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38 E L EC TR I C AL CONNEC T I ON
AU T UMN 20 1 7
LED BY EFFICIENCY
W
e live in a 24/7 world, in
which artificial lighting
represents a huge part of
humanity’s energy use.
Electric lighting is mainly provided
by alternating current (AC) with direct
current (DC) being sourced from
batteries and solar panels.
In Australia and elsewhere in the
‘first world’, the development path
has been to higher-efficiency
lighting, with light-emitting diodes
(LEDs) leading the way in energy
conservation. The luminous efficiency
of LEDs far surpasses other
technologies and drawbacks such as
colour and small angular dispersion
have been overcome.
However, we are not taking full
advantage of the technology. We
could be availing ourselves of more DC
energy than is commercially on offer.
There are substantial efficiencies to be
gained by using DC rather than AC as
primary input.
This article looks at short-term
and long-term developments in the
energising of LEDs and takes a brief
peek at a world in which much AC low-
voltage reticulation is being replaced
by DC.
DC is already being used with
USB-powered LED strings – clearly a
hobbyist’s venture but nevertheless an
indication of things to come.
LED illumination products have
become so well established in a
relatively short time that the basic
understanding of circuits and physical
properties now seems unimportant. Yet
these basics provide further insight into
development opportunities for custom
lighting designers and experimenters.
A BRIEF TOUR OF LEDS
The light-emitting diode is in many
ways barely a diode.
It quickly exhibits avalanche
breakdown under reverse polarities
of 5V reverse bias or more, therefore
requiring protection against such
events by means of clamping diodes.
The basic operation in the forward
direction is like that of any semi-
conductor diode. By increasing the
positive voltage in the P (acceptor)
region, electrons from the N (donor
region) are encouraged to flow to the
P region.
In LEDs electrons lose energy as
they jump back from the conduction
band to the ‘valence’, or bound region
(where they form part of the bonding
links between neighbouring atoms).
This energy is emitted in the form of
light quanta.
LEDs are therefore very similar to
photovoltaic cells, in which the
reverse process takes place. However,
the photovoltaic effect can also
happen in LEDs. In fact, it can be
used for testing LED wafers, thereby
avoiding damage to delicate copper
conduction terminals.
In Figure 1 the forward voltage
characteristic of an LED is shown. The
equation governing the current/voltage
relationship is standard for any semi-
conductor diode:
In this relationship, q is the charge
in coulombs of the electron, V is the
forward voltage on the P-N junction, k
is a constant (Boltzmann constant), and
T the junction temperature (in Kelvins,
i.e. absolute temperature). I
o
is the
saturation (dark) current also flowing at
reverse bias voltage.
Light output as a function of junction
temperature is shown in Figure 2, and
light output as a function of forward
THERE IS MUCH TO BE GAINED
FROM USING DC RATHER THAN
AC AS PRIMARY INPUT FOR LEDS.
PHILKREVELD
EXPLAINS THE
TECHNICALITIES.
0
0.0
1.0
2.0
3.0
4.0
5.0
100
200
300
400
500
600
700
800
900
1000
Forward Voltage (V)
I
1-
=
(
I
f
o
aV
kT
e
Figure 1: Forward voltage characteristic of an LED.
GUIDING LIGHT