User question: LED driver must use special LED driver IC? What are the advantages and disadvantages compared with ordinary IC?
Expert reply: The advantages of LED driver IC are wide Vin, large output current, high constant current accuracy, and frequency hopping can effectively reduce EMI.
The constant current source for LED driving is mainly to ensure that the brightness of the LED during the working period of the LED is uniform and does not flicker. Low-cost mobile phones use LDO to drive backlight LEDs are also commonly used, but the power utilization rate is not high, but in most mobile phone design lithium battery voltage drop to 3.3V shutdown. It is not a good idea to drive LED lighting with LDO. LDO can't keep constant current.
User question: So how do you choose domestic and foreign LED driver ICs?
Expert reply: LED driver ICs are many, the cost performance is different, the key is to see the target price of the design products, such as seeking competitiveness and more profits, local IC products are also good.
2. LED drive power selection and design issues
User question: What issues to consider when selecting and designing LED driver
Expert reply: LED drive power is a voltage converter that converts the power supply into a specific voltage and current to drive the LED to emit light. Normally, the input of the LED drive power includes mains, low voltage DC, high voltage DC, low voltage high frequency AC, and the like. The output of the LED driver is mostly a constant current source that changes the voltage as the LED's forward voltage drop changes.
According to the power consumption rules of the power grid and the characteristics of the LED drive power supply, the following issues should be considered when selecting and designing the LED drive power supply.
a. High reliability is especially like the driving power of LED street lamps . It is installed at high altitude, it is inconvenient to maintain, and the maintenance cost is also large.
b. The high efficiency power supply has high efficiency, its power consumption is small, and the heat generated in the lamp is small, which reduces the temperature rise of the lamp. It is beneficial to delay the light decay of LEDs.
c. There are two types of driving methods: one is a constant voltage source for multiple constant current sources, and each constant current source supplies power to each LED separately. The other is direct constant current supply, with LEDs running in series or in parallel. How to choose the startup method should be based on actual needs. Multi-channel constant current output power supply mode will be better in terms of cost and performance. Perhaps it is the mainstream direction in the future.
d. The ability of surge protection LEDs to resist surges is relatively poor, especially against reverse voltage capability. Due to the start of the grid load and the induction of lightning strikes, various surges will be invaded from the grid system, and some surges will cause LED damage. Therefore, the LED driver power supply must have the ability to suppress the intrusion of surges and protect the LEDs from damage.
e. Other factors, such as improving the power factor of the power supply; increasing the negative feedback function of the LED temperature; for external protection, it is waterproof and moisture-proof, and the outer casing is resistant to light; the life of the driving power supply should be compatible with the life of the LED; And electromagnetic compatibility requirements.
3. LED driver IC development direction
Experts explain: The development direction of LED driver IC for lighting
The LED light source is a long-life light source with a theoretical life of up to 50,000 hours. However, the application circuit design is unreasonable, the circuit components are not properly selected, and the LED light source is not well cooled, which will affect its service life. Especially in the application circuit, the electrolytic capacitor, which is the output filter of the AC/DC rectifier bridge, has a service life of less than 5,000 hours. This has become a roadblock for manufacturing long-life LED luminaire technology, which can be designed and produced in the application circuit. A new generation of LED driver ICs that save electrolytic capacitors is a viable solution.
In addition, the design of the new generation of LED driver ICs must break the traditional DC/DC topology design concept, such as constant voltage, step-down type without hysteresis control, constant frequency constant current control, and use of halogen-type electronic transformers. The generated light source flashes and the multiple lamps are not lit in parallel, etc.; the LED driver IC must also be able to pass EMC, safety, CE, UL and other certifications in various application circuits; the application circuit is intended to be simple and use less components. It is also necessary for customers to reduce costs and compete with the market; isolated and non-isolated applications have always been the focus of business security and efficiency; increase the duty cycle of PWM controllers and so on.
0.5W-3W LED light source and LED driver IC The next-generation chip integrated in a CMC package has been produced in small batches. The display LED driver IC is developing in a highly integrated multi-chip CMC package; it can be directly driven by alternating current (AC). The new generation of light-emitting, special topology AC LED light source production technology is becoming more and more mature, and will create another new era of LED lighting technology.
4. Comparison of isolated and non-isolated drive schemes
User question: At present, in the LED driver scheme with the mains as the input power source, what are the advantages and disadvantages of the isolated and non-isolated two drive schemes? How to choose
Expert reply: In general, the isolated drive is safe but less efficient, and the non-isolated drive is more efficient. The isolated or non-isolated drive should be selected according to the actual requirements.
As far as the circuit structure is concerned, the current isolation scheme is mostly a flyback circuit scheme of AC/DC, so the relative circuit is complicated and costly. The non-isolated type is basically a DC/DC boost or buck circuit, and the relative circuit is relatively simple, so the cost is relatively low.
Constant current accuracy: The isolation type can be achieved within ±5%, while the non-isolated type is difficult to achieve.
In the field of application: At present, in the LED lamps with the mains input power (especially the lamps that are integrated with the light source), the non-isolated scheme is basically no longer adopted in the principle of safety first. However, there are exceptions. Due to structural and space constraints, LED fluorescent tubes still use non-isolated solutions. In low-voltage-powered LED luminaires, non-isolated solutions are the best choice based on efficiency and cost priority.
5. AC direct drive LED technology
User question: What is the LED technology directly driven by AC?
Expert reply: AC LED light source is divided into five strings by a stack of LED micro-arrays. The AC LED die is composed of a rectifier bridge. The two ends of the rectifier bridge are connected to the AC power supply. A series of LED dies are connected at the end, the positive half of the alternating current flows along the blue path, the three strings of LED illuminate, the negative half cycle flows along the green path, and three strings of LED dies are illuminated, and the LED dies on the four bridge arms The light is emitted, and the LED crystal grains on the bridge arm emit light at the same time, and the middle LED strings are always illuminated due to sharing. In 60 Hz alternating current, it will be lit at a frequency of 60 times per second. The DC obtained by the rectifier bridge is pulsating DC, and the LED's illumination is also flashing. The LED has the characteristics of power-off afterglow, and the afterglow can be kept for tens of microseconds. Because the human eye is inert to the flow point memory, the result is The eye's interpretation of the working mode of the LED + light source + afterglow is continuous illumination. LEDs work half the time, half of the time is resting, so heat is reduced by 40-20%. Therefore, the life of the AC LED is longer than that of the DC LED.
Mature products of AC LEDs such as AX3201 and AX3211 for AC110C and AX3221 and AX3231 for 220V.
6. LED drive circuit working principle
User question: What is the role of each component in the LED driver circuit? How does it work?
Expert reply: The above figure is an electrical schematic diagram for driving three 1W white LEDs. It is AC and DC for 12~24V voltage range.
D1~D4 form a rectifier bridge. This bridge has two functions. When the input is AC power, the AC is rectified into DC. When the input is DC power, the polarity is switched, regardless of the polarity of the input power. Can ensure that the circuit works properly.
CIN is a filter capacitor that converts the pulsed DC into a smooth DC. RS is the sampling resistor that determines the absolute accuracy of the constant current source. L is a rectifying inductor that converts a 100KHz pulse current into a triangular wave current. The inductance of L affects the stability of the constant current source within the operating voltage range.
D5 is a freewheeling diode that provides a discharge loop for the current stored in the inductor when the MOS transistor inside the wafer is in an off state. Due to the high-frequency pulse state, D5 should use a Schottky diode with a small forward voltage drop and fast recovery.
The DIM end of the chip can be externally connected with PWM pulse or DC voltage dimming, or can be connected to the thermistor for auxiliary temperature control and automatic brightness control. If you do not use these features, the DIM terminal is left floating.
7. How to solve the problem of unevenness of LED fluorescent light?
User question: 23 string 12 and constant current power supply, there is obvious uneven brightness on the circuit board, the measured voltage is between 3.02-3.08, single lamp test, the current in the dark area is 3.1-15 bright in 3.1v The area is around 20ma. What is going on?
Expert reply: It may be that the PCB layout is not good, there is leakage of the live aluminum-based PCB board, and it can be found by means of segmentation and cutting, which is more time-consuming. 0.06W LED light source, the theoretical working current is 20mA, generally used to prevent LED premature aging, can be designed at 15-18mA.
8. PWM constant current LED driver chip design fluorescent lamp power supply problem
User question: Recently, using a PWM constant current LED driver chip to do fluorescent power, the following problems occur:
a, constant current source does not constant current, in the process of input 80V-250V voltage change, the current first increases, then decreases, and the range of variation is relatively large, the inductance is changed a lot (inductance is homemade, from 500uH-3mH) ,The problem is still
b, the MOS tube used is 5N5001, test the voltage across the DS, when the input is about 150V, the digital multimeter starts to jump.
Expert reply:
a. The LED fluorescent lamp power supply scheme composed of the PWM LED driver IC is constant current for a fixed load when the Vin range is changed. When Vin changes greatly, it needs a response time. When the circuit design is not good or the PCB design is not good, Iout should change 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.
9. LED flashing problem
User question: When the halogen lamp is driven, the voltage is lowered, and the brightness of the halogen lamp is lowered, and there is no other change. When the LED is driven to lower the voltage, the LED will flicker to a certain extent. why?
Expert reply: The halogen lamp is a purely resistive load, so the halogen lamp electronic transformer can work well with it. The LED luminaire is a capacitive impedance + inductive impedance load. When the halogen electronic transformer receives the LED lamp load, its output capability will be greatly reduced. The output voltage will drop from 12V to 7V. If the driver IC starts at 8V, it will enter the owed. The voltage protection state, at this time, the output voltage of the halogen lamp electronic transformer rises to 12V again, so that you can see the LED light flashing. The solution is to use a driver IC with a startup voltage lower than 6V.
10. How should the underwater light be designed?
User question: How should the underwater light be designed? What problems should be paid attention to? Is the LED lamp bead blown caused by the inrush current? How to solve it?
Expert reply: From a safety point of view, underwater lights require low-voltage isolated power supply, as far as possible to use the international AC 36V power supply, so AC24V, AC12V are optional standard power supply.
Long-distance power transmission will cause a voltage drop, which can increase the diameter of the copper wire to compensate. In addition, AC interline transmission is less than DC interline transmission loss.
Most underwater lights use AC24V safe power supply, so you can use Vin=6-30V, Iout=1.2A, and PT4115 with few system components to drive power. It uses four Schottky diodes as rectifier bridge and one filter. A storage capacitor, an output current setting resistor, a freewheeling Schottky diode, and a freewheeling inductor.
Instantaneous abrupt changes in the grid load can cause current surges, which can be effectively suppressed by adding protective devices to the circuit. The early 9910IC had a "fried lamp bead" problem that needed to be improved by the application circuit.
11. LED brightness unevenness problem
User question: To be a 60W power LED solar street light, use two 12V battery packs to store solar energy in series (single battery pack discharge voltage is about 10.8-15V), and the street lamp's power supply voltage is between 21.6-30V. Two drive units that can drive 30W respectively, using two 6-string 5-side modes. The driver selects a BUCK chip that can be PWM dimmed. However, the LED brightness unevenness problem actually occurs. How to solve this problem?
Expert reply: This is a very typical wrong 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 the 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 as the correctness of the basic laws of physics is not related to the nationality of the proponent.
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 the input voltage, but it is also available when it is out of range. It can be called a versatile architecture.
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