LED drive design of the five key
First, do not use bipolar power devices
Doug Bailey pointed out that bipolar power devices are cheaper than the MOSFET, usually about 2 cents a so some designers in order to reduce the cost of LED drive using bipolar power devices, this will seriously affect the circuit reliability, because with the LED Drive circuit board temperature rise, the effective working range of bipolar devices will be rapidly reduced, this will lead to failure in the device temperature rise and thus affect the reliability of LED lamps, the correct approach is to use MOSFET devices, MOSFET device life To be much longer than bipolar devices.
Second, try not to use electrolytic capacitors
LED drive circuit in the end do not use electrolytic capacitors? Supporters believe that if you can control the temperature of the circuit board, in order to achieve the purpose of extending the life of electrolytic capacitors, such as the use of 105 degrees life of 8000 hours of high-temperature electrolytic capacitors, according to the passage of the electrolytic capacitor life estimation formula “Temperature for each 10 degrees lower, life expectancy doubled,” then it 95 degrees in the environment working life of 16,000 hours in 85 degrees environment, the working life of 32,000 hours in 75 degrees of environmental life of 64,000 hours, if the actual Lower operating temperature, then the life will be longer! From this, as long as the choice of high-quality electrolytic capacitors on the power life of the drive is no impact!
Others argue that low-frequency flicker caused by high ripple currents caused by electrolytic capacitors can cause physical discomfort to some people’s eyes, and low-frequency ripples with large amplitudes can also cause some digital camera equipment to exhibit poor performance A light-dark raster. Therefore, high-quality light source lamp or the need for electrolytic capacitors. However, opponents argue that the electrolytic capacitor will naturally aging, In addition, LED lamps extremely difficult to control the temperature, so the electrolytic capacitor life is bound to reduce, thus affecting the life of LED lamps.
In this regard, DougBailey that, in the LED driver circuit input part can be considered without electrolytic capacitors, in fact, the use of PI’s LinkSwitch-PH electrolytic capacitor can be omitted, PI single-stage PFC / constant current design allows designers to save large capacity Capacitance, in the output circuit, you can use high-pressure ceramic capacitors instead of electrolytic capacitors to enhance reliability, “Some people in the design of two-stage circuit, the output using a 400V electrolytic capacitor, which will seriously affect the circuit Of the reliability of the proposed single-stage circuit with a ceramic capacitor on it. “He stressed. “I strongly recommend not using electrolytic capacitors for industrial applications that are less concerned with dimming, high temperature environments and high reliability requirements.”
Third, the MOSFET voltage should not be less than 700V
600V MOSFET is relatively cheap, a lot of that LED lighting input voltage is generally 220V, 600V enough pressure, but often the circuit voltage to 340V, when there is a surge, 600V MOSFET is easy to breakdown , Thus affecting the life of LED lamps, in fact, selected 600VMOSFET may save some cost but the cost is the entire circuit board, so, “Do not use 600V pressure MOSFET, the best selection voltage of more than 700V MOSFET. “He stressed.
Fourth, try to use single-level architecture circuit
Doug said that some of the LED circuit uses a two-tier architecture, the “PFC (power factor correction) + isolated DC / DC converter,” the structure, this 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%! “The LinkSwitch-PH devices from PI combine the PFC / CC controller, a 725 VMOSFET, and MOSFET driver into a single package, increasing the efficiency of the driver circuit to 87%!” Doug notes, “Such a device greatly simplifies the board layout Design, can save up to the traditional isolated flyback design used in the 25 components! Eliminate the components, including high-voltage high-capacity electrolytic capacitors and optocouplers. “Doug said that the LED two-stage architecture must be used for the second constant current drive Circuit to make PFC drive LED constant current of the old drive. These designs are obsolete and no longer cost effective, so a single-stage design is preferred in most cases.
Fifth, try to use MOSFET devices
If the design of the LED lighting power is not very high, Doug recommends the use of integrated MOSFET LED driver products, because the benefits of doing so is less integrated MOSFET on-resistance, resulting in less heat than the discrete, the other is the integrated MOSFET Is the controller and the FET together, generally have a thermal shutdown function in the MOSFET overheating will automatically turn off the circuit to protect the purpose of LED lamps, LED lamps and lanterns, which is very important because the LED lamps are generally very small and difficult to air heat . “Sometimes it can happen because of overheating and burning, but we never do that,” he said.