Introduction to Light Emitting Diodes (LEDs)
Introduction
Light-emitting diodes (LEDs) are promising lighting sources for general lighting applications with the
promise of being more than ten times as efficient as incandescent lighting. Such characteristic combined
with their long operating life and reliability has made them becoming a potential choice for next
generation of lighting systems including automotive, emergency, backlight, indoor, and outdoor. To
ensure proper operation and to control the light intensity, LEDs need an efficient driver, normally
implemented by power electronics-based conversion stages, to match the LED characteristics with the AC
grid voltage and to generate a controllable, high quality light.
Luminous flux is an attribute of visual perception in which a source appears to radiate or reflect light.
“Brightness” is a non-quantitative term that is often used to describe this characteristic. Luminous flux is
measured in lumen and is the light power measured multiplied with the V-λ scaling function which
compensates for the human eye’s sensitivity to different wavelengths. The luminous flux of LEDs is
largely governed by the current flowing through the device. Fig. 1 shows a typical curve characteristic of
an LED (luminous flux versus the current).
Fig. 1: LED Current vs. Luminous Flux [1]
Another variable that plays a significant role in the amount of luminous flux of the LED is the
temperature of the LED device. Thus, it is important to control the temperature of these devices in order
to maintain full control. Fig. 2 shows the relationship between the forward voltage and the forward
current of an LED. As expected, the LED has a threshold forward voltage beyond which it allow current
to flow through it. Once the LED voltage reaches its threshold value, its current becomes an exponential
function of its voltage.
, Fig. 2: LED Forward Voltage vs Current [1]
Fig. 3: Forward I–V characteristics of the 340 nm LED measured at 5-300 K [2]
As Fig. 2 shows, voltage control of LEDs might be possible. However, a slight increase in the
temperature results in a shift in the curve characteristic to the left as can be seen in Fig. 3. Therefore, over
time, the forward current of the LED increases which results in a higher temperature and subsequently a
shift in the current, i.e., a positive feedback. This process continues until the device fails. Therefore, a
current control is the preferred control of an LED.
LED Dimming Techniques
The capability of reducing the intensity of output light is called dimming. LEDs can be dimmed by
controlling the forward current flowing through them. This can be achieved in two ways: Amplitude
Modulation (AM) and Pulse Width Modulation (PWM).
Introduction
Light-emitting diodes (LEDs) are promising lighting sources for general lighting applications with the
promise of being more than ten times as efficient as incandescent lighting. Such characteristic combined
with their long operating life and reliability has made them becoming a potential choice for next
generation of lighting systems including automotive, emergency, backlight, indoor, and outdoor. To
ensure proper operation and to control the light intensity, LEDs need an efficient driver, normally
implemented by power electronics-based conversion stages, to match the LED characteristics with the AC
grid voltage and to generate a controllable, high quality light.
Luminous flux is an attribute of visual perception in which a source appears to radiate or reflect light.
“Brightness” is a non-quantitative term that is often used to describe this characteristic. Luminous flux is
measured in lumen and is the light power measured multiplied with the V-λ scaling function which
compensates for the human eye’s sensitivity to different wavelengths. The luminous flux of LEDs is
largely governed by the current flowing through the device. Fig. 1 shows a typical curve characteristic of
an LED (luminous flux versus the current).
Fig. 1: LED Current vs. Luminous Flux [1]
Another variable that plays a significant role in the amount of luminous flux of the LED is the
temperature of the LED device. Thus, it is important to control the temperature of these devices in order
to maintain full control. Fig. 2 shows the relationship between the forward voltage and the forward
current of an LED. As expected, the LED has a threshold forward voltage beyond which it allow current
to flow through it. Once the LED voltage reaches its threshold value, its current becomes an exponential
function of its voltage.
, Fig. 2: LED Forward Voltage vs Current [1]
Fig. 3: Forward I–V characteristics of the 340 nm LED measured at 5-300 K [2]
As Fig. 2 shows, voltage control of LEDs might be possible. However, a slight increase in the
temperature results in a shift in the curve characteristic to the left as can be seen in Fig. 3. Therefore, over
time, the forward current of the LED increases which results in a higher temperature and subsequently a
shift in the current, i.e., a positive feedback. This process continues until the device fails. Therefore, a
current control is the preferred control of an LED.
LED Dimming Techniques
The capability of reducing the intensity of output light is called dimming. LEDs can be dimmed by
controlling the forward current flowing through them. This can be achieved in two ways: Amplitude
Modulation (AM) and Pulse Width Modulation (PWM).
