Some key points in selecting and designing LED drivers

Regarding LED driver power supply, there are many issues that need to be paid attention to in selection and design. Only by mastering several key points of selection and design can we make better design.

　　LED driver IC features and selection

　　Q: Do I need to use a dedicated LED driver IC to drive the LED? What are the advantages and disadvantages compared to ordinary ICs?

　　Answer: The advantages of LED driver IC are wide Vin, large output current, high constant current accuracy, and the use of frequency jitter can effectively reduce EMI.

　　The main purpose of using a constant current source to drive an LED is to ensure that the brightness of the LED is consistent and does not flicker during the working period of the LED. It is also common to use LDO to drive the backlight LED of low-cost mobile phones, but the power utilization rate 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 because LDO cannot provide constant current.

　　Q: So how do you choose between domestic and foreign LED driver ICs?

　　Answer: There are many LED driver ICs, and their cost performance varies. The key is to see what the target price of the designed product is. If you want competitiveness and higher profits, local IC products are also good.

　　LED driver power supply selection and design issues

　　Q: What issues should be considered when selecting and designing an LED driver power supply?

　　A: LED driver power is a voltage converter that converts power supply into a specific voltage and current to drive the LED to emit light. Usually, the input of LED driver power includes AC 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 as the forward voltage drop value of the LED changes.

　　According to the power consumption rules of the power grid and the characteristic requirements of the LED driver power supply, the following issues should be considered when selecting and designing the LED driver power supply.

　　a. High reliability, especially for the driving power supply of LED street lamps, which are installed at high altitudes, making maintenance inconvenient and costly.

　　b. High efficiency power supply has high efficiency, low power consumption, low heat generation in the lamp, and thus low temperature rise of the lamp, which is beneficial to delaying the light decay of LED.

　　c. There are two common driving modes: one is a constant voltage source for multiple constant current sources, each constant current source supplies power to each LED separately. The other is direct constant current power supply, LEDs are connected in series or in parallel. How to choose the starting mode depends on actual needs. The multi-channel constant current output power supply mode will be better in terms of cost and performance. It may be the mainstream direction in the future.

　　d. Surge protection LEDs have poor surge resistance, especially reverse voltage resistance. Due to the induction of grid loads and lightning strikes, various surges will invade from the grid system, and some surges will cause damage to LEDs. Therefore, LED driver power supplies must be able to suppress surge intrusion and protect LEDs from damage.

　　e. Other factors, such as improving the power factor of the power supply; adding the LED temperature negative feedback function; in terms of external protection, it must be waterproof and moisture-proof, and the casing must be sun-resistant; the life of the driving power supply must be compatible with the life of the LED; it must comply with safety regulations and electromagnetic compatibility requirements.

　　LED driver IC development direction

　　Explanation: Development direction of LED driver IC for lighting

　　LED light source is a long-life light source, with a theoretical life of up to 50,000 hours, but unreasonable application circuit design, improper selection of circuit components, and poor heat dissipation of LED light sources will affect its service life. Especially in the application circuit, the electrolytic capacitor used as the output filter of the AC/DC rectifier bridge has a service life of less than 5,000 hours, which has become a stumbling block in the manufacture of long-life LED lamp technology. Designing and producing a new generation of LED driver ICs that can save electrolytic capacitors in the application circuit is a feasible solution.

　　In addition, the design of the new generation of LED driver ICs must break the traditional DC/DC topology design concept, such as using constant power, not using hysteresis control buck type but using fixed frequency and constant current control, solving the problem of light source flickering and multiple lamps not lighting up in parallel caused by using halogen lamp electronic transformers, etc.; it is also necessary to solve the problem that LED driver ICs can pass EMC, safety regulations, CE, UL and other certifications in various application circuits; the application circuit should be simple and use fewer components, which is also the dream of customers to reduce costs and is necessary for market competition; isolation and non-isolation applications have always been the focus of merchants' disputes over safety and efficiency; improving the duty cycle of the PWM controller, etc.

　　A new generation of chips that integrate 0.5W-3W LED light sources and LED driver ICs in a CMC package have been produced in small batches, indicating that LED driver ICs are developing towards highly integrated multi-chip CMC packages; the production technology of a new generation of special topology ACLED light sources that can be directly driven by alternating current (AC) is becoming more mature, which will usher in a new era of LED lighting technology.　Comparison of isolated and non-isolated drive solutions

Q: Among the current LED driver solutions 　　that use AC power as input power , what are the advantages and disadvantages of the isolated and non-isolated driver solutions? How to choose?

　　Answer: In general, isolated drives are safe but less efficient, while non-isolated drives are more efficient. You should choose isolated or non-isolated drives based on actual usage requirements.

　　In terms of circuit structure: the current isolation solutions are mostly AC/DC flyback circuit solutions, so the circuit is relatively complex and the cost is relatively high. The non-isolated type basically uses DC/DC boost or buck circuits, which are relatively simple and therefore relatively low in cost.

　　Constant current accuracy: The isolated type can achieve within ±5%, while the non-isolated type is difficult to achieve.

In the application field: At present, in LED lamps 　　that use AC power as input power (especially lamps with integrated driver and light source), based on the principle of safety first, non-isolated solutions are basically no longer used. However, there are exceptions. LED fluorescent tubes still use non-isolated solutions due to structural and space constraints. In low-voltage powered LED lamps, non-isolated solutions are the best choice based on the principle of efficiency and cost priority.

　　AC direct drive LED technology

　　Q: What is the technology of LED driven directly by AC?

　　Answer: ACLED light source is to divide a bunch of LED tiny grains into five strings by using staggered matrix arrangement process. The ACLED grain string is similar to a rectifier bridge. The two ends of the rectifier bridge are connected to the AC power supply respectively, and the other two ends are connected to a string of LED grains. The positive half cycle of the AC flows along the blue path, and 3 strings of LED grains emit light. The negative half cycle flows along the green path, and 3 strings of LED grains emit light. The LED grains on the four bridge arms emit light in turn, and the LED grains on the opposite bridge arm emit light at the same time. The middle string of LED grains is always emitting light because of sharing. In 60Hz AC, it will light up at a frequency of 60 times per second. The DC obtained by the rectifier bridge is pulsating DC, and the LED light is also flashing. LED has the characteristics of power-off afterglow, and the afterglow can last for tens of microseconds. Because the human eye has an inert memory of the flowing light spot, the human eye interprets the working mode of the LED light source + afterglow as continuous light. LED is working half of the time and resting half of the time, so the heat can be reduced by 40-20%. Therefore, the service life of ACLED is longer than that of DCLED.

　　ACLED's mature products include Seoul's AX3201, AX3211 for AC110C and AX3221, AX3231 for 220V.

Answers to 　　Engineers'

　　How to solve the uneven brightness of LED fluorescent lamps?

　　Question: There are 23 series and 12 parallel, constant current power supply, and there is obvious uneven brightness on the circuit board. The measured voltage is between 3.02-3.08. When testing a single lamp, the current in the dark area is 10-15ma at 3.1v and the current in the bright area is about 20ma. What is going on?

　　A: It may be that the PCB wiring is not good, or the aluminum-based PCB board has leakage. You can use the method of cutting it in sections to find it, but it is more time-consuming. The theoretical working current of a 0.06W LED light source is 20mA. Generally, in order to prevent LED premature aging, the lamp can be designed at 15-18mA.

　　PWM constant current LED driver chip design fluorescent lamp power supply problem.

　　Q: I recently used a PWM constant current LED driver chip as a fluorescent lamp power supply, and the following problems occurred:

　　a. The constant current source is not constant current. When the input voltage changes from 80V to 250V, the current increases first and then decreases, and the range of change is relatively large. The inductor has been changed many times (the inductor is self-made, ranging from 500uH to 3mH), but the problem still exists.

　　b. The MOS tube used is 5N5001. The voltage across DS is tested. When the input is around 150V, the digital multimeter starts to jump randomly.

　　Answer: a. The LED fluorescent lamp power supply solution composed of PWMLED driver IC is a constant current for fixed load when Vin changes in a small range. When Vin changes greatly, it needs a response time. When the circuit design or PCB board design is not good, Iout will also change to a certain extent 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 flickering problem

　　Q: When driving a halogen lamp, if I lower the voltage, the brightness of the halogen lamp will decrease, but there will be no other changes. When driving an LED, if I lower the voltage, the LED will flicker to a certain extent. Why?

　　Answer: Halogen lamps are pure resistance loads, so halogen lamp electronic transformers can work well with them. LED lamps are capacitive impedance + inductive impedance loads. When halogen lamp electronic transformers accept LED lamp loads, their output capacity will be greatly reduced, which is manifested as a sudden drop in output voltage from 12V to 7V. If the driver IC is started at 8V, it will enter the undervoltage protection state. At this time, the output voltage of the halogen lamp electronic transformer rises to 12V, and this cycle repeats, so that you can see the LED light flashing. The solution is to use a driver IC with a starting voltage lower than 6V.　　How should underwater lights be designed?

　　Q: How should underwater lights be designed? What issues should be paid attention to? Is the explosion of LED lamp beads caused by the impact current? How to solve it?

　　Answer: From a safety perspective, underwater lights require low-voltage isolated power supply, and the internationally accepted AC36V power supply should be used as much as possible. Therefore, AC24V and AC12V are both optional standard power supplies.

　　Transmitting electricity over long distances will cause voltage drop, which can be compensated by increasing the diameter of the copper wire. In addition, AC transmission between lines has less transmission loss than DC transmission between lines.

　　Most underwater lights use AC24V safe power supply, so Vin=6-30V, Iout=1.2A can be used as the driving power supply. The system uses PT4115 with very few components. It uses four Schottky diodes as a rectifier bridge, a filter energy storage capacitor, an output current setting resistor, a freewheeling Schottky diode and a freewheeling inductor.

　　The sudden change of grid load will cause current surge, which can be effectively suppressed by adding protection devices to the circuit. The early 9910 ICs all had the problem of "exploding lamp beads", which needed to be improved by the application circuit.

　　Uneven LED brightness problem

　　Q: To make a 60W LED solar street light, two 12V battery packs are connected in series to store solar energy (the discharge voltage of a single battery pack is about 10.8-15V), and the power supply voltage of the street light is between 21.6-30V. It is planned to use two drive units that can drive 30W each, using two 6-in-1 and 5-in-1 parallel modes. The driver selects a BUCK chip that can be dimmed with PWM. However, the actual LED brightness is uneven. How to solve this problem?

　　A: This is a typical wrong design. 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 the set current cannot be obtained even if the Buck enters the 100% duty cycle state.

　　Other proponents have suggested using a certain brand of components, but we know that the choice of architecture has nothing to do with brand, just as the correctness of the basic laws of physics has nothing to do with the nationality of their proponents.

　　The correct choice for this design is to use the Buck-Boost architecture. The best application condition for the Buck-Boost architecture is that the output voltage is within the range of the input voltage, but it is also usable when it is outside the range, so it can be called an all-round architecture.