The LED chip in the led display screen is a characteristic-sensitive semiconductor component and has negative temperature characteristics. Therefore, it needs to be stabilized and protected during the application process. It is precisely because of this that the concept of driving is born. The requirements of the LED display device on the driving power supply are almost harsh. Unlike ordinary incandescent bulbs, LEDs can be directly connected to 220V AC mains. The LED is driven by a low voltage of 2 to 3 volts. It is necessary to design a complicated conversion circuit. LED lights for different purposes must be equipped with different power adapters. In the international market, foreign customers have very high requirements for the efficiency conversion, effective power, constant current accuracy, power supply life, and electromagnetic compatibility of the LED drive power. The design of a good power supply must consider these factors, because the power supply is displayed in the entire LED. The role of the screen is as important as the human heart.

The power supply of the LED display is very important. Each LED display contains many dedicated power supplies, and the model and power of the power supply directly affect the effect and performance of the LED display. Simple knowledge of the LED display power supply brand There are many, many on the market, and the price varies greatly. If the performance of the LED display power supply can not keep up, it will cause many unpredictable consequences, such as: abnormal display, flickering, serious short circuit and fire. These accidents are not It’s not a new thing, it’s all real examples. Many customers like to compare prices when purchasing, and then keep lowering prices. In order to take orders, some manufacturers can only take orders at the reserve price and make a fuss on various accessories. Including power supply.

LED display power supply and constant current drive are inseparable. When using LED as display screen or other lighting equipment or backlight, it must be driven by constant current. The main reasons are as follows:

1. Avoid driving current exceeding the maximum rated value and affecting its reliability.

2. Obtain the expected brightness requirements and ensure the consistency of the brightness and chromaticity of each LED.

Constant current drive characteristics

According to the power rules of the power grid and the characteristic requirements of the LED drive power supply, the following points should be considered when selecting and designing the LED drive power supply:

1. High reliability: It is especially like the driving power supply of LED street lights, installed at high altitude, it is inconvenient to maintain, and the maintenance cost is also high.

2. High efficiency: LED is an energy-saving product, and the efficiency of the driving power supply should be high. It is especially important for the structure where the power supply is installed in the lamp. Because the luminous efficiency of the LED decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. The efficiency of the power supply is high, 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 LED.

3. High power factor: Power factor is the requirement of the grid on the load. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the power factor of a single electrical appliance with low power is lower, it has little effect on the power grid, but if everyone lights up at night, the similar loads are too concentrated, which will cause serious pollution to the power grid. For LED driving power supplies of 30 watts to 40 watts, it is said that in the near future, there may be certain index requirements for power factors.

4. In this way, the combination is flexible, and the failure of one LED will not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current power supply, with LEDs running in series or parallel. Its advantage is that the cost is a little lower, but the flexibility is poor, and it must solve the problem of a certain LED failure without affecting the operation of other LED display accessories. These two forms coexist for a period of time. Multi-channel constant current output power supply mode will be better in terms of cost and performance. Maybe it is the mainstream direction in the future.

5. Surge protection: The ability of LEDs to resist surges is relatively poor, especially the ability to resist reverse voltage. It is also important to strengthen protection in this area. Some LED lights are installed outdoors, such as LED street lights. Due to the onset of the grid load and the induction of lightning strikes, various surges will invade from the grid system, and some surges will cause damage to the LED. Therefore, the LED drive power supply must have the ability to suppress the intrusion of surges and protect the LED from damage.

6. Protection function: In addition to the conventional protection function of the power supply, it is best to add negative feedback of LED temperature to the constant current output to prevent the LED temperature from becoming too high.

7. Protection aspect: The lamp is installed outside, the power supply structure should be waterproof and moisture-proof, and the outer shell should be light-resistant.

8. The life of the driving power supply should be matched with the life of the LED.

9. To meet the requirements of safety regulations and electromagnetic compatibility.

The application range and usage of LED are becoming more and more extensive, and the performance of LED power supply and driving will be more and more suitable for the requirements of LED, so the performance of LED display will gradually improve and stabilize, which is the progress of the industry.

   Constant current drive principle

The relationship curve between the forward voltage drop (VF) and the forward current (IF). From the curve, we can see that when the forward voltage exceeds a certain threshold (about 2V), which is commonly referred to as the turn-on voltage, it can be approximately considered that IF and VF is proportional. See the table for the electrical characteristics of current main super bright LEDs. It can be seen from the table that the highest IF of current ultra-bright LEDs can reach 1A, while VF is usually 2 to 4V.

Since the light characteristics of LEDs are usually described as a function of current, rather than a function of voltage, the relationship curve between luminous flux (φV) and IF, the use of constant current source drive can better control the brightness. In addition, the forward voltage drop of the LED has a relatively large range (up to 1V or more). From the VF-IF curve in the above figure, it can be seen that a small change in VF will cause a large change in IF, which will cause a greater brightness. Big change. Therefore, the use of constant voltage source drive cannot guarantee the consistency of LED brightness, and affects the reliability, life and light attenuation of the LED. Therefore, ultra-bright LEDs are usually driven by a constant current source.

The relationship curve between LED temperature and luminous flux (φV). The figure below shows that luminous flux is inversely proportional to temperature. The luminous flux at 85°C is half of that at 25°C, and the light output at 40°C is 1.8 times that at 25°C. The temperature change also has a certain influence on the wavelength of the LED. Therefore, good heat dissipation is the guarantee for the LED to maintain a constant brightness.

Constant current drive circuit

Due to the limitation of the LED power level, it is usually necessary to drive multiple LEDs at the same time to meet the brightness requirements. Therefore, a special drive circuit is required to light the LEDs.

1. Resistance-capacitance step-down: Use the impedance of the capacitor under AC to limit the input current, thereby obtaining a DC level to power the LED. This drive mode has a simple structure and low cost, but the input is a non-isolated solution, which has potential safety hazards. Moreover, the conversion efficiency is very low, and constant current control cannot be achieved.

2. Isolation flyback circuit: using a flyback circuit, a DC level is generated on the secondary side through a transformer, and then the ripple of this level is fed back to the primary side through an optocoupler, thereby self-excited and stable. This kind of circuit meets the requirements of safety regulations, and the output constant current accuracy is better, and the conversion efficiency is higher. However, due to the need of optocoupler and secondary side constant current control circuit, the system is complicated, bulky, and costly.

3. Primary side plan: The primary side plan is to control the output power and current completely on the AC primary side, the most accurate can achieve 5% constant current accuracy, and the secondary side only needs a simple output circuit. The primary side mainly relies on the feedback of the auxiliary side to control the output voltage, relies on the current limiting resistor to control the primary side current, and at the same time multiplies the turns ratio to control the accuracy of the output current. The original-side solution inherits the advantages of the isolated flyback circuit, while the structure is simple, and it can achieve small size and low cost.

The problem of constant current accuracy of the primary side: The production accuracy of the transformation is difficult to control, resulting in a large output current drift when the primary side solution uses a low-quality transformer. Therefore, the primary-side solution is improved by adding a secondary-side constant current control circuit, which is more complicated than the ordinary primary-side solution, but compared with the flyback solution, the optocoupler can still be omitted, and the system has the highest cost performance.

 Constant current drive mode

Converting voltage through a linear regulator will face power consumption issues. This method is more suitable for use in conversion circuits that need to avoid noise (such as car audio) and cannot use switching methods. The characteristic of the switching method is that the conversion efficiency is very high, but it also has the problem of noise, so the choice of the conversion method depends on the application.

Generally, the efficiency of the charge pump driving method will change with the change of the input voltage. In applications with a wide range of voltage changes, its efficiency is relatively low; and in applications with a relatively small range of voltage changes, only when the input and output voltages are different When the relationship is integral multiples, its efficiency can be maximized, but this is difficult to achieve in practical battery-powered applications. In contrast, the conversion efficiency of an inductor is not affected by voltage interference, and its application restrictions are less than that of a charge pump.

Constant current design ideas

The status of LEDs in LED displays as backlights in portable products has been unshakable. Even in the backlights of large-size LCDs, LEDs have begun to challenge the mainstream status of CCFL (cold cathode fluorescent lamps); and in the field of lighting As the most critical component of semiconductor lighting, LED is sought after by the market because of its energy saving, environmental protection, long life, and maintenance-free aura. Drive circuit is an important part of LED (light emitting diode) products. Whether in the field of lighting, backlight or display panel, the choice of drive circuit technology architecture should match the specific application.

The light-emitting principle of LED is to add a forward voltage to both ends of it, so that the minority carriers and the majority carriers in the semiconductor recombine and release excess energy, thus causing the emission of photons. The main function of the LED drive circuit is to convert the AC voltage into a constant current power supply, and at the same time complete the matching of the voltage and current of the LED according to the requirements of the LED device. In addition to meeting safety requirements, the LED drive circuit should have two other basic functions:

One is to keep the constant current characteristics as much as possible, especially when the power supply voltage fluctuates by ±15%, the output current should be kept within ±10%.

1. Avoid driving current exceeding the maximum rated value and affecting its reliability.

2. Obtain the expected brightness requirements and ensure the consistency of the brightness and chromaticity of each LED.

The second is that the drive circuit should keep its own power consumption low, so that the system efficiency of the LED can be maintained at a high level.

PWM (Pulse Width Modulation) technology is a traditional dimming method. It uses simple digital pulses to repeatedly switch the LED driver. The system only needs to provide wide and narrow digital pulses to easily change the output current. Thereby adjusting the brightness of the LED. The advantage of this technology lies in its ability to provide high-quality white light, simple application and high efficiency, but a fatal disadvantage is that it is prone to electromagnetic interference and sometimes even noise that human ears can hear.

Boost is an important task of the LED drive circuit, and inductive boost and charge pump boost are two different topological modes. "Since the LED is driven by current, and the inductor has the highest efficiency when converting current, the biggest advantage of the inductor boost method is its high efficiency, which can exceed 90% if it is designed properly; but its disadvantage is also obvious, that is, electromagnetic interference. Very strong, the system requirements for communication products such as mobile phones are very high. With the advent of charge pumps, the efficiency of boosting methods using charge pumps will be lower than that of inductive boosting.

Whether in lighting applications or backlight applications, improving the conversion efficiency of the drive circuit is a problem that product designers must face. Improving the conversion efficiency is not only conducive to extending the standby time of portable products, but also an important means to solve the problem of LED heat dissipation. In the field of lighting, because of the use of high-power LEDs, it is particularly important to improve the conversion efficiency.

LEDs need to have constant current and voltage stabilizing components when they are working, but such components should have the characteristics of high voltage division but low power consumption. Otherwise, LEDs with higher efficiency will be caused by the working power of the drive circuit. Too much consumption will greatly reduce the efficiency of the overall system, which goes against the purpose of energy saving and high efficiency. Therefore, resistors or series voltage regulator circuits should not be used as the main current limiting circuit of the LED driver as much as possible, but high-efficiency circuits such as capacitors, inductors or active switching circuits should be used to ensure the high efficiency of the LED system. The use of series integrated constant power output circuit can keep the light output of the LED constant in a wide range of power supply, but the general IC circuit will therefore reduce the efficiency. The use of an active switch circuit can ensure that the constant power output is achieved when the power supply voltage changes greatly under high conversion efficiency.
But with its unique advantages, it can work efficiently under the condition of safe special voltage (swimming pool, underwater lamp in rowing pool, miner's lamp). In addition, it has unique advantages in the use of solar energy (wind energy), as well as in emergency lighting. Especially in the dimming aspect, LED can not only achieve 0 ~ 100% dimming, but also can ensure high lighting efficiency in the whole dimming process, and does not damage the life of the LED, which is difficult for gas discharge lamps.LED
Constant current driving requirements of display screen

In addition to meeting safety requirements, the LED drive circuit has two other basic functions:

The first is to maintain constant current characteristics as much as possible, especially when the power supply voltage changes by ±15%, the output current should still be maintained within ±10%. The main reasons are: 1. Avoid driving current beyond the maximum rated value. Affect its reliability. 2. Obtain the expected brightness requirements and ensure the consistency of the brightness and chromaticity of each LED.

The second is that the drive circuit should keep its own power consumption low, so that the LED system efficiency can be maintained at a high level.

DC control

LED is a device driven by current, and its brightness is proportional to the forward current. There are two ways to control the forward current.

The first method is to use the LEDV-I curve to determine the voltage that needs to be applied to the LED to produce the expected forward current. Its realization method generally adopts a voltage power supply and a ballast resistor. As described below, this method has several disadvantages. Any change in LED forward voltage will cause a change in LED current. If the rated forward voltage is 3.6V, the current of the LED in Figure 1 is 20mA. If the voltage becomes 4.0V, which is a specific voltage change caused by temperature or manufacturing changes, the forward current is reduced to 14mA. An 11% change in forward voltage will result in a larger forward current change, up to 30%. In addition, depending on the available input voltage, the voltage drop and power consumption of the ballast resistor will waste power and reduce battery life.

The second method and the preferred method of LED current adjustment is to use a constant current power supply to drive the LED.

high efficiency

Battery life is critical in portable applications. If the LED driver is practical, it must be highly efficient. The efficiency measurement of an LED driver is different from that of a typical power supply. The definition of a typical power supply efficiency measurement is the output power divided by the input power. For LED drivers, the output power is not a relevant parameter. What is important is the value of input power required to produce the expected LED brightness. This can be determined simply by dividing the LED power by the input power. Please note: If the efficiency is defined in this way, the power dissipation in the current sense resistor will cause power dissipation in the power supply. Through the formula shown in Figure 3, we can see that a smaller current sensing voltage will produce a higher efficiency LED driver. Figure 4 illustrates the efficiency improvement of the power supply with 0.25V reference voltage compared with the power supply with 1V reference voltage. A lower current-sensing voltage power supply is more effective. Regardless of the input voltage or LED current, as long as other conditions are the same, a lower reference voltage can improve efficiency and extend battery life.

"Conclusion" The constant current drive of the LED display power supply is a very important part, which directly affects the use effect, service life and safety hazards. Whether it is a manufacturer or a customer, it should be paid attention to instead of being important for ordering. Pull down.