Written by Richard Wilson April 19, 2016
Francois Mirand (Francois Mirand) explained the reasons and reasons for the direct AC drive of the LED lighting system
Like the history of other types of semiconductor products, LEDs provide better performance at a lower cost year after year.
In fact, other elements of traditional LED lighting fixtures—housing, connectors, PCBs, optics, and most importantly, power supplies—have already accounted for a large part of the total system cost five years ago and will only be dominated by the cost of LEDs.
This motivates people to try to create cheaper architectures, thereby significantly reducing power costs. Among the types of lamps such as low-end downlights, linear lamps and partitions, low cost is the most important attribute for buyers. Here, the concept of "AC Direct LED System" has been developed to completely eliminate the need for traditional (but bulky and expensive) AC-DC Switched Mode Power Supplies (SMPS).
However, the first implementation of the AC direct LED system brought thorny design problems to their designers to solve, and severely restricted their freedom to optimize the operating characteristics and the shape of the luminaire.
Now, the improved architecture for AC direct LED systems seems to provide competitive performance and design flexibility. But is its cost acceptable?
Figure 1: Simple exchange direct implementation with four LEDs
Why design SMPS? Traditional high-brightness LED is a low-voltage device, usually with a forward voltage of about 3V. The utility power is approximately 120V or 240V, depending on the user's region.
Therefore, lamps need a way to reduce the AC power supply voltage to the DC forward voltage of the LED or LED string, which is usually less than 60V.
So far, the most common method is to use SMPS. The main advantage of SMPS is its excellent efficiency, sometimes exceeding 90%, combined with its ability to provide electrical isolation (for safety purposes).
Its efficiency is an attribute of the switch-mode architecture. It saves and releases energy in inductors and capacitors at very high speeds, thereby storing energy at the peak of the AC voltage swing instead of dissipating it, acting as a Simple linear regulator.
Unfortunately, SMPS requires huge inductors or transformers and capacitors. In addition, the type of electrolytic capacitor used in the power supply has a much more limited lifespan than other components-it is usually the first component to fail in an LED lighting system. EMI countermeasures are also needed, because SMPS usually switches at high frequencies of 20kHz or higher.
And the SMPS cannot be easily installed on the same PCB as the LED. The single-sided copper structure of the metal core PCB (MCPCB) makes SMPS circuit wiring very difficult, and the required passive components are often not available in surface mount packages. FR4 (the most common type of PCB substrate) provides more wiring layers, but the isolation and thermal management of this type of PCB can become complicated. In any case, if the large passive components in the SMPS are placed too close to the LED, shadows will be created.
Therefore, SMPS is bulky, will bring some design difficulties, and its bill of materials and assembly costs are relatively expensive.
A simpler method is not to step down. This gives power conditioning IC manufacturers an incentive to develop a simpler architecture that drives high-voltage LEDs directly from the supply voltage without reducing that voltage.
The AC direct regulation scheme manages to achieve this by turning on the LEDs in sequence following the sinusoidal voltage of the power supply. Suitable LEDs for this application include Lumileds' LUXEON 3535 HV, and LG Innotek's 5630 HV, 5250 HV and 3030N HV series.
Figure 1 and Figure 2 schematically show the simplest implementation of this concept.
Figure 2: The AC direct regulation IC gradually turns on each LED until all the LEDs light up at the peak of the power supply voltage
The AC supply voltage is rectified (so the negative half of the cycle becomes positive). The rectified output is a 100Hz half sine wave. If the nominal power supply voltage is 230V, it usually swings around 0V and 325V.
In each half cycle, once the voltage reaches about 70V, the first high-voltage LED (or LED string) is turned on, the second is added at 140V, and the third is added at 210V. As long as the voltage is equal to or higher than 280V, all four LEDs will light up.
This regulation scheme means that the excess voltage must be dissipated through the linear regulator, which generates waste heat.
Compared with SMPS solutions, this power loss will inevitably reduce system efficiency. But in practice, this AC direct LED design can achieve a total system efficiency of up to 100lm/W, while the equivalent lamp using SMPS is 130lm/W.
In a simple, low-cost application, such as a 600lm downlight, it may be more enlightening to compare with an equivalent dichroic halogen bulb. It provides a typical system efficiency of about 10lm/W: if such a bulb can be greatly reduced The power consumption is replaced by AC pass-through high-voltage LED lights.
Compared with the equivalent SMPS, the AC direct design is smaller, simpler, and easier to assemble because it only requires a PCB (see Figure 3).
Figure 3: LED bulbs using the AC direct adjustment scheme
However, in addition to the reduced efficiency, the simple form of AC direct LED regulation has other disadvantages, which are very important in certain applications.
One is heat. As shown in Figure 1, current regulation and power consumption are concentrated in a single IC. In normal operation, this makes the IC an extreme "hot spot". This requires special countermeasures to avoid the risk of overheating damage, including the use of MCPCB and circuit board layouts with large gaps between other components and the regulating IC.
This centralized form of direct communication adjustment also lacks flexibility in terms of function and performance optimization and component selection. For example, in the constraints of circuit board layout, this is obvious: a single regulator IC takes up a lot of board space, so it is difficult to adapt to linear formats, such as strip lights and fluorescent tube alternatives. This solution is also limited in the flexibility of implementing functions such as dimming.
These shortcomings led to the development of an enhanced version of the AC direct LED regulation scheme: Power IC manufacturer Exar (through its iML subsidiary) developed a distributed architecture instead of the centralized implementation shown in Figure 1. (Figure 4)
Figure 3: LED bulb using SMPS
The working principle is the same as the centralized control scheme: the high-voltage LED is turned on and off sequentially in each half cycle of the power supply. But in a distributed control scheme, a separate IC (such as iML8684) regulates each high-voltage LED string.
Balancing cost and efficiency, most implementations use a three-step adjustment scheme (with three HV LED strings), but two-step or four-step schemes, or even more than four-step schemes, are also possible.
The main advantage of the distributed control scheme lies in its design flexibility. For example, the flexibility of the circuit topology allows system designers to adapt to various LED arrangements and achieve the best balance between performance, functionality, and cost.
The distributed method allows the choice of almost any LED, including classic low-voltage types as well as multi-junction high-voltage LEDs. This provides designers with a wider choice of package styles and performance specifications (flux, efficacy, color temperature and color rendering).
It also supports compact implementations that use a small number of high-flux LEDs, and designs that use more low-flux LEDs to achieve more diffuse light output.
In addition, the balanced circuit structure supported by the distributed architecture or the addition of a single capacitor can reduce flicker and phase dimmer compatibility, and a bleeder can be easily added for better compatibility with the triac dimmer solution .
The flexibility of the distributed architecture of the AC-direct LED system is also achieved by increasing the freedom of choice of PCB type, shape and layout.
Figure 4: Three-step regulation, with three regulator ICs and a ballast transistor, is the most common way to achieve distributed AC direct regulation
This is because the distributed architecture can be easily and completely implemented using surface mount components, and its simple circuit can be relatively easily arranged on a single-sided PCB. In the distributed architecture, the heat dissipation range of a large number of driver ICs is wider, eliminating the single hot spot in the traditional AC direct LED system.
This means that distributed solutions can usually use FR4 PCBs, while achieving a more compact layout and smaller spacing between components. In addition, the small, thin components used in the distributed solution allow designers to implement lamps of any form factor, including linear strips and uniform diffuse light engines without shadows.
Compared with centralized supervision, the distributed solution has other advantages. The Exar topology has an inherent high surge (transient voltage) immunity of >750V. In contrast, in a centralized solution, surge immunity requires the addition of a large, discrete metal oxide varistor (MOV). MOV is prone to long-term reliability problems because of its limited ability to absorb the energy of transient voltage surges commonly found on AC power sources. To make matters worse, these varistors are not always available in surface mount versions, and they are expensive.
Concerns about flicker In the centralized and distributed implementation of AC direct LED regulation, lighting designers have expressed some concerns about flicker. In both scenarios, when the voltage is equal to or close to 0V, the LED will dim every half cycle of the power supply (ie, every 1/100 second in a 50Hz power system).
In fact, the time interval between flickers is so short that in most applications, no user will notice it. In low-cost applications where direct AC adjustment is intended, low flicker levels are often tolerable.
However, using a balanced circuit topology and adding a small storage capacitor can not only reduce flicker, but also improve efficiency and LED utilization, thereby balancing part of the additional bill of materials cost of the distributed architecture.
Distributed solution: cost/performance trade-off Exar's first and patented distributed version of AC direct adjustment provides designers with unique freedom to achieve design optimization, and to a large extent control the trade-offs between performance, function and cost .
The simplest implementation of distributed solutions is often cheaper than centralized methods, and more complex designs provide opportunities to add valuable additional features. For example, functions such as presence detection or DALI dimming control can all be accommodated in a distributed AC direct LED system.
Each application has its own balance of performance and cost requirements, but the unparalleled design flexibility of the distributed approach is worth considering for system designers who wish to implement direct AC adjustment in a new generation of low-cost LED lamps.
Francois Mirand is the technical director of Future Lighting Solutions Europe.
Marked as: Future Electronics LED Driver LED Lighting
Hi, I hope you all go well. I have some questions about the design of the AC direct driver for the 12w LED lamp with a 28*35 LED chip. Please suggest me to find out: 1- Are there any ic drivers for driving only? Please introduce 2- How many smd chips 0.5 watts can I consider? 3-I need lumen 1200 4-beam angle 200 Thank you for guiding me, please send me if you have a plan
Your email address will not be published. Required places have been marked *
"Electronic Weekly" collaborated with RS Grass Roots to focus on introducing the brightest young electronic engineers in the UK today.
Send our news, blogs and comments directly to your inbox! Sign up for the e-weekly newsletter: style, gadget guru, and daily and weekly roundups.
Read our special supplement celebrating the 60th anniversary of Electronic Weekly and look forward to the future of the industry.
Read the Electronics Weekly @ 60 supplement »
Read the first issue of Electronic Weekly online: September 7, 1960. We have scanned the first edition so that you can enjoy it.
Read the first edition »
Listen to Xilinx Interview: Responding to Platform-Based Embedded Design
Listen to this podcast and listen to Chetan Khona (Director of Industry, Vision, Healthcare and Science, Xilinx) talk about how Xilinx and the semiconductor industry respond to customer needs.
Read our special supplement celebrating the 60th anniversary of Electronic Weekly and look forward to the future of the industry.
Read the Electronics Weekly @ 60 supplement »
Read the first issue of Electronic Weekly online: September 7, 1960. We have scanned the first edition so that you can enjoy it.
Read the first edition »
By using this website, you agree to the use of cookies. Electronics Weekly is owned by Metropolis International Group Limited, a member of the Metropolis Group; you can view our privacy and cookie policy here.