LEDs are making their entrance into the lighting field using modern high-efficiency semiconductor material compounds and structures. Solid-state lighting (SSL), offers new possibilities and advantages for the end-user. By using appropriate drivers, control strategy and LEDs, the qualitative and quantitative aspects of the light can be fully controlled. LED drivers are low-voltage devices that convert the line-voltage 120/220/277 V power to the low voltage needed for the LEDs, and may also interpret control signals to dim the LEDs. LED drivers come in either constant current or constant voltage. These two types of drivers are NOT interchangeable, and it is the design of the LED load that determines which driver is appropriate. Both LED lamps and LED fixtures require LED drivers.
There are two ways to control the brightness of an LED. The first method is using Analog dimming, which involves varying the forward current through the LED to adjust the brightness. The second method uses a digital dimming technique involving switching the forward current on and off for short periods of time. The human eye averages these on and off times together for a perceived brightness.
The cheapest and most basic way to drive LEDs is to use a constant voltage power supply and a resistor in series with the LED to limit the current flowing through it. The selected resistance depends on the magnitude of the voltage source (VIN), on the value of the LED’s forward voltage and the forward current of the LED.
Linear power supply (LPS) is an economical, simple and reliable way of driving LEDs. LPSs are based on either integrated circuit (IC) linear regulator or on bipolar junction or field effect transistors operating in the linear region. Switched-mode power supplies (SMPS) lack the main drawbacks of linear power supplies and are therefore the main solution to drive LEDs. Because LEDs are DC components, just DC/DC and AC/DC SMPS types are considered. Efficiency, controllability, small size and low weight are their main advantages over the linear power supplies. An SMPS can provide, if necessary, high currents (e.g., more than 30A) at very low voltages.
The selection of the most appropriate topology to drive LEDs depends on the standards, specifications and application requirements like operation environment conditions, system input voltage, LEDs’ forward voltage, number of LEDs and circuit array.
Intelligent drivers are usually based on ASICs switching microcontrollers which include programmable flash memory (EEPROMs), several on-chip Pulse-Width Modulation (PWM) controllers, ADCs (analogue-to-digital converter) and DACs (digital-to-analogue converter) channels. Microcontroller-based LED drivers bring additional benefits such as operational flexibility, efficiency, reliability, controllability and intelligence to the system.
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