5 Keys to LED Driver Design
Time:2023-08-12
Views:768
To popularize LED lighting fixtures, it is not only necessary to significantly reduce costs, but also to solve technical problems. How to solve the challenges of energy efficiency and reliability, Doug Bailey, Vice President of Marketing at PowerIntegrations, shared his insights on efficient and reliable LED driver design.
1、 Do not use bipolar power devices
DougBailey pointed out that due to the fact that bipolar power devices are cheaper than MOSFETs, usually around 2 cents per device, some designers use bipolar power devices to reduce the cost of LED driving, which can seriously affect the reliability of the circuit. As the temperature of the LED driving circuit board increases, the effective working range of bipolar devices will quickly shrink, leading to device failures when the temperature rises, thereby affecting the reliability of LED lamps, The correct approach is to choose MOSFET devices, which have a much longer lifespan than bipolar devices.
2、 Try not to use electrolytic capacitors as much as possible
Should electrolytic capacitors be used in LED driver circuits? At present, there are supporters and opponents who believe that if the temperature of the circuit board can be controlled well and the purpose of extending the lifespan of the electrolytic capacitor can be achieved in sequence, such as selecting a high-temperature electrolytic capacitor with a lifespan of 8000 hours at 105 degrees Celsius. According to the commonly used formula for estimating the lifespan of electrolytic capacitors, "every 10 degrees Celsius decrease in temperature doubles its lifespan", then its working lifespan is 16000 hours at 95 degrees Celsius and 32000 hours at 85 degrees Celsius, The working life in a 75 degree environment is 64000 hours. If the actual working temperature is lower, the service life will be longer! From this perspective, as long as high-quality electrolytic capacitors are selected, there is no impact on the lifespan of the driving power supply!
Some supporters believe that low-frequency flicker caused by high ripple currents without electrolytic capacitors can cause physiological discomfort to certain human eyes, and large low-frequency ripples can also cause some digital camera devices to exhibit bright and dark grids of differential frequency flicker. So, high-quality light source lamps still require electrolytic capacitors. However, opponents argue that electrolytic capacitors will naturally age, and in addition, the temperature of LED lighting fixtures is extremely difficult to control, so the lifespan of electrolytic capacitors will inevitably decrease, thereby affecting the lifespan of LED lighting fixtures.
DougBailey believes that in the input part of LED driver circuits, electrolytic capacitors can be considered not to be used. In fact, using PI‘s LinkSwitch-PH can eliminate electrolytic capacitors. PI‘s single stage PFC/constant current design can help designers eliminate 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 as the output when designing a two-stage circuit, which seriously affects the reliability of the circuit. It is recommended to use a ceramic capacitor for a single-stage circuit, "he emphasized. For industrial applications that do not pay much attention to dimming function, 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
MOSFETs with a voltage resistance of 600V are relatively cheap. Many people believe that the input voltage of LED lamps is generally 220V, so a voltage resistance of 600V is sufficient. However, many times the circuit voltage can reach 340V. In case of surges, 600V MOSFETs are easily broken down, which affects the lifespan of LED lamps. In fact, choosing 600VMOSFETs may save some costs, but it pays the price of the entire circuit board. Therefore, Do not choose MOSFETs with a voltage resistance of 600V, it is best to choose MOSFETs with a voltage resistance exceeding 700V, "he emphasized.
4、 Try to use single-stage architecture circuits as much as possible
Doug stated that some LED circuits adopt a two-stage architecture, namely a "PFC (Power Factor Correction)+Isolated DC/DC Converter" architecture, which reduces the efficiency of the circuit. For example, if the efficiency of PFC is 95% and the efficiency of DC/DC part is 88%, the efficiency of the entire circuit will decrease to 83.6%! The LinkSwitch PH device of PI integrates PFC/CC controller, a 725VMOSFET, and MOSFET driver into a single package, improving the efficiency of the driving circuit to 87%! "Doug pointed out, This type of device can greatly simplify circuit board layout design, eliminating up to 25 components used in traditional isolation flyback design! The omitted components include high-voltage high-capacity electrolytic capacitors and optocouplers. "Doug said that the LED two-stage architecture is suitable for old-fashioned drivers that must use a second constant current driving circuit to drive the PFC to constant current LED. These designs are outdated and no longer cost-effective, so it is best to adopt a single stage design in most cases.
5、 Try to use MOSFET devices as much as possible
If the power of the designed LED lamp is not very high, Doug suggests using LED driver products that integrate MOSFETs, because the advantage of doing so is that the integrated MOSFET has less conduction resistance and generates less heat than discrete ones. In addition, the integrated MOSFET is a controller and FET together, and generally has an overheat shutdown function. When the MOSFET overheats, it will automatically turn off the circuit to protect the LED lamp, which is very important for LED lamps, Because LED lamps are generally small and difficult to dissipate air. Sometimes there may be situations where LEDs burn excessively and cause harm to people, but our plan has never been like this, "he said.
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