Five suggestions for solving LED lighting design problems

In 2011, Australia has taken the lead in banning the use of incandescent lamps, which has opened the prelude to the large-scale popularization of LED lamps. In addition, as EU countries, Japan, Canada and other countries will ban the use of incandescent lamps in 2012, the popularity of LED lamps will further increase, which makes thousands of Chinese LED lamp manufacturers who are digging for the green lighting revolution excited - because a huge market is about to be opened, and this time it is Chinese manufacturers who play the leading role. However, it should be noted that in order to popularize LED lamps, it is necessary not only to significantly reduce costs, but also to solve the problems of energy efficiency and reliability. How to solve these problems, Doug Bailey, vice president of marketing at Power Integrations, shared five suggestions for the design of efficient and reliable LED lamps.

1. Do not use bipolar power devices Doug Bailey pointed out that since bipolar power devices are cheaper than MOSFETs, generally about 2 cents each, some designers use bipolar power devices to reduce the cost of LED drivers. This will seriously affect the reliability of the circuit, because as the temperature of the LED driver circuit board increases, the effective operating range of the bipolar device will shrink rapidly, which will cause the device to fail when the temperature rises, thus affecting the reliability of the LED lamps. The correct approach is to use MOSFET devices, which have a much longer service life than bipolar devices.

2. Do not use 600V MOSFET.

600V MOSFET is relatively cheap. Many people think that the input voltage of LED lamps is generally 220V, so 600V is enough. However, the circuit voltage often reaches 340V. When there is a surge, 600V MOSFET is easily broken down, thus affecting the life of LED lamps. In fact, using 600V MOSFET may save some costs, but the price paid is the price of the entire circuit board. Therefore, "do not use 600V MOSFET, it is best to use MOSFET with a voltage of more than 700V." He emphasized.

3. Try not to use electrolytic capacitors. Should electrolytic capacitors be used in LED drive circuits? There are both supporters and opponents. Supporters believe that if the temperature of the circuit board can be controlled well, the purpose of extending the life of the electrolytic capacitor can be achieved in turn. For example, a high-temperature electrolytic capacitor with a life of 8,000 hours at 105 degrees is selected. According to the common electrolytic capacitor life estimation formula "the life doubles for every 10 degrees drop in temperature", its working life in a 95-degree environment is 16,000 hours, in an 85-degree environment is 32,000 hours, and in a 75-degree environment is 64,000 hours. If the actual working temperature is lower, the life will be longer! From this point of view, as long as high-quality electrolytic capacitors are selected, there will be no effect on the life of the driving power supply! Other supporters believe that the low-frequency flicker caused by the high ripple current brought by the electrolytic capacitor will cause physiological discomfort to some people's eyes, and the large-amplitude low-frequency ripple will also cause some digital camera equipment to have a light and dark grid with a difference frequency flicker. Therefore, high-quality light source lamps still need electrolytic capacitors. However, opponents believe that electrolytic capacitors will age naturally. In addition, the temperature of LED lamps is extremely difficult to control, so the life of electrolytic capacitors will inevitably decrease, thus affecting the life of LED lamps. In this regard, Doug Bailey believes that electrolytic capacitors can be considered not to be used in the input part of the LED driver circuit. In fact, using PI's LinkSwitch-PH can save electrolytic capacitors. PI's single-stage PFC/constant current design allows designers to save large-capacity capacitors. In the output circuit, high-voltage ceramic capacitors can be used to replace electrolytic capacitors to improve reliability. "Some people use a 400V electrolytic capacitor at the output when designing a two-stage circuit, which will seriously affect the reliability of the circuit. It is recommended to use ceramic capacitors for a single-stage circuit." He emphasized. "For industrial applications that do not pay much attention to dimming functions, high temperature environments, and require high reliability, I strongly recommend not using electrolytic capacitors for design." Figure: 5 W A19 driver circuit solution designed using LinkSwitch-PL (RDK-251) 4. Try to use integrated MOSFET If the power of the designed LED lamp is not very high, Doug recommends using LED driver products with integrated MOSFET, because the advantage of doing so is that the on-resistance of the integrated MOSFET is small, and the heat generated is less than that of the discrete one. In addition, the integrated MOSFET is the controller and FET together, and generally has an overheating shutdown function. When the MOSFET is overheated, it will automatically shut down the circuit to protect the LED lamp. This is very important for LED lamps because LED lamps are generally very small and difficult to dissipate air. "Sometimes LEDs will burn and injure people due to overheating, but our solution will never be like this." He said. 5. Try to use single-stage architecture circuit Doug said that some LED circuits use a two-stage architecture, that is, the "PFC (power factor correction) + isolated DC/DC converter" architecture, which will reduce the efficiency of the circuit. For example, if the efficiency of the PFC is 95% and the efficiency of the DC/DC part is 88%, the efficiency of the entire circuit will be reduced to 83.6%! "PI's LinkSwitch-PH device integrates the PFC/CC controller, a 725 V MOSFET and a MOSFET driver into a single package, increasing the efficiency of the drive circuit to 87%!" Doug pointed out, "Such a device can greatly simplify the circuit board layout design, and can save up to 25 components used in traditional isolated flyback designs! The components that are saved include high-voltage large-capacity electrolytic capacitors and optocouplers." Doug said that the LED two-stage architecture is suitable for old drivers that must use a second constant current drive circuit to enable the PFC to drive the LED constant current. These designs are outdated and no longer cost-effective, so it is best to use a single-stage design in most cases. Doug revealed that PI's market share in 2010 ranked first in the world in terms of LED driver shipments! The use of a large number of customers allows PI to deeply understand the application trends of LEDs and continuously launch new products that meet customer needs. Looking into the future, he said that China's indoor LED lighting, LED fluorescent lamps and LED street lamps will have good applications in 2011. Figure 2. 14W LED driver circuit solution with power factor greater than 0.9 designed using PI LinkSwitch-PH