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mart lighting is one of the
keys to the smart home.
Radio and network control can be
used to alter the level of lighting
either by direct control from the user,
employing a dedicated remote control
or with the help of a smartphone
app, or automatically based on
time of day. However, LED power
control for lighting requires special
consideration. Circuitry developed for
traditional dimming control does not
work well with the requirements of
solid-state lighting.
Most dimmers installed in homes are
based on a phase-cut circuit. The
advantage of this type of circuit is
that it is very simple to design and
is robust. The circuit works by simply
cutting the supply to the bulb for
a portion of the AC cycle, reducing
the overall voltage supplied to the
filament. This, in turn reduces the
filament’s light output. Because of the
comparatively long time that it takes
for the filament to heat up and cool
down, this switching is not noticeable
to the user except possibly at the
very lowest settings of the dimmer.
However, with LEDs, which have
much shorter response times, the
low switching frequency of the main
supply means that attempting to
use a similar strategy to control the
light output will result in significant
flickering. As a result, LEDs require
specialized power supply circuits
that can deliver a more consistent
voltage and current output to provide
power that minimizes flicker without
compromising efficiency.
Although it is possible to manipulate
light levels from LEDs by altering the
current supplied to them, this results
in a drop in power efficiency at lower
current levels. A power LED provides
its best efficiency at the highest
rated current. So, dimming behavior
is still best obtained using pulse-
width modulation (PWM) techniques,
turning the LED on and off at higher
frequencies than would be supported
by traditional mains-based dimmer
circuits.
In a PWM-based power converter,
a reference signal fed back to the
controller is used to determine
whether the transistor that feeds
power to the load should be on or
off. The reference is compared to a
periodic signal created by a ramp
generator. While the reference
signal is higher than the ramp, the
transistor is switched on. When the
reference signal falls below the ramp,
the transistor is switched off. The
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Power Control for LED Lighting
European Editors - Publitek Marketing Communications
30 l New-Tech Magazine Europe