Five Key Points for LED Driver Design

    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 LED driver costs. 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 lamp . The correct approach is to use MOSFET devices, which have a much longer service life than bipolar devices.

    2. Try not to use electrolytic capacitors

    Should electrolytic capacitors be used in LED driver 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 at 95 degrees is 16,000 hours, at 85 degrees is 32,000 hours, and at 75 degrees 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, the life of the driver power supply will not be affected!

    Some supporters believe that the low-frequency flicker caused by the high ripple current without electrolytic capacitors 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 light and dark grids with 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 be reduced, thus affecting the life of LED lamps.

    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."

    3. The withstand voltage of MOSFET should not be lower than 700V

    MOSFET with a withstand voltage of 600V is relatively cheap. Many people think that the input voltage of LED lamps is generally 220V, so a withstand voltage of 600V is sufficient. However, in many cases, the circuit voltage can reach 340V. When there is a surge, a 600V MOSFET can easily be broken down, thus affecting the life of the LED lamp. In fact, choosing a 600V MOSFET may save some costs, but the price paid is the entire circuit board. Therefore, "do not choose a 600V MOSFET. It is best to choose a MOSFET with a withstand voltage of more than 700V." He emphasized. 

    4. 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. 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 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 725VMOSFET and a MOSFET driver into a single package at the same time, which increases 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 PFC to drive LED 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 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 overheat shutdown function. When the MOSFET is overheated, it will automatically shut down the circuit to protect the LED lamp, which is very important for LED lamps, because LED lamps are generally very small and difficult to dissipate air. "Sometimes LEDs burn and injure people due to overheating, but our solution will never be like this." He said.