by the lowest thermal impedance
path(s) to the external thermal environment. Once
thermal equilibrium is achieved the Vf, wavelength,
and light output of the device stabilizes.
Overheating of the diode junction adversely affects
the lamp performance for many reasons. First,
the efficiency of an LED drops with increasing
temperature, second, the lifetime of an LED is
reduced at higher temperatures, and finally, the
packaging material that surrounds the diode can
be catastrophically damaged at high temperatures.
This last effect can be very significant since
it is inherently non-linear. The encapsulants
used to package LED reach a glass transition temperature
above which the plastic flows very easily, which
can cause several deleterious effects in a lamp.
To the end user of the LED lamp this implies that
there is a maximum junction temperature, Tj, which
should not be exceeded whether using DC or pulsed
There are two basic ways to control the current
through a diode. In one case one or more LEDs
in series are connected to a standard current
supply which is set up to deliver the amount of
current specified for a specific application.
An advantage of this method is that it accommodates
variation in the junction voltage of the LEDs
at a specific current level. In the second case
LEDs can be run in parallel with a single voltage
supply. Normally a one or more series resistors
are used to control the current through the diode.
The primary advantage of this scheme is that
it’s simple and is readily used where a DC power
source is already available (e.g. from a battery
in a car). The primary difficulty is that the
drive current through the diode will change with
its Vf. A practical example will illustrate this.
Assume that standard off-the-shelf diodes have
a voltage range of 1.90V to 2.2V at a current
of 20mA. One design could have six diodes connected
in series (i.e.no series resistor is used to limit
the current), and they’re driven with a 12V battery.
If the Vf values for each of the diodes is 2.0V,
then exactly 20mA flows in the circuit. However,
for diodes with Vf of 1.9V the drop at 20mA will
be only 11.4V, which implies that they will draw
more than the designed 20mA. For diodes with high
values of Vf the situation is reversed and less
than the design current flows.
The situation is made worse by the fact that
in cars the allowed variation in voltage across
the battery is very large. In general, it is impractical
to drive LEDs with a voltage source without a
current-limiting series resistor. The disadvantage
of this is that excess power is dissipated in
the series resistor which lowers the overall efficiency
of the circuit. The conclusion is that each design
needs to be optimized to a specific application,
and either current or voltage drive circuits can
be used with LEDs.
Reverse Voltage Considerations
An LED, like any diode, conducts current easily
under forward bias, but blocks the current flow
when reverse biased. However, since the devices
are optimized for high light output their characteristics,
as blocking diodes, are not very good. Typically
a device will conduct a few microamps (uA)at –5V,
but this leakage current becomes significant at
more negative voltages. Therefore, using LEDs
in an AC circuit (e.g. at 120VAC) where they both
emit light and block current in the reverse direction
is not normally recommended. Using LEDs under
these conditions can lead to unpredictable performance
and a significant reduction in the lifetime of