Light-emitting diodes (LEDs) have now supplanted most other lighting technologies. Thanks to their versatility, low cost, and efficiency, LEDs are now used in any type of application in the most disparate cases: Status indicators, backlights for LCD displays, and traditional or atmospheric lighting are all areas in which it is now more convenient (both economically than technologically) to use LEDs rather than classical light-emission sources. Let’s have a look at the characteristics that have imposed LEDs as a standard for light sources and the related types of applications.
Physics of the LED
An LED is an active semiconductor electronic component that lays its foundation on the diode. The phrase “light-emitting diode” underlines how this technology is nothing more than a p-n junction with geometric and physical characteristics designed to exploit the effect of the electroluminescence of semiconductors. In fact, by directly biasing a p-n junction with an adequate voltage greater than the threshold (or forward) voltage, the charges near the junction move from energy level to energy level; when holes and electrons recombine, if the released energy is sufficiently high, photons will be emitted, whose frequency (therefore, color) and luminous intensity depend on the physical characteristics of the material and on the voltage level applied across the device. This is the basis behind color LEDs, allowing engineers to use these components in the most varied ways and exploit them in a multitude of applications.
In addition to the color of the light emitted, the minimum threshold voltage that triggers the current flow also depends on the type of semiconductor the device is made of; the most used materials for the realization of LEDs are AlGaAs, GaAlP, GaAsP, SiC, GaN, GaP, Si, and C.
Driving of LED devices
The LED is an electronic component driven in current. For this reason, a current-limiting resistance must always be provided in the driving circuit, without which the only resistance seen by the component would be the internal one of the junction itself; not limiting the current would run the risk of damaging the component and of obtaining deviating behavior from one device to another.
Figure 1 shows the basic scheme for driving an LED. In this circuit, a positive voltage is supplied to the base of a MOSFET, triggering a flow of current between drain and source, thus conducting the LED on the drain. Note the presence of the limiting resistor. Assuming that the optimal forward current of the component is If, the threshold voltage Vth, and the supply voltage of the LED Vin, the sizing of the limiting resistor is calculated as R = (Vin – Vth) / If. For example, a classic LED used as a power status indicator can have Vth = 1.8 V and If = 20 mA; supposing to supply the circuit with a voltage Vin = 5 V, the limiting resistor will have the value R = (5 – 1.8) / 0.02 = 160 Ω.
Figure 1: LED driver base circuit (Source: Davide Di Gesualdo)
The proposed scheme is used when the driving takes place through a microcontroller; in this case, it is always recommended to adopt a transistor (or similar component) capable of withstanding the forward currents of the LEDs: If the component were connected directly to a GPIO of the microcontroller, the risk of damaging the chip (due to the currents at stake) would be extremely high. Obviously, if there is a need to drive power LEDs (which can absorb even more than 3 W or 5 W), it will be necessary to adopt drivers that are congruent to the required current. In this regard, the Cypress Power PSoC line of microcontrollers is worthy to note, as it is capable of supplying a power MOSFET directly in the chip, which is extremely useful for driving power LEDs by minimizing the surrounding circuitry.
One characteristic that has allowed LEDs to become popular in the world of lighting is undoubtedly the possibility of exploiting the pulse-width–modulation) technique to achieve dimming, i.e., raising or lowering the current flowing in the LED (therefore, check its luminous flux). This technique consists of applying a control signal having a square wave with variable duty cycle: The current used will therefore be proportional to the time Ton of the applied wave, thus allowing the electronic control of the brightness of the LED.
Figure 2: PWM operation
Clearly, the PWM technique, which is extremely simple to apply, can be ineffective if a precise brightness control is needed; in fact, remember that, because the LED is a diode in all respects, its voltage/current characteristic is non-linear and, therefore, the variations of the current obtained by modifying the duty cycle are also non-linear.
To overcome this drawback, it is necessary to use specially designed LED drivers to provide constant current against a proportional voltage signal. This solution avoids the continuous switching on and off cycles of the luminous components, improving both the duration of the devices and the quality of the light emitted, as it does not flicker.
Common applications of LEDs
As mentioned above, LEDs are very versatile, and this is also due to the fact that there are different types available.
Classifying them according to the dissipated power (therefore, the luminous flux produced), we can basically find three types of LEDs: low-power LEDs, high-brightness LEDs, and power LEDs.
Low-power LEDs have a typical forward current of 15 mA and are used as status indicators in electronic devices (power-on indicator, connection status, communication indicator between devices, etc.). Their use is the most classic and old, and their packages are both PTH and SMD. The angle of illumination is not essential for this type of device.
High-brightness LEDs have a typical forward current ranging from 30 mA to 100 mA and can be used as elements of weak lighting (e.g., pedestrian path indicators), although the main use is as a backlight in segment displays and in LCD panels. The latter has given a significant boost to the diffusion of LEDs, as most of the LCD panels of modern TVs adopt LED backlighting.
Power LEDs have forward current ranging from 100 mA upward. It’s easy to imagine that this type of device has a significantly higher cost compared with the other two categories, and its thermal characteristics require careful design of the device’s cooling methods. The typical application of power LEDs is undoubtedly that of functional and atmospheric lighting; a single device of this type can emit even more than 350 lumens, and by combining several LEDs, it is possible to obtain real street lamps, so much so that it is no longer rare to find lighting fixtures equipped with this technology on the roadsides of our cities; to tell the truth, public lighting with LED street lamps is one of the cornerstones of smart cities, as they allow the reduction of costs through dimming during the hours in which the regulations allow the possibility of lowering the expected brightness.
Figure 3: RGB power LED device (Source: Super Bright LEDs)
The new frontiers of LEDs
In recent years, a technology has appeared on the market that makes use of organic components (to be precise, plastic conductive polymers) and that exploits the electroluminescence of these materials. This is the technology called organic LED (OLED), used in the construction of displays and having the characteristic of allowing the creation of thin, therefore flexible devices, particularly suitable for wearables and the mobile world. Unlike using LEDs as LCD backlights, OLEDs form the active matrix of the display itself! To date, there are still no economies of scale capable of making this technology economically competitive, but research has moved forward and numerous production processes have been carried out (AMOLED, PHOLED, PLED, SM-OLED, SOLED, TOLED), which means it’s only a matter of time before consumer electronics are permeated with objects with OLED displays.
This article was originally published on EEWeb.
