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PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 84 NR 8/2008
Grażyna GILEWSKA, Marian GILEWSKI
Bialystok Technical University
Digital control methods of LED and LED lamps
Abstract. This paper presents an overview of digital control methods of LEDs that are currently available. There are a few methods of control: PWM
method, pulse level correcting method and digital control of RGB LEDs. Circuit’s examples of supply LED of every method are presented. Optical
frequency response of pulse driving LED is discussed. The questions of thermal and long time stabilization of LEDs power are described too. In the
final part, measure problem of switched light source is presented.
Streszczenie. W referacie opisan
o cyfrowe metody sterowania diod elektroluminescencyjnych i ich zespołów. W szczególności scharakteryzowano
następujące metody: regulacji współczynnika wypełnienia impulsu, regulacji amplitudy impulsu oraz cyfrowego sterowania diod RGB. Przedstawiono
wybra
ne układy pracy diod elektroluminescencyjnych. Omówiono również częstotliwościowe charakterystyki emisyjne diod, zagadnienie stabilizacji
mocy emitowanej oraz techniczne problemy pomiaru parametrów świetlnych źródeł sterowanych impulsowo. (Cyfrowe metody sterowania diod
elektroluminescencyjnych i lamp LED).
Keywords: LED driving techniques, variable current source, digital dimming of LED lamps.
Słowa kluczowe: metody zasilania LED, regulowane źródło prądowe, cyfrowe przygaszanie lamp LED.
Introduction
Radiant flux of LED depends on supply power. LEDs
and LED lamps will be controlled by constant value of
forward voltage or forward current.
Value of forward voltage should enable to overcome
bandgap energy. Bandgap energy value depending on light
colour determines the light colour of LED (figure 1).
Fig. 1. Typical diode forward voltage versus bandgap energy for
LEDs made from different materials [1].
In practice, rarely (hardly ever) voltage control method
of LED is applied. Current control permits more stable
diode’s work. It comes from a gradient of current-voltage
characteristic of p-n junction. Circuit with dropping resistor
is the most simple solution. Constant value of diode current
is extracted by dropping resistance. Such solution puts big
power supply losses. Besides, this solution can’t control of
LED’s or LED lamp’s output light. Current sources are
applied more often than not in current driven method. They
extract constant value of diode current independently of
temperature and voltage supply changes. Digital methods
of LEDs control are applied much often. A human eye can’t
detect the high frequency ripple light. It can’t detect
changes smaller than 20% of average supply current too
[2]. Hence in digital dimming of LED lamps is applied pulse
width modulation (PWM) of supply current. When we
change value of duty cycle we have got a visual effect of
linear regulation of output power, because a human eye
integrates and averages pulsed light above 120 Hz (figure
2).
PWM dimming drives at only one current supply level.
Alternative digital driving method is an amplitude
modulation current pulse of supply. Value of duty cycle (D)
of current supply in this method is constant. It is about 10%.
Fig. 2. Convergent visual sensation of linear current and digital
control.
(1)
D = t
on
/T
where:
t
on
– turn on time, T – period of switching frequency f.
Switching frequency f is usually higher than 10 kHz.
Value of pick forward current is 4 to 6 times higher than
maximum DC forward current. Changes of pulse amplitude
causes similar visual effect of luminance changing like in
PWM method. Practical applications of LEDs drivers are
applied either first or second method. This paper presents
an idea of driving circuit which joined both methods.
An idea of alternative circuit driver
Block diagram of the proposed solution is presented in
figure 3. It includes: pulse generator, inverter and variable
current source. Pulse generator produces signal PWM with
controlled frequency.
Fig. 3. Block diagram of the proposed driver.
