While the LED industry has been traditionally dominated by direct current (DC) designs, new alternating current (AC) options are becoming more prevalent. This article will explore the difference between AC and DC LED systems, and provide insights into which applications might benefit from AC powered solutions.
AC and DC are different types of current flow within a circuit. While DC flows in one direction, AC periodically reverses direction, resulting in a sinusoidal wave pattern. AC sine curves include both positive and negative crests/troughs.
Alternating current is the preferred form of large-scale electric distribution, as voltage can be increased or decreased easily (with a transformer) at end user locations. This feature of AC enables the distribution of high voltages at lower current, which in turn minimizes resistance losses and optimizes transferred energy. Power lines utilize AC to distribute electricity around the grid at hundreds of kilovolts, before it is converted to 100-240V for standard usage.
LEDs are traditionally DC devices, passing current in one polarity or direction. These LEDs require a power supply or "driver" to convert AC power to a DC constant current suitable for powering the luminaire.
Drivers feature an AC-to-DC converter and often include a large electrolytic capacitor. High power applications can utilize 20+ different driver board components. The AC energy available in a building is converted to DC using a bridge rectifier, turning AC in to pulsed DC. The pulsed result is then filtered and smoothed into a continuous DC.
Power supplies are often the weak link in an LED luminaire, as the converter circuit increases costs and shortens the tech lifespan. According to a DOE and Next Generation Lighting Industry Alliance report, Solid-State Lighting Product Quality Initiative, drivers are responsible for a 52% failure rate of the luminaires tested.
Alternatively, AC LEDs operate directly out of AC line voltage and do not require the conversions associated with DC LEDs.
Passive components (resistors, inductors, controllers and capacitors) are used to smooth a rectified signal to a constant DC voltage. Without control, the output voltage will fluctuate with input voltage.
• Inductors – Robust coils of wire wrapped around a piece of iron.
• Controllers – Semiconductor chips usually constructed to handle high temperatures.
• Capacitors – Able to handle harsh environments and heat.
To connect a system of LEDs across an AC voltage source, certain conditions apply:
Total Forward Voltage (the amount of voltage required to conduct electricity) of the LEDs has to be comparable to the AC voltage source ~ Low (12V) or high (277V) voltage. Any voltages below this amount will cause the LED to remain “open” and non-conductive. This also means any components in-series with the LED will not have current flowing through them. Once the forward voltage is achieved within the LED, it will begin to conduct electricity.
Proper AC LED circuit (topology) selection must match the source voltage. The AC LED circuit options vary based on factors such as chip type, final product requirement, voltage of AC power source and end-user lighting application.
In an AC LED light engine, the pulsed voltage (rectified AC) is presented to the LEDs with no electronic storage elements. Control is achieved through the careful use of simple components (typically non active). A Current Limiting Resistor is put in series with the LED to control the current. The LED topology and the control scheme are determined by the AC line voltage.
Two types of AC infrastructure are common:
• Line voltage AC – 120V/230V ~ residential, commercial and industrial lighting
• Low voltage AC – 12V/24V ~ track, recessed/cove, linear and garden lighting.
For high voltage designs, the AC voltage is typically rectified while low voltage designs utilize both rectified and direct AC LEDs. In all designs, reverse voltages and peak current must be properly considered and managed to create a quality AC LED.
Reverse Voltage – The maximum voltage that an LED can be fed reverse biased (positive voltage supplied to cathode instead of anode) without being damaged or destroyed.
Peak Forward Current – The maximum current that an LED can be fed without being damaged or destroyed.
AC systems enjoy a simplified circuit design and do not experience lifespan reductions as a result of secondary power supply components.
Removing the AC-to-DC driver reduces:
System cost
System size
System weight
Maintenance required
Heat generation
Failure rate
Conversion loss
Each of these reductions provides a host of environmental and financial benefits that should be very compelling to property owners making lighting decisions. Shipping logistics and eco footprints can be reduced with the smaller system size and weight inherent with AC LEDs.
Total Harmonic Distortion (THD) - The numeric representation of distortion in the current waveform relative to the sinusoidal voltage waveform on the AC main. Harmonics are unwanted currents at multiples of the fundamental line frequency that create power losses (and an annoying buzzing sound). Different types of conditioning circuits (including resistors and switch-mode power supplies) are used to limit THD in AC light engines. Unwanted harmonics from LEDs can degrade electrical line and affect the performance of other equipment that shares the circuit with offending luminaires.
Power-Factor Correction - The power factor refers to the ratio of the real power used by the luminaire versus the apparent power. In AC LEDs, the load is non-linear so Power Factor (PF) is a bigger concern. Market requirements for LED bulbs and fixtures are driving PF>0.9 for consumptions as low as 5W.
Heat - Current regulation and power dissipation are all concentrated in a single Integrated Circuit (IC) that becomes a hot spot. To avoid the risk of over-temperature damage, some designs utilize a Metal Core Printed Circuit Board (MCPCB) or board layouts with wide gaps between other components.
Few Suppliers - Limits the options available and bottlenecks the distribution on larger scales. When large orders start coming in, the manufacturing capabilities will need to expand to meet demand.
ObstACles
Many existing dimmers have been designed for traditional DC lighting fixtures, and compatibility issues can exist between these controls and LED light source components on the same circuit. Provided the minimum load conditions of the dimmer are maintained, driverless AC LED light engines dim linearly with leading and trailing edge dimmers.
Though the relationship between the dimmer logic and LED behavior can be complex, LEDs are being designed to mesh with existing dimming systems to ensure a broader application advantage with retrofit projects.
An LED is a device which includes N-type and P-type semiconductors. The semiconductor nature of LEDs allows them to respond quickly to inputs. Any current or voltage fluctuation at the input is immediately reflected to the output.
When the input variation is the line frequency modulation, the light produced will be modulated (varied). This can result in a flickering (stroboscopic) effect in the lighting, which can cause headaches and nausea. Even though flicker may not be visible to the naked eye, there is evidence that the human brain can detect light flicker frequencies as high as 200Hz.
Potential problems include headaches, eye strain, impaired visual performance or, in extreme cases, epileptic seizure. In AC LEDs, the output current inevitably comes as a series of half sine waves. With 50 Hz power, the presence of these half sine wave pulses of current at twice the power line frequency produces a flicker effect.
In recent designs, high voltage integrated circuits (HVICs) are included to correct the flicker effect. HVICs automatically connect the correct number of LEDs to match the power line voltage. An electronic current limiter is used to maintain a constant current between switch operations. Additional stages of switching can be added.
Since voltage is being dropped across the current limiter, power is dissipated which limits the attainable efficiency. Dimming can be achieved by varying the current level programmed for each stage of switching.
Currently, the design of AC LED light engines involves numerous compromises. Mass-adoption of driverless AC LEDs will follow newer approaches that can balance efficiency, power factor and light quality as measured by flicker index.
While flicker remains an issue in human centric applications, the horticulture industry could benefit immensely from current AC LED technology. It remains to be seen whether or not the flickering inherent with sinusoidal AC light output affects the growth of indoor cultivated crops. If the growth is comparable, then there will be a major advantage in utilizing AC LED technologies in horticulture grow operations.
AC powered LEDs have varied applications and are preferred over DC LEDs when complex control is not needed. Some of the applications are:
• Small form factor replacement lamps – such as the GU 10, MR 11 and MR16 lamps; halogen capsules; festoons (space, cost and dimmability)
• 12V lighting systems –lamps, recessed, track, linear and garden lighting and for safe low voltage retail shelf lighting (magnetic and high frequency electronic transformer compatibility, reliability)
• High Bay, Flood lights and Street lights (Power Supply Unit (PSU)/driver weight, cost and reliability)
• Low and line voltage track lighting (PSU/driver bulk and cost)
• Signage (PSU/driver bulk, cost and reliability)
• Recessed and surface mount down-lighting for commercial use (cost, reliability and convenient form factor) and residential and hospitality applications (cost, reliability, dimming and warm-on-dim).
LiteSheet (www.litesheet.com)
15 year warranty (Lloyd’s of London). BriteCor LED Luminaires and SmartCor LED Dimming Systems. First to deliver patented, commercially viable AC direct LED lighting technology.
ZegaLED: (www.zegaled.com): Develops and distributes AC LED Driverless Engines and Modules using its patent process of electronic design and control. Technology is REAC™ (Rare Earth Alternating Current).
Lync Labs Inc (http://www.lynklabs.com/) Tesla® AC LEDs are direct AC LED COBs for low voltage and direct main applications. Low voltage Tesla AC LED components can connect directly to the 12V AC infrastructure provided by a magnetic transformer or a low voltage “LV” BriteDriver® electronic transformer/driver.
Seoul Semiconductor Inc (http://www.seoulsemicon.com/en/application/Lighting/) AC Modules: - Compact design and easy to implement in lighting systems. Full range dimming with TRIAC dimmers. Highest efficiency in the market for AC module technology.
ERG Lighting (http://www.erglighting.com/) AC LED Light Modules have dimming down to 3% brightness with trailing edge dimmer. UL-Recognized feature LM-80 tested LEDs and a 5-Year warranty.
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