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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.
LEDs are a typical current-driven device that precisely controls the LED drive current and determines many parameters including light efficiency, power efficiency, heat dissipation, and product brightness. Driving the LED is mainly to control its current. Whether it is directly increasing or decreasing the drive current, or the duty cycle (PWM) reduces the switching time ratio, it is the control current mode, but the purpose is not the same. This article will explain the differences between different drivers in different applications.
1. Introduction to distributed constant current drive principle
In the past incandescent and energy-saving lamps market, the mainstream models of lamps with limited specifications formed by large companies, LEDs are difficult to continue to comply. LEDs have the flexibility of application and will bring more power specifications in future designs.
The principle of distributed constant current is to establish independent constant current sources at each parallel branch point to manage, maintain and control the stability of branches and branches, branches and overall lines. The distributed constant current circuit can be regarded as a complete line structure in use, and the practical application is distributed in each node of the line, which is a circuit structure that can be controlled by constant current and can communicate with each other.
At present, LED products claim a big gap with the actual service life. The stability of the drive line will directly affect the overall stability of the product.
The reason why the distributed constant current technology has high reliability is that the AC power supply section continues to use the conventional switching power supply, and the constant voltage power supply mode is adopted. The accumulation of switching power supply technology will create quality conditions for LED power supply design. Under the same power supply specification, it is no longer necessary to develop a new power supply model, the power can be backward compatible, greatly reducing power supply specifications and improving power supply uniformity.
2, the combination of soft and hard precision control ideas
In the daily drive power supply design, the cumulative error of peripheral devices is very difficult to handle, resulting in a far cry from the original design of the drive power parameters. Constant current driving requires current detection. The usual method is to connect the millicouple resistors in the branch to obtain feedback information. To achieve high efficiency, the resistance value will be smaller. Too small millicouple resistors will cause inconvenience to production and testing. The general instrument can not verify the correct value, the production process will also affect the accuracy, and the resistance mode setting current is a fixed mode, and the adjustment is not convenient.
The combination of software and hardware will open the leap in LED application technology. The LED constant current accuracy value is softwareized, which can greatly enhance the flexibility of LED applications. The circuit current setting can be done with intuitive digital writing through the microcomputer operating software.
It is our goal to drive the zero components around the line. Peripheral zero devices do not introduce error in the design device parameters, which greatly improves the accuracy of constant current.
China's IC manufacturing process can not meet the LED drive accuracy requirements at present, but we can use the new technology and new methods to achieve the world's top constant current accuracy level. Driving precise control is one of them.
When driving precise control, the first thing to look at is whether the design is based on the highest light efficiency or the consistency of the luminaire. If it is limited to the accuracy of the drive current, it is actually very easy to do. For example, the drive current is stable and accurate, or protected with temperature changes. Customers require that all parameters meet the requirements, such as product consistency and efficiency.
For these requirements of customers, we need to work hard on design drivers. In the final analysis, how to control the accuracy, and finally adjust the current according to our design intent to improve the stability of the product.
The value is implemented. The built-in non-volatile E2PROM can be selected. It is believed that any register can perform its task, and the type selection of the memory can be determined according to the application requirements and the conditions allowed by the process.
The division and design of the current levels can vary from market to market. There is always an error in the output current due to manufacturing processes, which will be improved after software.
When the driver IC of Changyuntong is delivered from the factory, it can provide different current output values ​​according to different needs of customers, eliminating the batch calibration process. Small-volume customers can also override the current value by the included microcomputer software.
3. New design method to improve driving efficiency
AC power drive LEDs are feasible in a single series of branches, but single series is only a small part of LED drive applications, most applications have parallel conditions. In the case of parallel LED driving, the branch LEDs in the overall constant current design do not necessarily work in a constant current state, and the LED current of the entire product interacts with each other.
In high current designers, such as LED streetlight designs, designers do not directly connect multiple LEDs in parallel because such dangers occur immediately. The usual practice is to first constant voltage and then DC constant current, through the two-stage design. We know that DC drive efficiency is under reasonable voltage and load conditions, then how to ensure the number of load LEDs or LEDs with temperature changes within a reasonable range, how to make customers change the number of LED drivers to solve the above problems, need to design AC to DC constant The feedback mechanism of the flow, but so far does not have the technical conditions.
Changyun has adopted another approach: the company's distributed constant current driver can provide optocoupler drive capability, one of which can be used as a representative of all branches. The distributed constant current branches can communicate with each other to realize an adaptive linkage mechanism, and at the same time, it is compatible with the control and data read/write interface functions. In addition, the peripheral design is zero-deviceized, and the power supply output voltage is matched with the load impedance to achieve integration of the constant current source and the light source.
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