In 2011, Australia has taken the lead in banned the use of incandescent lamps, which kicked off the large-scale popularization of LED lamps. In addition, with the EU countries, Japan, Canada and other countries will ban the use of incandescent lamps in 2012, the lighting of LED lamps The penetration rate will be further enhanced, which makes the thousands of LED lighting manufacturers in China's Nuggets green lighting revolution rejoice - because a huge market is about to open, and this time the main players are Chinese manufacturers. However, it should be noted that LED lamps should be popularized, not only need to significantly reduce costs, but also need to solve the problem of energy efficiency and reliability. How to solve these problems, Doug Bailey, vice president of marketing for Power Integrations, shared the design of high-efficiency and high-reliability LED lamps. Five advices.
First, do not use bipolar power devices
Doug Bailey pointed out that because bipolar power devices are cheaper than MOSFETs, typically around 2 cents, some designers use bipolar power devices to reduce LED drive costs, which can seriously affect circuit reliability because The temperature of the LED driver board increases, and the effective working range of the bipolar device will shrink rapidly. This will cause the device to malfunction when the temperature rises, which will affect the reliability of the LED lamp. The correct way is to use the MOSFET device and the MOSFET device. Lifespan is much longer than bipolar devices.
Second, do not use MOSFET withstand voltage 600V
MOSFETs with a withstand voltage of 600V are relatively cheap. Many people think that the input voltage of LED lamps is generally 220V, so the withstand voltage of 600V is enough, but in many cases the circuit voltage will reach 340V. In the case of surge, the 600V MOSFET is easily broken down. This affects the life of LED lamps. In fact, the choice of 600V MOSFETs may save some cost but the cost of the entire board, so, "Do not use 600V withstand voltage MOSFET, it is best to use MOSFET with over 700V. "He stressed.
Third, try not to use electrolytic capacitors
Do you want to use electrolytic capacitors in the LED driver circuit? At present, there are supporters and opponents. Supporters believe that if the temperature of the 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 105 degrees and a lifetime of 8000 hours is selected, according to the estimated life expectancy of the electrolytic capacitor. “Every time the temperature is lowered by 10 degrees, the life is doubledâ€, then it has a working life of 16,000 hours in a 95 degree environment, a working life of 32,000 hours in a 85 degree environment, and a working life of 64,000 hours in a 75 degree environment, if Lower operating temperature, longer life! From this point of view, as long as the choice of high-quality electrolytic capacitors has no effect on the life of the drive power!
Other proponents believe that the low-frequency flicker caused by the high ripple current caused by electroless capacitors will cause physiological discomfort to some people's eyes. The large amplitude of low-frequency ripple will also cause some digital camera equipment to appear poor. A brightly lit grid of flickering lights. Therefore, high-quality light source lamps still need electrolytic capacitors. However, opponents believe that electrolytic capacitors will naturally age. 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 the input part of the LED driver circuit can be considered without electrolytic capacitors. In fact, PI's LinkSwitch-PH can save electrolytic capacitors. PI's single-stage PFC/constant current design allows designers to save large Capacitance capacitors, in the output circuit, high-voltage ceramic capacitors can be used instead of electrolytic capacitors to improve reliability. "Some people design a two-stage circuit, using a 400V electrolytic capacitor at the output, which will seriously affect For the reliability of the circuit, it is recommended to use a ceramic capacitor for a single-stage circuit," he stressed. “For industrial applications that do not pay much attention to dimming, high temperature environments, and high reliability, I strongly recommend not using electrolytic capacitors for design.â€
Fourth, try to use integrated MOSFET
If the LED luminaires are not designed to be very powerful, Doug recommends using an LED driver with integrated MOSFETs because the benefit of this is that the integrated MOSFETs have less on-resistance and generate less heat than discrete, in addition to integrated MOSFETs. The controller and the FET are generally together, and generally have a thermal shutdown function. When the MOSFET overheats, the circuit is automatically turned off to protect the LED lamp. This is very important for the LED lamp, because the LED lamp is generally small and difficult to dissipate air. . "Sometimes LEDs can cause people to burn because of overheating, but our solution has never been like this," he said.
Fifth, try to use single-stage architecture circuit
Doug said that some LED circuits use a two-stage architecture, 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 section is 88%, the efficiency of the entire circuit will be reduced to 83.6%! “PI's LinkSwitch-PH device combines a PFC/CC controller, a 725 V MOSFET, and a MOSFET driver into a single package, increasing the efficiency of the driver circuit to 87%!†Doug points out, “such devices greatly simplify the circuit. The board layout design eliminates up to 25 components used in traditional isolated flyback designs! The omitted components include high voltage bulk electrolytic capacitors and optocouplers.†Doug said the LED two-stage architecture is suitable for the second constant The flow driver circuit allows the PFC to drive the LED's constant current to the old driver. These designs are outdated and no longer cost effective, so in most cases it is best to use a single stage design.
Doug revealed that PI's market share in terms of LED driver shipments in 2010 ranked first in the world! The use of a large number of customers allows PI to have a deep understanding of LED application trends and to continuously introduce new products that meet customer needs. Looking forward to the future, he said that in 2011 China's indoor LED lighting, LED fluorescent lamps and LED street lights will have a good application.
Edit: Leo
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