Low Power LED Lighting Trend

LED lighting applications will start with three basic input power levels: low power for less than or equal to 20W, medium power for applications greater than 20W: see Figure 1. Keep in mind that real world applications will not fit neatly into these three categories, but these three power levels are consistent when considering LED driver solutions. LED applications focus on high brightness LED designs.

The subject of this article is low-power applications ≤ 20W, especially replacement or retrofit of bulb-type lighting fixtures - replacing existing lamps and luminaires. This also includes new construction lighting fixtures.

Trends in Low-Power LED Lighting

In 2010, global sales of high-brightness LEDs were estimated at $890 million, and the CAGR is expected to be 39% from 2010 to 2015, showing great market potential. However, for LED drivers, the main trends are efficiency improvements, cost reductions, and long operating life related to drivers. Efficacy is the ratio of lumens to watts.

The DOE SSL ( Department of Energy Electronics World http://www.eepw.com.cn/article/133636.htm Solid State Lighting ) Program predicts the potential of high brightness LEDs to exceed the traditional technologies of today and the past; Figures 2 and 3 show the trend of increasing efficacy. In the definition of efficacy, input power is in the denominator, and the input power and the efficiency of delivering energy to the LED string are related to the LED driver solution. A single driver topology is not the best choice for the full range of LED load powers and load possibilities, but a minimum of topologies can be considered to meet all LED driver development needs.

Selecting the most efficient semiconductors can be a basis for selecting the topology, but the cost of the driver is also a constraint. The DOE SSL program estimates today's costs as shown in Figure 4; the driver accounts for 10% to 20% of the total manufacturing cost.

This is the overall cost target seen by the end user, which has become the most common barrier to the adoption of LED lighting solutions regardless of performance improvements. The cost target requirements proposed by the US Department of Energy at the 2011 Solid-State Lighting Market Introduction Workshop are shown in Figure 5; the cost has dropped by 50% almost every 4 years. The LED driver topology selection can also lead to the best cost solution.

The operating 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 components used, temperature or power loss. Using the number of components 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 the operating temperature, so thermal design is also important, as is reducing the power losses associated with the LED driver components and the topological control method. The trend is to eliminate electrolytic capacitors, as well as other components such as optoisolators , and integrate the functions into the silicon controller .

Low Power LED Driver Design Challenges

LED driver designs face the following challenges today; the listed items will become design constraints that designers must balance, and their order will vary from company to company.

● Shorten the development cycle;

● Reduce costs;

● Design complexity;

● Find a power topology that meets input and output voltage-current parameters, thermal design, safety regulations, and protection requirements;

● Efficiency and efficacy;

● Meet global regulatory requirements, namely reducing power losses, using power factor correction (PFC) and low THD (total harmonic distortion) in LED drivers;

● The reliability and service life of the drive;

● Constant current output tolerance;

● Dimming and dimming range (phase-cut dimmer requirements, dimming ratio, inrush current limiting, damping circuits, voltage dividers, etc.);

● No flicker;

● Limited printed circuit board ( PCB ) space or volume (height) restrictions;

● Protection functions—OVP, OCP, OTP, short-circuit LED, open-circuit LED;

● Operating temperature;

● Multiple suppliers make the supply chain complex.

Analysis of Low Power LED Applications

The following will review low power LED lighting; structure, function, design challenges and application trends.

MR11/16 LED Light

MR11/16 lamp is a typical halogen lamp, and its common type has rated power of 20W, 35W and 50W.

System Structure

The typical design of existing halogen lamps is shown in Figure 6

The input voltage can be DC 12V or 24V, or plugged 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 lamp holder input. The LED replacement product needs to be controlled as a constant current source. A 4W LED MR lamp is equivalent to a 20W halogen lamp. Some models have dimming features and the trend is for more of these products to be available.

Driver Design Challenges

The biggest challenge of MR11/16 design is the lack of standards, including lamp and bulb form factors, power factor and total harmonic distortion requirements (≥0.9 for Energy Star LED lamps , ≥0.7 for integrated lamps >5W), and low system power efficiency. Considering that the size of the Figure 7 lamp must include the driver, LED drivers with a small footprint are welcomed.

There are two printed circuit board (PCB) form factors, one is circular as shown in Figure 8, using the back of the LED module . The circular diameter should be less than 30mm, with the tallest components within 5mm of the center connector .

Another PCB board has a vertical dimension as shown in Figure 9. It needs to be smaller than 30 mm x 20 mm.

Fairchild Semiconductor's Solution

Fairchild Semiconductor has proposed a new LED driver device to solve the AC-DC problem, the FL7701 shown in Figure 10. It is an intelligent non-isolated PFC buck LED driver solution. Using the AC line input voltage directly, it is possible to achieve a small PCB size that can be used for MR lamp housings. This LED driver design eliminates all electrolytic capacitors: usually used for input, output, and IC Vcc voltage. By using only a few external components, PF and THD requirements can be met while achieving high efficiency of more than 80%. The buck topology also has the advantage of continuous output current (reduced ripple current) relative to the boost design because the inductor is in series with the output, and the buck topology looks like a constant current source to the LED load. 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-type bulbs. Most are incandescent bulbs, and CFL or LED alternatives can meet most application requirements.

System Structure

Its input voltage comes directly from the AC power supply. The lamp holder types are: E14/17 (candle type), A19/E26/27 screw-on type. The rated power is: 1~5W for candle type lamps, 4~17W for incandescent lamp replacement. The overall dimensions are shown in Figure 11.

Design Challenges

The challenge of LED driver design for candle type lamp is small PCB space, which is smaller than MR lamp space and works on AC input voltage power supply. Using LED driver design to replace incandescent lamp, its PCB space is larger than candle type lamp or MR lamp, the rated power is also larger, so the LED driver is also larger, the actual result is that the PCB space is still limited, similar to candle type lamp. For screw-type bulb design, PF and THD are almost mandatory. There is also an additional dimming function requirement.

The PCB dimensions for an E26/27 lamp with a parabolic shape on the base side are 20 mm on the base side, 35 mm on the LED module side, and 70 mm in length, see Figure 12.

Efficiency greater than 75% is required. A few quick notes on compatible dimmer designs include compatibility with a wide range of holding currents, linear operation over a wide range of light amplitudes, and no flicker.

Fairchild Semiconductor Solutions

Fairchild Semiconductor's primary regulation controller is shown in Table 1. When operating in constant current regulation mode, the output current can be estimated using the peak drain current IPEAK and the inductor current discharge time TDIS, because the output current is the same as the average diode current in steady state. Using Fairchild Semiconductor's innovative TRUECURRENT technology, the constant current output can be accurately controlled.

PAR16, 20, 30, 38 lamp system structure

These lamps are AC voltage input, with rated power ranging from 4W to 20W, and the lamp holders are screw-type E26/27 or 2-pin GU10, as shown in Figure 13.

With larger lamp sizes, there is considerable space to accommodate the LED driver solution, and PF and low THD remain mandatory requirements.

Design Challenges

The higher wattage of these LED lamps causes a higher Vds, peak (spike) across the MOSFET, thus requiring a MOSFET with a higher BVDss rating. For high voltage spikes, the BVDss rating must be reduced due to the higher input current. Figure 14 shows the voltage spike as the summed Vds, peak = Vin + nVo + Vos, where nVo is the reflected output voltage, also known as Vro.

A snubber is usually used to limit the Vos peak voltage, but the snubber consumes energy, which reduces the LED driver efficiency:

Fairchild Solutions

Fairchild Semiconductor's solution is shown in Table 2, and a comparison of the single-stage flyback and 2-stage approaches is shown in Table 3.