1 Overview
1.1 Foreword:
Global energy shortages and improving the efficiency of electrical appliances are effective methods. Lighting power accounts for 21% of the world's total electricity consumption. If it can improve the efficiency of lighting, it can effectively alleviate energy shortages. How to improve the energy efficiency of lighting systems, extend the life of lighting systems, and be green and pollution-free? What is the fourth generation of lighting fixtures that replace incandescent, fluorescent, and energy-saving lamps? The answer given by the industry is LED lighting. LED lighting can reach 120lm per lumen. It is much higher than incandescent lamps and fluorescent lamps. In addition, LED lamp beads can last up to 100,000 hours and are green and non-polluting. These advantages of LED lighting determine its application prospects are very broad. The limitation of LED lighting applications is that the LED has a fixed forward voltage drop and the current has an upper limit (the operating current is the main factor affecting the life of the LED). The forward voltage drop on high-power white LEDs is typically 3-4V, and it is not possible to use the mains drive directly. Therefore, an efficient, environmentally friendly, long-life power supply that matches the LED bead is a must. This is the meaning and purpose of this topic.
1.2 Research Status
The technology of switching power supplies is very mature, because most of the LED-driven buck technology uses switching power supplies. Therefore, even if the LED driver power supply really enters the research time is not long, it does not hinder the maturity of its technology. The technical features required for LED driving are: long life and small size (especially for commercial lighting and home lighting, preferably embedded in the lamp head).
As we all know, most switching power supplies require an output filter electrolytic capacitor. Even high-quality electrolytic capacitors work at around 100 degrees Celsius and have a lifetime of only about 1Wh. There is no doubt that electrolytic capacitors are the bottleneck of the overall life of LED lamps. The electrolytic capacitor on the in-line driver board, because of the heat generated by the LED and the heat generated by the driver board itself, works at high temperatures for a long time, and the life of the electrolytic capacitor is shortened. At present, there are already integrated circuits, no need to output electrolytic capacitors, and only a few peripherals can directly drive LED illumination. This makes the long life of LED lighting really guaranteed.
Another point is to limit the life of the LED lamp to the temperature at work. At present, a technical solution in Taiwan is to make the LED lamp be plugged and unplugged like a lamp, making the LED lamp a maintainable product.
In addition to technical innovations, there are also combinations of innovations, such as the addition of dimming technology (analog dimming, digital dimming, Triac dimming); the use of three-color LEDs to form a color-light modulation system; the use of frequency jitter technology to reduce EMI; Add power factor correction circuit, etc.
1. 3 system performance indicators:
1. The maximum output power of the main output is 14W, the auxiliary output is 0.5W, and the total output is less than 15W.
2. The output current is constant at 350mA;
3, the maximum output voltage is 40V;
4. The conversion efficiency under full load is greater than 85%;
5. The maximum ripple is 5mV when the load is 350mA;
6, because there is no power factor module added, so the PF value is only about 0.45;
7. At least one 1W high-brightness LED can be connected, and up to 12 1W high-brightness LEDs can be driven (TNY280 should add heat sink for full power output).
1.4 System composition and design ideas:
1 Design ideas:
Considering that the home is commonly used 5 × 1W, or 6 × 1W, commercial lighting is commonly used 1 × 1W to 12 × 1W, this design uses constant current output, the output voltage is automatically adjusted with the load size for a wide range of designs.
Considering the domestic use of 50HZ, 220V power supply system, while the United States, Europe, Japan residents use electricity from 110V-240V, and then consider the 10% fluctuation of the network pressure, the system to relax the input to 85V-265V.
In line with the idea of ​​designing a truly energy-saving, cost-saving, long-life LED driver power supply that meets social requirements. In the choice of energy efficiency conversion and product cost, the conversion efficiency requirement is above 75%; the component selection adopts the common model as much as possible. When the requirements are met, the domestic components are used as much as possible (for example, the commonly used 1N4007, 1N4148, PI's TinySwitch-III series, the capacitor uses domestic BHA, Jwco). For mass production, it can effectively reduce costs and make LED lighting easier to enter people's lives.
2 system components:
System block diagram as shown
The system adopts a single-ended flyback topology commonly used in LED driving circuits, and the topology design is relatively simple. The input adopts four 1N4007 as full-wave rectification, and then rectifies by π-type filter. The TNY280 with the largest power capacity in PI's TinySwich-III series is used as the switch control IC, and the EE19 core is used for primary and secondary isolation. The output of the circuit realizes independent power supply of the op amp, and the op amp and voltage comparator are used as feedback control to achieve high-precision constant current control. The Schottky rectifier ER303 with low turn-on voltage drop reduces diode heating. The filter capacitor uses a common electrolytic capacitor acceptable for ESR to reduce the temperature rise of the capacitor.
1. 5 Overall function description:
1, achieve isolated output
(1) Transformer T1 provides the functions of isolation, energy storage and voltage transformation. Using a high-frequency transformer, the primary and secondary energy transmission channels are magnetic channels for isolation.
(2) The optocoupler 817B provides isolation and feedback functions. Using an optocoupler, the secondary feedback signal transmission channel is an optical channel for isolation.
(3) The primary and secondary uses the safety capacitor CT7 to achieve electrostatic discharge to prevent high-voltage ignition caused by static electricity accumulation. At the same time, the capacitance value is small, and the secondary does not cause a touch electric shock.
2, achieve LED constant current, achieve overvoltage protection
(1) The secondary uses LM358 dual op amp to achieve constant current and overvoltage protection. The LED's rising volt-ampere characteristics shift to the left with temperature (Figure 1.1). Assume that the drive is a constant voltage V2 (as shown in Figure 1.2). During operation, the lamp temperature reaches T2 from T1, the current flowing through the lamp bead increases significantly, and the current increases and the temperature rises. This vicious cycle will eventually burn out. Lamp beads. There are two solutions. One is to connect a resistor in the output loop at the same time as the constant voltage drive (as shown in Figure 1.3). When the LED lamp bead increases due to the temperature increase, the voltage drop of the series resistor increases. Large, the LED terminal voltage can be reduced to protect the LED. However, this method will dissipate a certain amount of power at the series resistance, reducing the conversion efficiency of the entire circuit, and has the original intention of design. Therefore, this method is not used.
The most ideal method is to use the overall idea to adjust the output voltage by controlling the duty cycle of the switch IC, and the control secondary always outputs a constant current. This design is the use of this program.
