Characteristics and Selection of LED Driver IC
Q: Do LED drivers require dedicated LED driver ICs? What are the advantages and disadvantages compared to ordinary ICs?
Answer: The advantages of LED driven dedicated ICs are wide Vin, high output current, high constant current accuracy, and the use of frequency jitter to effectively reduce EMI. The constant current source used for LED driving is mainly to ensure that the brightness of the LED light is consistent and does not flicker during the working time period. Low priced mobile phones also commonly use LDO to drive backlight LEDs, but the power utilization is not high. Fortunately, most mobile phones are designed to shut down when the lithium battery voltage drops to 3.3V. Using LDO to drive LED lighting fixtures is not a good method, as LDO cannot maintain constant current.
Q: So how should we choose between domestic and foreign LED driver ICs?
Answer: There are many LED driven ICs, and their cost-effectiveness varies. The key lies in the target price range of the designed product. If we are looking for competitiveness and more profits, local IC products are also good. Selection and design issues of LED driver power supply
Q: What issues should be considered when selecting and designing LED driver power supplies?
Answer: LED driver power supply is a voltage converter that converts the power supply into a specific voltage and current to drive LED lighting. Generally, the inputs of LED driver power supply include mains power, low-voltage DC, high-voltage DC, low-voltage high-frequency AC, etc. The output of LED driver power is mostly a constant current source that can change the voltage with the change of LED forward voltage drop value. According to the electricity usage rules of the power grid and the characteristic requirements of LED driving power supplies, the following issues should be considered when selecting and designing LED driving power supplies: a. High reliability, especially like LED street light driving power supplies, installed at high altitudes, makes maintenance inconvenient and expensive. b. The high-efficiency power supply has high efficiency, low power consumption, and low heat generation inside the lamp, which reduces the temperature rise of the lamp. It is beneficial for delaying the light decay of LEDs. c. There are two driving methods currently in use: one is a constant voltage source that supplies multiple constant current sources, with each constant current source supplying separate power to each LED. The other is direct constant current power supply, with LED series or parallel operation. How to choose the startup method should be based on actual needs. The multi-channel constant current output power supply method will have better cost and performance. Perhaps it is the mainstream direction in the future. d. The ability of surge protection LED to withstand surges is relatively poor, especially in terms of resistance to reverse voltage. Due to the load shedding of the power grid and the induction of lightning strikes, various surges can invade the power grid system, some of which can cause damage to LEDs. Therefore, the LED driver power supply should have the ability to suppress the invasion of surges and protect the LED from damage. e. Other elements, such as improving the power factor of the power supply; Add LED temperature negative feedback function; In terms of external protection, it should be waterproof and moisture-proof, and the shell should be sun resistant; The lifespan of the driving power supply should match the lifespan of the LED; To comply with safety regulations and electromagnetic compatibility requirements.
LED driven IC development direction
Explanation: LED driving IC development direction for lighting: LED light source is a long-life light source, with a theoretical lifespan of up to 50000 hours. However, unreasonable application circuit design, improper selection of circuit components, and poor heat dissipation of LED light sources can all affect its service life. Especially in application circuits, electrolytic capacitors, as output filters for AC/DC rectifier bridges, have a service life of less than 5000 hours, which has become a roadblock in the manufacturing of long-life LED lighting technology. Designing and producing a new generation of LED driver ICs that can eliminate electrolytic capacitors in application circuits is a feasible solution. In addition, the design of the new generation LED driver IC must break away from the traditional DC/DC topology design concept, such as using constant power, reducing voltage without hysteresis control and using fixed frequency and current control, solving the problems of lamp source flickering and multiple lamps not working together caused by the use of halogen lamp electronic transformers, and so on; It is also necessary to ensure that LED driver ICs can pass EMC, safety regulations, CE, UL and other certifications in various application circuits; Striving for simplicity in application circuits and using fewer components is also a dream for customers to reduce costs and a necessity for market competition; The application of isolation and non isolation has always been the focus of contention among businesses in terms of security and efficiency; Increase the duty cycle of the PWM controller, and so on. The new generation of chips with 0.5W-3W LED light sources and LED driver ICs integrated in one CMC package have been produced in small quantities, indicating that LED driver ICs are developing towards highly integrated multi-chip CMC packaging; The increasingly mature production technology of new generation and special topology ACLED light sources that can be directly driven by alternating current (AC) to emit light will usher in another era of LED lighting technology. Comparison between isolated and non isolated driving schemes: What are the advantages and disadvantages of the isolated and non isolated driving schemes in LED driving schemes that currently use mains power as the input power? How to choose? Answer: Overall, isolated drives are safe but inefficient, while non isolated drives have higher efficiency. Therefore, the selection of isolated or non isolated drives should be based on actual usage requirements. In terms of circuit structure, the current isolation type schemes are mostly AC/DC flyback circuit schemes, which are relatively complex and costly. The non isolated type basically uses DC/DC boost or buck circuits, which are relatively simple and therefore have relatively low costs. Constant current accuracy: Isolation type can achieve within ± 5%, while non isolation type is difficult to achieve. In the application field: Currently, in LED lighting fixtures that use mains power as the input power source (especially those that integrate the driver and light source), based on the principle of safety first, non isolated solutions are basically no longer used. However, there are also exceptions. Due to structural and spatial constraints, LED fluorescent tubes still use non isolated solutions. In low-voltage powered LED lighting fixtures, non isolated solutions are the best choice based on the principle of prioritizing efficiency and cost.
LED technology driven directly by AC
Q: What is the issue with LED technology driven directly by AC?
Answer: ACLED light source is a method of evenly dividing a pile of small LED grains into five strings using an interleaved matrix arrangement process. The ACLED grain strings form a rectifier bridge, with two ends of the rectifier bridge connected to AC power and the other end connected to a string of LED grains. The positive half of AC power flows along the blue path, three strings of LED grains emit light, and the negative half flows along the green path. There are also three strings of LED grains emit light, and the LED grains on the four bridge arms emit light in turns, The LED grains on the relative bridge arm emit light simultaneously, while the middle string of LED grains continuously emit light due to sharing. In 60Hz AC, it will rotate at a frequency of 60 times per second. The direct current obtained by the rectifier bridge is a pulsating direct current, and the LED's luminescence is also flashing. The LED has the characteristic of power-off and afterglow, which can last for tens of microseconds. Due to the inertia of the human eye in remembering flowing light points, the interpretation of the working mode of the LED light source's luminescence and afterglow by the human eye is continuous luminescence. LED spends half of its time working and half of its time resting, thereby reducing heat generation by 40-20%. Therefore, the service life of ACLED is longer than that of DCLED.
How to solve the uneven lighting of LED fluorescent lamps?
Question: 23 series and 12 parallel, constant current power supply, with obvious light and dark imbalance on the circuit board. The measured voltage is between 3.02 and 3.08, and a single lamp is tested. At 3.1V, the current in the dark area is 10-15ma, and the current in the bright area is about 20ma. Can you please explain this?
Answer: It is possible that the PCB wiring is poor, and there is leakage on the aluminum based PCB board. The method of segmented cutting can be used to find it, but it is quite time-consuming. A 0.06W LED light source, with a theoretical working current of 20mA, is generally used as a lamp to prevent LED premature aging, and can be designed at 15-18mA. Design of PWM constant current LED driver chip for fluorescent lamp power supply.
Q: Recently, when using a PWM constant current LED driver chip as a fluorescent lamp power supply, the following problem occurred: a. The constant current source is not constant, and during the input voltage change process of 80V-250V, the current first increases and then decreases, with a relatively large range of changes. Many inductors have been replaced (all of them are self-made, ranging from 500uH-3mH), but the problem still persists. b. The MOS transistor used is 5N5001, and the voltage at both ends of the DS is tested. When inputting around 150V, the digital multimeter starts to jump randomly. Answer: A. The LED fluorescent lamp power supply scheme composed of PWMLED driver IC is constant current for fixed loads when the Vin changes in a small range. When Vin changes significantly, it needs to have a response time. When the circuit design is poor or the PCB board design is poor, there should be a certain change in Iout when Vin changes. b. It is unreasonable to test the DS voltage with a digital multimeter. If you want to see the voltage, you should use an oscilloscope. LED flashing question: When driving the halogen lamp, the voltage is reduced, and the brightness of the halogen lamp decreases without any other changes. When driving the LED, reducing the voltage will cause the LED to flicker to some extent. Why? Answer: Halogen lamps are pure resistive loads, so their electronic transformers can work well in conjunction with them. LED lamps and lanterns are loads of capacitive impedance and inductive impedance. When the halogen lamp electronic transformer receives the LED lamp load, its output capacity will greatly decrease, manifested as a sudden drop in output voltage from 12V to 7V. If the driver IC starts at 8V, it will enter an undervoltage protection state. At this time, the output voltage of the halogen lamp electronic transformer will rise to 12V, and so on, making you see the LED lamp flickering. The solution is to choose a driver IC with a starting voltage below 6V. How should underwater lights be designed?
Q: How should underwater lights be designed? What are some issues to pay attention to? Is the explosion of LED lamp beads caused by impulse current? How to solve it?
Answer: From a safety perspective, underwater lights require the use of low-voltage isolation power supply, and international commonly used AC36V and below power supply should be used as much as possible. Therefore, AC24V and AC12V are both optional standard power sources. Long distance power transmission can cause voltage drop, which can be compensated by increasing the diameter of the copper wire. In addition, the transmission loss between AC lines is smaller than that between DC lines. Most underwater lights use AC24V safety power supply, so it is completely possible to choose DU2401 or DU2402, which has Vin=6-30V and Iout=1.2A, and few system components, as the driving power supply. It uses four Schottky diodes as rectifier bridges, a filtering energy storage capacitor, an output current setting resistor, a freewheeling Schottky diode, and a freewheeling inductor. Instantaneous sudden changes in power grid load can cause current surges, which can be effectively suppressed by adding protective devices to the circuit. Early 9910ICs had the problem of "exploding lamp beads" and needed to be improved by applying circuits. Question on uneven LED brightness: To make a 60W power LED solar street light, two 12V battery packs are connected in series to store solar energy (with a single battery pack discharging at around 10.8-15V), and the power supply voltage of the street light is between 21.6-30V. The plan is to use two driver units that can drive 30W each, using two 6-series and 5-parallel methods. The driver selects a PWM dimmable BUCK chip. But in reality, there is an issue of uneven LED brightness. How can this problem be resolved? Answer: This is a typical incorrect design. The Buck circuit can only be used when the forward voltage of the LED string is lower than the battery voltage, and a certain voltage margin must be reserved, otherwise even if the Buck enters a 100% duty cycle state, the set current cannot be obtained. Other proponents suggest using a certain brand of device, but we know that the choice of architecture is not related to the brand, just like the correctness of the fundamental laws of physics is not related to the nationality of the proposer. The correct choice for this design is to use the Buck Boost architecture. The best application condition of the Buck Boost architecture is that the output voltage is within the range of input voltage variation, but it is also available when it exceeds the range, which can be called an all-around architecture.
