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. It ensures that the LED receives the correct voltage and current for proper operation. Consequently, the LEDs that count on an external driving circuit to continuously operate at a constant current level are known as DC LEDs.
LED downfall: how driving circuits rob LED systems of performance
The use of a driving circuit in an LED lighting system brings in many adverse effects. First of all, the service life of the electronic circuit 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 systems 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 lights such that they may not satisfy the operational requirements.
What is an AC LED
An AC LED, when we interpret this term literally, is an LED array that can be AC powered without using an external driving circuit. Nevertheless, there're no LEDs that operate directly on electrical power supplied by standard household or commercial electrical systems. An AC LED is basically an integrated assembly of the LED emitter and the LED driver. The integrated driver circuit performs the essential function of converting incoming AC line power to a regulated DC output current. Combining the LED and the driver simplifies the overall design of the LED lighting system. Instead of dealing with separate components, designers have a compact, pre-integrated unit that can be easily incorporated into lighting fixtures or systems. This integration streamlines the design process, reducing the complexity of the system. The integration of the LED and driver into a single assembly often results in a more space-efficient design. This is especially important in applications where size constraints or compact designs are crucial, such as in smaller lighting systems or areas with limited space for electrical components.
To create an AC LED, the driver and control circuitry must be physically small enough to accommodate the integration at the package- or module-level. Typically, an LED driver is a switching-mode power supply (SMPS), implementing a DC-DC converter to regulate the LED current. The main inconvenience of the SMPS LED drivers consists of their dependence on bulky, expensive and unreliable reactive components. Additional circuits used for EMI (electromagnetic interference) filtering and screening can double the overall cost and volume of the SMPS LED driver. The use of linear power supplies can offer simplicity in terms of circuitry. AC LEDs typically operate by using a linear regulator for the conversion from the incoming AC line power to the regulated DC output current. Linear regulators operate in the linear region of a pass element (such as a transistor) to control the output voltage. The components involved are typically straightforward, consisting of an error amplifier, a pass transistor, a reference voltage, a feedback voltage divider, and an output capacitor. The absence of EMI radiation is a major advantage for linear power supplies because it helps in maintaining a cleaner and interference-free electromagnetic environment. This is particularly important in sensitive electronic systems, such as medical devices, communication equipment, and audio/video systems, where any interference could have a detrimental effect on performance. The reduction in components for EMI attenuation also contributes to a simpler overall circuit.
In the past, LED manufacturers worked on the package-level platform to create AC LEDs. This attempt faced inherent challenges and limitations. The impracticality of this approach has led LED manufacturers to explore alternative methods or more comprehensive solutions. As the integrated design of LED systems becomes a trend, the board-level AC LED platform has established its dominance. The driver-on-board (DOB) LED technology combines the linear power supply and the LED packages into the same assembly. The driver circuit components and the LEDs are mounted on the same metal-core printed circuit board (MCPCB). The driver circuit components are surface-mount solid state devices that can be assembled on circuit boards with a single-sided copper construction. This simplified design can result in cost savings and reduced complexity in luminaire manufacturing. The integration of the driver at the board level can streamline assembly, reduce the number of components that need to be handled, and improve overall manufacturing efficiency. With fewer points of failure, DOB LEDs may experience fewer issues related to connectivity or compatibility. By eliminating the need for a separate driver and associated components, DOB LEDs can contribute to cost savings in terms of materials, labor, and manufacturing processes. The overall cost-effectiveness makes them an attractive option for lighting manufacturers. DOB LED technology can contribute to a cleaner and more aesthetically pleasing luminaire design. The absence of external drivers and associated wiring can result in a neater appearance, especially in fixtures where aesthetics is a crucial consideration. Designers have more freedom to create innovative and compact lighting solutions without the constraints imposed by external driver placement.
Beyond the hype
While DOB-based AC LED systems offers benefits in terms of space efficiency, cost savings, and potentially improved reliability, there are challenges that remain to be addressed. Linear power supplies may introduce flicker due to their design characteristics. The limited current-regulating capability of linear power supplies can result in inconsistent current supply, leading to flicker. DOB LED systems may have limited compatibility with external control systems or dimmers. The regulation of voltage in linear power supplies involves adjusting the output voltage by dissipating excess power as heat. The heat generated during the dissipation process contributes to the thermal load in the system. In other words, it adds to the overall heat produced by the components in the LED system. Excessive heat can affect the efficiency and reliability of electronic components, including the co-located LEDs. In linear power supplies, there is always some dropout voltage, which is the voltage that must be maintained for the power supply to function. The larger the dropout voltage, the less efficient the power supply is in delivering the required voltage to the LED array. This inefficiency can have implications for energy consumption and the overall performance of the LED lighting system. The presence of a high-voltage breakdown path through the control circuitry can compromise electrical safety.
The selection between AC LED and DC LED should be made based on practical considerations related to the electrical infrastructure of the application and the type of power source available. If light flicker, dimming and controllability, power conversion efficiency, electrical safety, and the operating voltage range are serious concerns, LED lights operated by SMPS LED drivers are the preferred choice. AC LED systems are likely to provide a basic and cost-effective lighting solution suitable for applications that prioritize affordability over advanced features or capabilities. They also find applications where EMI filtering and space efficiency are crucial requirements.