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To popularize LED luminaires, not only need to significantly reduce costs, but also need to solve technical problems. How to solve the problems of energy efficiency and reliability, Doug Bailey, vice president of marketing for PowerIntegrations, shared the experience of efficient and reliable LED driver design.
First, do not use bipolar power devices Doug Bailey pointed out that because bipolar power devices are cheaper than MOSFETs, generally about 2 cents, so some designers use bipolar power devices to reduce the cost of LED driving, which will seriously affect The reliability of the circuit, because as the temperature of the LED driver board increases, the effective working range of the bipolar device will shrink rapidly, which will cause the device to malfunction when the temperature rises, thus affecting the reliability of the LED lamp. The correct approach is to With MOSFET devices, MOSFET devices have a much longer lifetime than bipolar devices.
Second, try not to use electrolytic capacitors. In the LED drive circuit, do you want to use electrolytic capacitors? 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. For every 10 degrees decrease in temperature, the life is doubled, 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 the actual operating temperature Lower, then life will be longer! 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 in the input part of the LED driver circuit can be considered without electrolytic capacitors, in fact, the PI LinkSwitch-PH can save electrolytic capacitors, PI's single-stage PFC / constant current design allows designers to save large capacity Capacitor, 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 with a 400V electrolytic capacitor at the output, which will seriously affect the circuit. For reliability, it is recommended to use a ceramic capacitor for a single-stage circuit. He stressed. For industrial applications that are less concerned with dimming, high temperature environments, and where high reliability is required, I strongly recommend not using electrolytic capacitors for design.
Third, the MOSFET's withstand voltage should not be lower than 700V
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. Therefore, the life of the LED lamp is affected. In fact, the selection of the 600V MOSFET may save some cost but the cost of the whole circuit board is paid. Therefore, do not use a 600V withstand voltage MOSFET, and it is better to use a MOSFET with a withstand voltage exceeding 700V. Emphasize.
Fourth, try to use a single-level architecture circuit Doug means that some LED circuits use a two-stage architecture, that is, PFC (power factor correction) isolated DC / DC converter architecture, such a design 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 725V MOSFET, and a MOSFET driver into a single package, increasing the efficiency of the driver circuit to 87! Doug pointed out that such a device can greatly simplify the layout of the board, and can eliminate up to 25 components used in the traditional isolated flyback design! The components that are omitted include high voltage bulk electrolytic capacitors and optocouplers. Doug said that the LED two-stage architecture is suitable for older drivers that must use a second constant current drive circuit to enable the PFC to drive the LEDs at constant current. These designs are outdated and no longer cost effective, so in most cases it is best to use a single stage design.
5. Try to use MOSFET devices. If the LED lamp design is not very high, Doug recommends using an LED driver with integrated MOSFET, because the benefit of this is that the integrated MOSFET has less on-resistance and generates less heat than discrete. In addition, the integrated MOSFET is a controller and FET together, generally has a thermal shutdown function, when the MOSFET overheats, it will automatically shut down the circuit to protect the LED luminaire. This is very important for LED luminaires, because LED luminaires are generally very Small and difficult to dissipate air. Sometimes LEDs can hurt people due to overheating, but our solution has never been like this. He said.