LED driver basics
LED drive LED constant current drive
Since the LED is a characteristic sensitive semiconductor device and has a negative temperature characteristic, it needs to be stably operated and protected during the application process, thereby generating a driving concept. LED devices have almost strict requirements for driving power. Unlike ordinary incandescent bulbs, LEDs can be directly connected to 220V AC mains. The LED is a low voltage drive of 2 to 3 volts. It is necessary to design a complicated conversion circuit. LED lamps for different purposes must be equipped with different power adapters. In the international market, foreign customers have very high requirements for efficiency conversion, effective power, constant current accuracy, power supply life and electromagnetic compatibility of LED drive power. To design a good power supply, these factors must be considered comprehensively because the power supply is in the entire luminaire. The role is just as important as the human heart.
When using LEDs as displays or other lighting devices or backlights, they need to be driven with constant current. The main reasons are:
1. Avoid driving current exceeding the maximum rating and affect its reliability.
2. Obtain the expected brightness requirements and ensure the consistency of brightness and chromaticity of each LED.
LED drive features
According to the power consumption rules of the power grid and the characteristics of the LED drive power supply, the following points should be considered when selecting and designing the LED drive power supply:
High reliability
Especially like the driving power supply of LED street lamps, it is installed at high altitude, the maintenance is inconvenient, and the maintenance cost is also large.
2. High efficiency
LEDs are energy-saving products, and the efficiency of driving power supplies is high. It is especially important for structures where the power supply is installed in the luminaire. Since the luminous efficiency of the LED decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. The 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.
3. High power factor
The power factor is the grid's load requirements. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the power factor of a single power consumer with low power has little effect on the power grid, at night, everyone lights up, and the similar load is too concentrated, which will cause serious pollution to the power grid. For 30 watts to 40 watts of LED driver power, it is said that in the near future, there may be certain indicators for power factors.
4. Drive mode
There are two types of traffic: one is a constant voltage source for multiple constant current sources, and each constant current source supplies power to each LED separately. In this way, the combination is flexible, and all the LED faults do not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current supply, with LEDs running in series or in parallel. Its advantage is that the cost is lower, but the flexibility is poor, and it is necessary to solve a certain LED failure without affecting the operation of other LEDs. These two forms coexist for a while. 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.
5. Surge protection
The ability of LEDs to withstand surges is relatively poor, especially against reverse voltage capability. It is also important to strengthen protection in this area. Some LED lights are installed outdoors, such as LED street lights. 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.
6. Protection function
In addition to the conventional protection function, the power supply preferably increases the LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high.
7. Protection
The external installation of the lamp, the power supply structure should be waterproof and moisture-proof, and the outer casing should be light-resistant.
8. The life of the drive power should be matched to the life of the LED.
9. To comply with safety and electromagnetic compatibility requirements.
With the increasing use of LEDs, the performance of LED drive power will be more and more suitable for LED requirements.
LED drive principle
The relationship between forward voltage drop (VF) and forward current (IF). It can be seen from the curve that when the forward voltage exceeds a certain threshold (about 2V), that is, the so-called turn-on voltage, it can be approximated as IF and VF is proportional. See the table for the electrical characteristics of the current main super bright LEDs. It can be seen from the table that the highest IF of the current super bright LED can reach 1A, and the VF is usually 2 to 4V.
Since the light characteristics of LEDs are usually described as a function of current, not as a function of voltage, the relationship between luminous flux (φV) and IF, therefore, the use of constant current source driving can better control brightness. In addition, the forward voltage drop of the LED varies widely (up to 1V or more), and the VF-IF curve in the above figure shows that a small change in VF causes a large, IF change, which causes a comparison of brightness. Big change. Therefore, the use of constant voltage source drive can not guarantee the consistency of LED brightness, and affect the reliability, life and light decay of LED. Therefore, super bright LEDs are usually driven by a constant current source.
The relationship between the temperature of the LED and the luminous flux (φV) is shown by the following figure. The luminous flux is inversely proportional to the temperature. The luminous flux at 85 °C is half that at 25 °C, and the light output at 18 °C is 1.8 times that at 25 °C. The change in temperature also has a certain effect on the wavelength of the LFD. Therefore, good heat dissipation is a guarantee that the LED maintains a constant brightness.
LED drive circuit
Due to the limitation of LED power level, it is usually necessary to drive multiple LEDs at the same time to meet the brightness requirement. Therefore, a special driving circuit is needed to illuminate the LED. Here are a few of the current mainstream LED driver methods:
Resistance to pressure reduction
The input current is limited by the impedance of the capacitor under AC to obtain a DC level to power the LED. The driving method is simple in structure and low in cost, but the input non-isolation scheme has security risks. Moreover, the conversion efficiency is very low and constant current control cannot be achieved.
2. Isolated flyback circuit
The flyback circuit is used to generate a DC level on the secondary side through the transformer, and then the ripple of this level is fed back to the primary side through the optocoupler to be self-oscillated. Such circuits comply with safety certification requirements, and the output constant current accuracy is better, and the conversion efficiency is higher. However, due to the need for optocoupler and secondary constant current control circuits, the system is complicated, bulky, and costly. It has gradually been replaced by the original side plan.
3. Primary plan
The primary side scheme is to control the output power and current completely at the primary side of the AC. 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, and relies on the current limiting resistor to control the primary current, and multiplies the turns ratio to control the accuracy of the output current. The primary side scheme inherits the advantages of the isolated flyback circuit, and at the same time, the architecture is simple, can be small in size and low in cost, and has become a mainstream driver.
Primary constant current accuracy problem
Since the production accuracy of the transformer is difficult to control, the output current drifts greatly when the primary side scheme uses a low-quality transformer. Therefore, the primary side scheme has improved the secondary side constant current control circuit through improvement, which is more complicated than the ordinary primary side scheme. However, compared with the flyback scheme, the optocoupler can be omitted, and the system has the highest cost performance.
Edit: Xiang Zuo
Fan Motor,Air Cooler Motor,Ac Motor Air Cooler,Asynchronous Fan Motor
WUJIANG JINLONG ELECTRIC APPLIANCE CO., LTD , https://www.jinlongmotor.com