When it comes to LED driver accuracy, constant current error is usually thought of. In fact, the drive accuracy is not limited to current accuracy. 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.
Introduction to distributed constant current drive principle
In the past incandescent and energy-saving lamps market, LEDs have become a mainstream lamp type with limited specifications, and it is difficult for LEDs to continue to comply. LEDs have the flexibility of application and will bring more power specifications in future designs. We have to avoid too many power supply specifications and do not bring many obstacles to mass production in the future. Based on the idea of ​​not limiting design flexibility while also taking into account as few power supply design specifications as possible, we propose a distributed constant current architecture.
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. Distributed constant current design LED products have very high product stability and unique design advantages.
At present, LED products claim a big gap with the actual service life. In the case where the accumulation of drive line design technology is limited, the actual service life is measured by the method of evaluating the life of the product, which is easy to cause errors. 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.
Soft and hard combination 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 constant current driver current setting software is actually setting up the register inside the IC, and setting the output current according to the actual product application storage method. All of these are software-based processes, and there is no need to change the circuit design. 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 precision control, the first thing to look at is the design purpose. Is it based on the highest light efficiency, or is it designed according to the consistency of the lamps? 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. But in the eyes of customers, it is not limited to these. 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.
In order to achieve the above purpose, we can equally divide several levels in the refined constant current output range, and digitally describe each order current, and drive the IC to read the corresponding number and execute the corresponding current.
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. After the distributed constant current built-in registers, the current value division will make up for the lack of precision in the process. 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. In addition, the customer can also change the brightness of the product according to the needs through the product design interface, and after a long time, the drive current value and use are determined again due to the attenuation of the LED light or according to the actual use.
New design method to improve drive 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 general, parallel use is chosen in low current designs because low current products are not immediately affected and damaged, but the hidden dangers still exist.
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 the DC drive efficiency is under reasonable voltage and load conditions. How to ensure that the number of load LEDs or LEDs are within a reasonable range with temperature changes? How flexible is it for customers to change the number of LED drivers? To solve the above problems, it is necessary to design a feedback mechanism for AC to DC constant current, but this technical condition is not available so far.
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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