原标题：低功率 LED 照明趋势解决方案-第四部分

　　A19, E14 / 17, E26 / 27 bulb LED Driver Design

　　LED lighting trend in our blog series, we discussed the MR11 / 16 LED driver design problems. Now, let’s look at A19, E14 / 17, E26 / 27 bulb LED driver designs.

　　Some bulb type is called “screw caps” and “candle light.” Most with CFL or LED incandescent replacement, has won the majority of application requirements.

　　A19, E14 / 17, E26 / 27 screw bulb structure

　　Input voltage directly from the AC power socket type: E14 / 17 (Candle), A19 / E26 / 27 (screw type) (for candle lamp, rated 1~5W; to replace incandescent lamps, rated power 4~17W.) dimensions shown in Figure 1 and Figure 2.

　　Figure 1. Example candle lamps ((L: 99 毫米, D: 26 毫米, E: 17 mm) of

　　Figure 2. Example Edison light bulb (L: 105 毫米, D: 55 毫米, B: 26 mm) of

　　A19, E14 / 17, E26 / 27 screw bulb LED Driver Design Challenge

　　For candle lights, the LED driver design challenge is a small printed circuit board space. The printed circuit board space is less than the MR lamp space, and works in AC input voltage source. With LED driver designed to replace incandescent lamps, which printed circuit board space than a candle lamp or lamps MR large, rated power is large, and therefore LED driver as well. Printed circuit board space is limited, like candle lamp. For screw bulb design, power factor and total harmonic distortion is almost mandatory requirements, there are additional dimmers operating requirements.

　　For a lamp holder side parabolic E26 / 27 bulbs, printed circuit board dimensions of lampholders side: 20 mm; LED module side: 35 mm; width: 70 mm (see FIG. 3 ) .

　　Figure 3. E26 / E27 PCB Dimensions Example

　　It must be greater than 75% efficiency. Dimmer compatible with a variety of design requirements include holding current, in a wide range of optical amplitude linear manner, and without blinking.

　　A19, E14 / 17, E26 / 27 screw bulb Fairchild Solutions

　　In terms of security, isolated drive is preferred. In this power range, the preferred LED driver solution is flyback topology. For candle lights, power factor and total harmonic distortion, although low-power applications, but still mandatory. Many designers use a single-stage flyback solutions. Single-stage power factor correction flyback topology reduces the printed circuit board size, because it eliminates the need for bulky input electrolytic capacitor. Use a single-stage primary side regulation (PSR) flyback solutions can further reduce the number of components. With its low bill of materials (BOM) cost, isolation, power factor correction and a wide input voltage range, PFC PSR flyback topology is expected to become the first choice of LED driver topologies.

　　Table 1. Fairchild primary side regulation controller

　　In PSR topology without secondary-side feedback, so no optical isolator, the error amplifier (eg TL431), as well as compensation and biasing resistors and capacitors. Figure 4 shows a simplified schematic of PSR.

　　Figure 4. Schematic primary side regulation

　　Advantage of primary side regulation flyback topology include:

　　· Single-level solutions limit the number of components and eventually for smaller design space.

　　· FL103 50kHz operating switching frequency helps flyback transformer magnetic for volume-constrained applications.

　　· Integrated MOSFET having FSEZ1317 reduces the number of components, thus saving additional printed circuit board space.

　　· Primary side regulation topology element to reduce help address pressure to reduce costs.

　　· Without secondary feedback circuitry, which will immediately reduce the number of components and improve reliability (eliminating the need for optical isolators or TL431).

　　· Fairchild’s PSR topologies include TRUECURRENT ™ technology, the industry-leading performance constant <± 3%, to provide consistent, high-quality light irradiation.

　　· Solutions have been adopted isolation mode.

　　· Single-stage flyback topology can meet

　　the power factor and total harmonic distortion requirements.

　　Primary side regulation flyback topology operates in two modes: constant voltage (CV) and constant current (CC). LED drive should run in constant current mode, in order to better control the LED string lighting luminance output. Figure 5 shows the PSR flyback regulator IV characteristics.

　　Figure 5. The primary side regulation flyback LED driver output characteristics IV

　　PSR preferably discontinuous conduction mode (DCM), because this mode supports better output regulation. Typical waveforms shown in FIG. 6.

　　Figure 6. DCM flyback converter waveforms

　　When operating at a constant voltage regulation mode, the current in the inductor discharge time t DIS period, the output voltage and diode forward voltage drop, and will be reflected back to the auxiliary winding end. Since the diode forward voltage drop and decrease with the current decrease, the auxiliary winding voltage reflects the diode conduction time t DISat the end of the output voltage. Through the end of the diode conduction time auxiliary winding voltage is sampled, the output voltage information can be obtained.

　　When operating at constant current regulation mode, peak drain current I PEAK and inductor current discharge time t DIS to estimate the output current because the output current is in a stable state and the average diode current is the same. Using Fairchild’s TRUECURRENT ™ technology, constant current output can be obtained more accurately controlled.