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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.

PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 84 NR 8/2008 141

General-purpose generator Sony AFG320 was used to

measurements. Output signal of generator was shaped by
inverter with Schmitt Trigger Input type 74AC14 [3]. Output
current of inverter can drive small power LED.

This current switches current source which is supplying

LEDs. Source works as current diode and it can change an
amplitude of supply current I

reg

. It is made of SIPMOS

transistor type BS107 [4] and variable feedback resistor.
This solution allows to get pulse current with variable
frequency, duty cycle and switched value. Practically we
can change external generator by control circuit (figure 4).

Fig. 4. The modified driver circuit.

The control circuit may be constructed in Programmable

Logic Devices (PLD). PLD chip can create PWM frequency
variable signal and control LED current I

reg

.

Measurement results

Measuring path (figure 5) included: generator AFG320,

tested circuit, examined LED RGB type OSTA5131A-C [5],
Si photodiode type S1226-18BQ, 4 k



load resistor and

oscilloscope TDS714L. Photodiode S1226-18BQ measured
output light of LED. Voltage signal across photodiode was
registered by digital oscilloscope TDS714L.

Fig. 5. Structure of measuring path.

Examples

of

measurement

characteristics

were

presented on following figures. Figure 6 shows timings
green LED supplied by 11 mA current source. The duty
cycle of pulse current had 50%.

Fig. 6. Timing of green LED driven by 11 mA and 50% duty cycle
current.

Figure 7 shows timings the same LED driven by 5 mA

current. We can see different voltage of photodiode (curves
No 3).

Fig. 7. Timing of green LED driven by 5 mA and 50% duty cycle
current.

The next figure shows timings of blue LED which was

driven by 11 mA and 25% duty cycle current. This diode
has larger forward voltage than green LED (curve No 4).

Fig. 8. Timing of blue LED driven by 11 mA and 25% duty cycle
current.

Driving signals of red photodiode are shown in figure 9.

The first curve represents input voltage of circuit and the
fourth curve displays voltage across red LED.

Fig. 9. Timing of red LED driven by 8 mA and 75% duty cycle
current.

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PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 84 NR 8/2008

Conclusions

Presented driver circuit is very si

mple. It isn’t

commercial solution. When we use RGB LED to synthesis
of colour light we must control work of monochromatic
components (red, green and blue LEDs). It is difficult in
digital driving method. Figure 10 shows frequency spectral
distribution of green LED light. The LED was driven by 100
Hz pulsed current. We can see wide frequency spectral
band. We should have large frequency band apparatus to
measure and control this signals.

Fig. 10. Frequency spectral distribution of green LED light.

Figures 11 and 12 show spectral distributions for different
duty cycles of blue LED.

Fig. 11. Frequency spectral distribution of blue LED driven by 50%
duty cycle current.

Fig. 12. Frequency spectral distribution of blue LED driven by 75%
duty cycle current.

The PWM method is more spectral sensitive than pulse

amplitude modulation method.


This paper was prepared under S/WE/1/2006 grant.

REFERENCES

[1] S c h u b e r t E.F., Light Emitting Diodes and Solid-State

Lighting, Rensselaer Polytechnic Institute, Troy, NY 12180,
(2008),

www.LightEmittingDiodes.org

[2] R i c h a r d s o n C h . , LED Applications and Driving Techniques,

The Sight & Sound of Information, (2007),

www.national.com

[3] Fairchild Semiconductor Corporation:

74AC14 • 74ACT14 Hex

Inverter with Schmitt Trigger Input, DS009917,

www.fairchildsemi.com

[4] Semiconductor Group: BS107 SIPMOS

® Small-Signal

Transistor, Siemens Corporation,

www.siemens.com

[5]

OptoSupply: Superjasne diody pełnokolorowe OSTA5131A-C,

www.tme.com.pl

[6] Hamamatsu Photonics K.K.: Si pin photodiode S1226-18BQ,

www.hamamatsu.com

Authors

: dr inż. Grażyna Gilewska, Bialystok Technical University,

Faculty of Electrical Engineering, 45D Wiejska Street, 15-351
Bialystok, Poland, Phone: [phone], Fax: +48 085
7469400, e-mail:

[e-mail]

; dr inż. Marian Gilewski, Bialystok

Technical University, Faculty of Electrical Engineering, 45D
Wiejska Street, 15-351 Bialystok, Poland, Phone: +48 085
7469352, Fax: [phone], e-mail:

[e-mail]

.