LED lighting applications will begin with three basic input power levels: low power less than or equal to 20W, medium power between 20W and 50W, and high power greater than 50W applications: see Figure 1. Keep in mind that real-world applications don't fit exactly into these three divisions, but when considering LED driver solutions, these three power levels are consistent. LED applications focus on high brightness LED design.
The subject matter of this paper is ≤20W low power applications, especially replacement or retrofit of bulb-type luminaires - replacing existing luminaires and lighting fixtures. This also includes new structural lighting fixtures.
Trends in low power LED lighting
In 2010, global sales of high-brightness LEDs are estimated at US$890 million. It is estimated that from 2010 to 2015, the average annual growth rate (CAGR) will be 39%, which shows that the market potential is very large. But for LED drivers, the main trends are driver-related improvements in power, cost reduction, and long working life. The effect is the ratio of lumens to watts.
"DOE SSL" is expected to exceed the traditional technology of today and the past; Figure 2 and Figure 3 show the trend of increasing efficiency. In terms of power efficiency, the input power is in the denominator, and the input power and the efficiency of transferring energy to the LED string are related to the LED driver solution. With a full range of LED load power and load possibilities, a single drive topology is not the best choice, but the smallest topology can be considered to meet all LED driver development needs.
Choosing the most efficient semiconductor can be used as a basis for selecting a topology, but the cost of the driver is also a constraint. The DOE SSL plan estimates today's costs as shown in Figure 4; drives account for 10% to 20% of total manufacturing costs.
This is the overall cost goal seen by end users, and this has become the most common barrier to performance improvement for LED lighting solutions. The cost targets set by the US Department of Energy at the 2011 Solid State Lighting Market Introduction Symposium are shown in Figure 5; costs are reduced by almost 50% every four years. LED driver topology selection also offers the most cost-effective solution.
The working life is also related to the reliability of the power supply. Reliability is affected by the number of components of the LED driver, the type of component used, temperature or power loss. Using the component number method, the reliability of the LED driver can be calculated and the number of components can be reduced according to the target. Reliability is also affected by operating temperature, so thermal design is also important, as is the reduction of power losses associated with LED driver components and topology control methods. The trend is to eliminate electrolytic capacitors, as well as other components such as opto-isolators, and to integrate functions into silicon control devices.
Low Power LED Driver Design Challenge
LED driver design faces the following challenges today; the listed projects will be design constraints that designers must balance, and the order will vary from company to company.
â— Shorten the development cycle;
â— Reduce costs;
â— design complexity;
â— Find power supply topologies that meet input and output voltage-current parameters, thermal design, safety regulations, and protection requirements;
â— efficiency and efficacy;
â— Meet global regulatory requirements, ie reduce power loss in LED drivers, use power factor correction (PFC) and low THD (total harmonic distortion);
â— Reliability and service life of the drive;
â— Constant current output tolerance;
â— Dimming and dimming range (phase-cut dimmer requirements, dimming rate, inrush current limit, damping circuit, voltage divider, etc.);
â— No flicker;
â— Limited printed circuit board (PCB) space or volume (height) limits;
◠Protection function—OVP, OCP, OTP, short-circuit LED, open LED;
â— working temperature;
â— Multiple suppliers make the supply chain complex.
Low power LED application analysis
Below we review low-power LED lighting; structure, functionality, design challenges, and application trends.
MR11/16 LED light
The MR11/16 lamp is a typical halogen lamp with a typical power rating of 20W, 35W and 50W.
system structure
The typical design of the existing halogen lamp is shown in Figure 6.
The input voltage can be DC 12V or 24V, or plug directly into a 120V or 230V AC mains supply. The 12V or 24V voltage can come from a simple transformer that uses the mains AC voltage and outputs 12 V/24V AC as the lampholder input. LED replacement products need to be controlled as a constant current source. A 4W LED MR lamp is equivalent to a 20W halogen lamp. Some models have dimming characteristics, and the trend is to increase the supply of such products.
Drive design challenge
The biggest challenge of the MR11/16 design is the lack of standards, including luminaire and bulb shape, power factor and total harmonic distortion requirements (Energy Star LED luminaire ≥ 0.9, for a 5W integrated lamp, ≥ 0.7), and low system power efficiency . Considering that the size of the lamp in Figure 7 must include a driver, LED drivers with small footprints are welcome.
There are two printed circuit board (PCB) form factors, one shown in Figure 8, which is round and uses the back of the LED module. The circular diameter should be less than 30 mm and the higher component is located within 5 mm of the center connector.
The outer dimensions of another PCB board are shown in Figure 9, which is vertical. It needs to be less than 30mm x 20 mm.
Fairchild's solution
Fairchild Semiconductor has proposed a new LED driver device to solve the AC-DC problem, as shown in Figure 10 of the FL7701. It is a smart, non-isolated PFC buck LED driver solution. Direct use of the AC line input voltage makes it possible to achieve a small PCB size that can be used for MR lamp covers. This LED driver design eliminates all electrolytic capacitors: typically used for input, output, and IC Vcc voltages. By using only a small number of external components, PF and THD requirements can be met while achieving over 80% efficiency. The buck topology also has the advantage of continuous output current (reducing ripple current) compared to the boost design because the inductor is in series with the output, and for LED loads, the buck topology looks like a constant current source. The output current of the boost topology is discontinuous unless an output capacitor is used to filter the ripple current.
A19, E14/17, E26/27 bulbs
Some bulb types are also referred to as Edison sockets and candle lamps. Most are incandescent bulbs, and CFL or LED replacements are available for most applications.
system structure
The input voltage is directly from the AC power supply. The lampholder type is: E14/17 (candle type), A19/E26/27 screw type, rated power: 1~5W for candle type lamp, 4~17W for incandescent lamp replacement. . The external dimensions are shown in Figure 11.
Design challenge
The LED driver design challenge for candle lamps is a small PCB space that is smaller than the MR lamp space and operates on an AC input voltage source. The LED driver design replaces the incandescent lamp. Its PCB space is larger than that of the candle lamp or the MR lamp, and the rated power is also large. Therefore, the LED driver is also large. The actual result is that the PCB space is still limited, similar to a candle lamp. . For screw bulb designs, PF and THD are almost mandatory. There are additional dimming features required.
The PCB dimensions for the E26/27 lamp with a parabolic profile on the lampholder side are 20 mm on the lampholder side, 35 mm on the LED module side and 70 mm in length, see Figure 12.
The required efficiency is greater than 75%. A few short notes on compatible dimmer designs include compatibility with various holding currents, linear operation over a wide range of light amplitudes, and no flicker.
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