LED lighting applications will begin with three basic input power levels: low or equal to 20W power, medium power between 20W and 50W, and power larger than 50W applications: see Figure 1. Keep in mind that real-world applications do not exactly match these three kinds of methods, but when considering led driver solutions, these three power levels are consistent. LED applications focus on high-brightness led design.
The theme of this paper is ≤20w low power applications, especially the replacement or modification of light bulb lamps-replacing existing lamps and lighting fixtures. This also includes new structural lighting fixtures.
The trend of low power LED illumination
2010 Global High-brightness LED sales estimated at $890 million, estimated from 2010 to 2015, the average annual growth rate (CAGR) is 39%, visible market potential. But for LED drives, the main trend is drive-related improvements, cost reductions, and long working life. The effect is the ratio of lumen to Watt.
"Doe SSL (American Electronic products World http://www. eepw. com. cn/article/133636. htm source Solid State lighting) plan" The potential of high-brightness LEDs is expected to exceed the traditional technologies of today and in the past; Figures 2 and 3 show the increasing trend of efficacy. In the definition of efficacy, the input power is located in the denominator, the input power and the efficiency of passing energy to the LED lamp string are related to the LED driver solution. In the full range of LED load power and load possibilities, a single drive topology is not the best choice, but you can consider a minimal topology to meet all the LED driver development requirements.
Choosing the most efficient semiconductor can be used as a basis for choosing a topology, but the cost of the drive is also constrained. The DOE SSL program estimates the current cost as shown in Figure 4, and drives the $number of the overall manufacturing cost.
This is the total cost target seen by end users, and the adoption of LED lighting Solutions has become the most common obstacle to performance improvement. The cost target requirements presented by the Department of Energy at the 2011 Solid State Lighting Market Presentation Workshop are shown in Figure 5, with costs falling by almost 50% per 4 years. LED drive topology selection also offers the best cost 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 drive, the type of components used, the temperature, or the power loss. Use the number of components method to calculate the reliability of the LED drive and to determine the number of components to be reduced according to the target. Reliability is also affected by the temperature of the work, so thermal design is also important, and it is also important to reduce the power loss associated with the LED driver components and topology control methods. The development trend is to eliminate electrolytic capacitors, as well as other components such as optical isolators, and to integrate functions in silicon control devices.
Low power LED driver Design Challenge
LED driver design today faces the following challenges; the listed items will become design constraints that designers must balance, in the order of different companies.
shorten the development cycle;
Looking for power topologies that can satisfy input and output voltage-current parameters, thermal design, safety rules and protection requirements;
efficiency and effectiveness;
To meet global regulatory requirements, it is also in the LED drive to reduce power loss, the use of power factor correction (PFC) and low THD (total harmonic distortion);
Drive reliability and service life;
Constant current output tolerance;
Dimming and dimming range (the requirements of phase-cut dimmer, dimming rate, inrush current limit, damping circuit, voltage regulator, etc.);
Limited printed Board (PCB) space or volume (height) restrictions;
protection function-OVP, OCP, OTP, short-circuit led, open circuit led;
Multiple vendors complicate the supply chain.
Analysis of low power LED application
Below is a review of low power LED lighting, structure, function, design challenges and application trends.
MR11/16 LED Light
MR11/16 lamp is a typical halogen lamp, its ordinary type rated power is 20W, 35W and 50W.
Typical designs of existing halogen lamps are shown in Figure 6
The input voltage can be DC 12V or 24V or directly plugged into the 120V or 230V AC mains power supply. The 12V or 24V voltage can come from a simple transformer that uses the mains AC voltage and outputs a v/24v AC as the lampholders input. LED substitution 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 development trend is that the supply of such products increases.
Drive Design Challenges
The biggest challenge of the MR11/16 design is the lack of standards, including lamp and bulb shape, power factor and total harmonic distortion requirements (ENERGY STAR LED lamps ≥ 0.9, for the "5w of one lamp, ≥ 0.7), and low system power efficiency." Considering that the size of the figure 7 lamp must contain a drive, the small footprint of the LED driver is welcome.
There are two types of printed circuit board (PCB) dimensions, a kind as shown in Figure 8, is round, using the back of the LED module. The circular diameter should be less than 30mm and the higher component is within the 5mm range of the center connector.
Another kind of PCB board in the shape of the size of fig. 9, is vertical. It needs to be less than 30mmx20 mm.
The scheme of Fairchild
Fairchild presents a new LED drive piece to solve the AC problem, as shown in Figure 10 FL7701. It is an intelligent non isolated PFC step-down led driver solution. Directly using AC line input voltage, it is possible to achieve the small size of the PCB can be used for Mr Lampshade. This led drive design eliminates all electrolytic capacitors: typically used for input, output, and IC VCC voltages. By using only a small number of external components, the PF and THD requirements can be met while more than 80% efficiency is achieved. In contrast to the step-up design, the step-down topology also has the advantage of continuous output currents (reduced ripple currents), because the inductance and output in series, for the LED load, the buck topology looks like a constant current source. The output current of the boost topology is discontinuous unless the output capacitor is used to filter the ripple current.
A19, E14/17, E26/27 bulbs
Some bulb types are also referred to as a screw outlet (Edison socket) and candle-type lamps. Most of them are incandescent bulbs, using CFL or LED substitution products to meet most application requirements.
Its input voltage directly from the AC power supply, the type of lampholders are: E14/17 (candle type), A19/e26/27 screw, rated power: 1~5w for candle-type lamps, 4~17w for incandescent replacement. The overall dimensions are shown in Figure 11.
The challenge for the LED driver design for candle-type lamps is the small PCB space, which is less than Mr Lamp space and works on AC input voltage. The use of LED driver design to replace the incandescent lamp, its PCB space than candle-type lights or Mr Lights, the power rating is also larger, the actual result is the PCB space is still limited, similar to the candle-like lamp. For the design of the screw bulb, pf and THD are almost mandatory. There are additional dimming functions.
For lampholders side is parabolic shape of the E26/27 lamp of the size of the PCB, lamp side for MM, in the LED module side mm, the length of 70mm, see Figure 12.
Requirement efficiency is greater than 75%. A few brief notes on the design of a compatible dimmer include compatibility with a variety of hold currents, linear in a wide range of light, and no flicker.
The primary regulator controller for Fairchild is shown in table 1. When working in constant current adjustment mode, the output current is estimated by using the peak drain current Ipeak and the inductor current discharge time TDIs, because the output current is the same as the diode current average in steady state. Using the Truecurrent technology of Fairchild, the constant current output can be controlled accurately.
PAR16, 20, 30, 38 light system structure
These lamps are AC voltage input, rated power is between 4w~20w, Lampholders are E26/27 or 2-pin GU10, as shown in Figure 13.
With a larger lamp size, there is considerable room to accommodate the LED drive solution, and PF and low THD are still mandatory requirements.
The higher wattage of these LED lamps will cause higher VDS, peak (spikes) through MOSFET, and thus require a BVDSS MOSFET with higher ratings. For high voltage spikes, BVDSS ratings must be lowered due to high input current. Figure 14 shows the voltage spike as a summary of the VDS, peak = Vin+nvo+vos, where nvo is the output voltage of the reflection, also known as the Vro.
Buffers are typically used to limit Vos peak voltages, but buffers consume energy, reducing the efficiency of LED drives:
The Fairchild solution is shown in table 2, where the comparison between Single-stage and 2-level methods is shown in table 3.
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