A light emitting diode (LED) is a semiconductor device which includes an N-type semiconductor and a P-type semiconductor, and emits light by way of recombination of holes and electrons. LEDs are intrinsically direct current (DC) devices that only pass current in one polarity and are typically driven by DC voltage sources using resistors, current regulators and voltage regulators to limit the voltage and current delivered to the LED. Because of this, a power supply or "driver" is required for the purpose of converting the mains AC power to a DC voltage or current suitable for driving the LEDs. An LED driver is a self-contained power supply that features outputs corresponding to the electrical characteristics of the array of LEDs. Most LED drivers are designed to provide constant currents to operate the array of LEDs. Consequently, the LEDs that count on a driving circuit to continuously operate at a constant current level are known as DC LEDs.
However, an alternating current (AC) source may be employed to drive the LED lighting system. An AC LED is an LED that operates directly out of AC line voltage instead of utilizing a driver to transform the line voltage to direct current (DC) power. An AC LED chip has a plurality of LED units formed on one chip and is assembled into a circuit loop or a Wheatstone bridge to be directly used in an alternating current field. An AC LED is also referred to as a high voltage light emitting diode (HV LED) since it is clear of a current conversion driving component and can be directly employed in mains electricity which is high voltage (220V in Europe or 110 V in the USA) and alternating current (AC).
The typical LED luminaire includes a complex driving circuit, which may result in an increase in manufacturing costs, a substantial loss of operating life, less design flexibility as a consequence of increased volume with additional driving and dimming circuits, low power efficiency and system stability.
The introduction of the driving circuits in a DC LED lighting system brings in many adverse effects. First of all, the service life of the electronic cuircuit is significantly less than that of the LED. Moreover, considering that the input load characteristics of an LED don't stay constant throughout the LED's lifetime, but rather change with age and environmental conditions, the compatibility between an LED and its driver may deteriorate ultimately, and thus leading to unstable LED performance. The power converter reduces the efficiency of the light emitting device. The power losses inherent to such a power converter reduces overall efficiency of the light source. A driver circuit may include components like resistive loads, inductive coils, capacitors, switching transistors, clocks, and the like to modulate the operational parameters. In the course of operation, LED lamps and their LED drivers encounter a number of parasitic losses which include heat, vibration, radio frequency or electromagnetic interference, switching losses, and so on. As time goes by, the environmental factors and parasitic losses may lead to dropping of operational performance of the LED lamps such that they may not satisfy the operational requirements.
For AC LEDs, additional voltage transformers or rectifiers are not required, and AC LEDs can operate by applying alternating current directly. Because of this, the cost of an AC LED lamp is reduced when compared with its DC counterpart, and the circuit related quality issues are minimized. Electromagnetic Interference (EMI), in particular, is no longer a concern since the linear power supply does not require high frequency switching operation. The transformation for lower voltage direct current is not needed, hereby cutting down energy consumption occurred in power transformers. The power converter reduces the power factor and increases the total harmonic distortion of the current. The inherent efficiency of an AC-direct design makes it possible for a high power factor over 0.9 with no additional power conditioning or power factor correction circuitry required. A further benefit of the AC LED configuration is its intrinsic full-range dimmability, without resorting to a dimming circuit. One of the core features of AC LED approaches is the compatibility with phase-cut (triac) dimmers. It is often desired to implement LED lamps with a dimming functionality to deliver varying light output.
But nevertheless, there still has been a challenge of improvement in manufacturing the AC LED. The light produced by AC-LEDs driven from the AC mains supply can present an unacceptably high degree of optical flicker, as a consequence of the accelerated alteration in polarity at mains frequency. This flicker can be irritating, particularly when it comes to indoor lighting applications. The flicker problem can be fixed by employing a rectifier and a capacitor, which are typical components in DC LED drivers. Furthermore, LED lights with a driver circuitry can be designed to convert the AC mains voltage, in a wide range (e.g. 100-277V), into the possibly constant load voltage and the possibly constant load current. The AC LEDs are only able to accept a narrow range of input voltage, say for example 220-240V, which limited their operation in applications with radical voltage fluctuations.
LEDs powered by AC power sources make a non-linear load. Attributable to the non-linearity, LEDs powered by AC power sources may likely have a lower power factor, and may have a higher total harmonic distortion. The power factor of an alternating current (AC) electric power system is described as the ratio of real power to the apparent power flowing to a load.