LED technology features


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Light-emitting diodes (LEDs) have now replaced most other lighting technologies. Thanks to their versatility, low cost and efficiency, LEDs are now used in any type of application in the most contrasting situations: status indicators, backlights for LCD screens, conventional lighting or in the atmosphere are all areas where it is now more convenient (both economically and not technically) to use lamps LED instead of traditional light sources. Let’s take a look at the characteristics that imposed LEDs as a standard for light sources and the types of related applications. LED Physics An LED is an active semiconductor electronic component that lays its foundation on a diode. The phrase “light-emitting diode” emphasizes how this technology is nothing more than a p-n junction with engineering and physical properties designed to exploit the electro-luminescence effect of semiconductors. Indeed, by direct bias of a p-n junction with a sufficient voltage greater than the threshold voltage (or forward), charges near the junction go from power level to power level; When the holes and electrons combine, if the emitted energy is high enough, then photons will be emitted, whose frequency (and thus color) and intensity of illumination depend on the physical properties of the material and on the level of voltage applied across the device. This is the basis behind color LED lights, allowing engineers to use these components in the most versatile way and to exploit them for a multitude of applications. In addition to the color of the emitted light, the minimum voltage that leads to the flow of current also depends on the type of semiconductor that the device is made of; The materials most used to realize LEDs are AlGaAs, GaAlP, GaAsP, SiC, GaN, GaP, Si and C. Leading LED devices. LED is an electronic component that is driven in current. For this reason, the limiting resistance of the current must always be provided in the driving circuit, without which the only resistance that the component sees is the internal resistance of the junction itself; Failure to restrict the current may damage the component and cause skew behavior from device to device. Figure 1 shows the basic LED driver diagram. In this circuit, a positive voltage is supplied to the base of the MOSFET, causing current to flow between the drain and the source, thus connecting the valve on the drain. Note the presence of the specified resistor. Assuming the component’s optimum forward current is IF, threshold voltage Vth, and the supply voltage of the LED Vin, the size of the specified resistor is calculated as R = (Vin – Vth) / If. For example, a classic LED used as a power state indicator can have Vth = 1.8 V and If = 20 mA; Assuming the circuit is supplied with a voltage of Vin = 5 V, then the specified resistor will have the value R = (5 – 1.8) / 0.02 = 160 Ω. Figure 1: LED driver base circuit (Source: Davide Di Gesualdo) The suggested diagram is used when driving through a microcontroller; In this case, it is always recommended to use a transistor (or similar component) capable of withstanding the forward currents of the LEDs: if the component is connected directly to the GPIO of the microcontroller, the risk of damaging the chip (due to currents at stake) will be extremely high. Obviously, if there is a need to power LEDs (which can absorb even more than 3W or 5W), then it will be necessary to adopt drivers compatible with the required current. In this regard, the Cypress Power PSoC line of microcontrollers is noteworthy, because it is able to provide power MOSFETs directly into the chip, which is very useful for driving power LEDs by reducing the surrounding circuits. One of the characteristics that allowed LEDs to become popular in the lighting world is undoubtedly the possibility of exploiting pulse width modulation technology to achieve dimming, that is, to raise or lower the current flowing in the LED (so, check its luminous flux). This technique consists of applying a control signal that has a square wave with a variable duty cycle: thus the current used will be proportional to time per tonne of the applied wave, allowing for electronic control of the LED brightness. Figure 2: PWM Process PWM technology, which is very easy to apply, can be ineffective if precise brightness control is needed; In fact, remember that since an LED is a diode in all respects, its voltage / current characteristics are non-linear and, therefore, the current variations obtained by modulating 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 the relative voltage signal. This solution avoids continuous on and off cycles of luminous components, thus improving instrument life and the quality of the light emitted, as it does not flicker. Common Applications of LED As mentioned earlier, LEDs are quite versatile, and this is also due to the fact that there are different types available. Classifying them according to the energy dispersed (and thus, the luminous flux was produced), we can mainly 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 (operation indicator, connection status, connection indicator between devices, etc.). Their use is most classic and old, and their packages are PTH and SMD. Lighting angle is not necessary for this type of device. High-brightness LEDs have a typical forward current of 30 mA to 100 mA and can be used as weak lighting elements (for example, pedestrian path indicators), although the main use is as a backlight in displays and in LCD panels. The latter has given a major boost to the spread of LED lights, as most LCD panels in modern televisions adopt LED backlights. Powered LEDs have a forward current of 100 mA upwards. It is easy to imagine that this type of device has a much higher cost compared to the other two classes, and its thermal properties require careful design of the device’s cooling methods. The typical application of power LED lamps is undoubtedly the application 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 lights, so much so that it is no longer rare to find lighting fixtures equipped with this technology on the roadside of our cities to tell the truth, LED street lighting is one of the cornerstones of smart cities, as it allows costs to be reduced by dimming during hours when regulations allow the expected brightness to be reduced. Figure 3: RGB Powered LED (Source: Super Bright LED) The New Frontier of LED In recent years, a technology has appeared on the market that uses organic components (to be precise, conductive polymers for plastics) that exploit the electrostatic luminescence of these materials. This is a technology called organic LED (OLED), used to build displays and has the advantage of allowing the creation of thin and flexible devices, particularly suitable for wearables and the world of mobile devices. Unlike using LEDs as backlights for LCD screens, OLEDs form the active matrix of the screen itself! So far, there are still no economies of scale capable of making this technology economically competitive, but research has gone ahead and several production processes have been implemented (AMOLED, PHOLED, PLED, SM-OLED, SOLED, TOLED). ), Which means it’s only a matter of time before OLED displays are permeated by consumer electronics. .


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