LED constant current precise control-EEWORLD

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When it comes to LED drive accuracy, constant current error usually comes to mind. In fact, drive accuracy is not limited to current accuracy. LED is a typical current-driven device. Accurately controlling the LED drive current can determine many parameters including light efficiency, power efficiency, heat dissipation, and product brightness. Driving an LED is mainly about controlling its current. Whether it is directly increasing or decreasing the drive current, or reducing the switching time ratio by the duty cycle (PWM) , they are all ways to control current, but the purposes they achieve are different. This article will explain the differences between different drivers in different applications.

Introduction to the principle of distributed constant current drive

In the past incandescent lamp and energy-saving lamp markets, the mainstream lamp models with limited specifications formed by large companies are difficult for LEDs to continue to comply with. LEDs have their application flexibility, which will bring more power specifications in future designs. We must avoid too many power specifications and avoid many obstacles to future mass production. In line with the idea of not limiting design flexibility while taking into account as few power design specifications as possible, we proposed a distributed constant current architecture.

The principle of distributed constant current is to set up independent constant current sources at each parallel branch point to manage, maintain and control the stability of branches and branches, branches and the overall line. The distributed constant current circuit can be regarded as a complete line structure in use, and the actual application is distributed at each node of the line. It 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, there is a big gap between the claimed lifespan of LED products and their actual lifespan. With limited accumulation of drive circuit design technology, using the method of evaluating product lifespan to measure actual lifespan is prone to errors. The stability of the drive circuit will directly affect the overall stability of the product.

The reason why distributed constant current technology has high reliability is that the AC power supply part continues to use the traditional switching power supply and adopts a constant voltage power supply mode. The accumulation of switching power supply technology will create quality conditions for LED power supply design. Under the same power supply specification, there is no need to develop new power supply models, and the power can be backward compatible, which greatly reduces the power supply specifications and improves the uniformity of the power supply.

The idea of precision control combining software and hardware

In daily drive power supply design, it is very difficult to handle the accumulated errors of peripheral devices, resulting in the drive power supply parameters being far from the original design intention. Constant current drive requires current detection. The usual practice is to connect a milli-resistance in series in the branch to obtain feedback information. To achieve high efficiency, the resistance value will be smaller. Too small milli-resistance brings inconvenience to production and testing. General instruments cannot verify the correct value, and the production process will also affect the accuracy. The resistance method is a fixed method to set the current, and it is not convenient to adjust.

The combination of software and hardware will open up a leap in LED application technology. The softwareization of LED constant current precision value can greatly improve the flexibility of LED application. The softwareization of constant current driver current setting is actually to set up registers inside the IC and set the output current size according to the actual product application storage method. All of this is a software process and does not require changing the circuit design. The circuit current setting can be completed through the microcomputer operation software with intuitive digital writing.

Our goal is to reduce the number of peripheral components in the driving circuit. The peripheral components will not cause the accumulation of design device parameter errors, thereby greatly improving the accuracy of constant current.

China's IC manufacturing process currently cannot meet the LED drive accuracy requirements, but we can use new technologies and methods to achieve the world's top constant current accuracy level. Precision drive control is one of the methods.

When performing precise control of the driver, we must first consider the purpose of the design. Is it designed for the highest light efficiency or for the consistency of the lamp? If it is limited to the precision of the driving current, it is actually very easy to do. For example, the driving current is stable and accurate, or there is protection with temperature changes. But in the eyes of customers, it is not limited to these. Customers require that all parameters can meet the requirements, such as product consistency and efficiency. For these requirements of customers, we need to work hard on the design of the driver. In the final analysis, it is still how to control the accuracy, and finally adjust the current according to our design intention to improve the stability of the product.

In order to achieve the above purpose, we can divide the constant current output range into several levels equally, describe each level of current digitally, and drive IC to read the corresponding number and execute the corresponding current value. You can choose to build in non-volatile E2PROM . I believe that any register can complete its task, ^and the type of memory can be determined according to the application needs and the conditions allowed by the process.

The division and design of current levels may vary depending on the market. After the distributed constant current built-in register, the current value division will make up for the lack of precision in the process. Due to the manufacturing process, the output current always has errors, which will be improved after software.

Changyuntong's driver IC can provide different current output values according to different customer needs when it leaves the factory, eliminating the batch calibration process. Customers with small quantities can also rewrite the current value by themselves through the included microcomputer software. In addition, customers can also change the brightness of the product according to their needs through the product design interface, and after years, due to LED light attenuation or according to actual usage, re-determine the drive current value and purpose.

New design method to improve drive efficiency

It is feasible to drive LED with AC power supply in single series branch, but single series connection is only a small part of LED driving applications, and most applications have parallel connection. In the case of parallel LED driving, the branch LED in the overall constant current design does not necessarily work in a constant current state, and the LED current of the entire product affects each other. Usually, parallel connection is chosen in low current design, because low current products will not be immediately affected and damaged, but the hidden danger still exists.

In high current designers, such as LED street light designers, designers will not directly connect multiple LEDs in parallel, because this will cause danger immediately. The usual practice is to first constant voltage and then DC constant current, through these two stages of design. We know that DC drive efficiency is under reasonable voltage and load conditions, so how to ensure that the number of load LEDs or the change of LEDs with temperature are within a reasonable range? How to flexibly allow customers to change the number of LED drivers? Solving the above problems requires the design of an AC to DC constant current feedback mechanism, but so far there is no technical condition for this.

Changyuntong adopts another approach: the company's distributed constant current driver can provide optocoupler driving capabilities, and one branch can be used as a representative of all branches. Distributed constant current branches can communicate with each other to achieve an adaptive linkage mechanism, while being compatible with control and data read and write interface functions. In addition, the peripheral design is zero-device, and the power supply output voltage matches the load impedance, thereby realizing the integration of constant current source and light source.

At present, the AC power supply combined with control methods mainly include the following: 1. Primary feedback power supply. This technology is used to drive low-power LED lamps and is transplanted from the original primary feedback charger switching power supply. This part of the AC driver has not changed its specifications due to LED applications, and there are many models to choose from. Except for the limited size, there is no essential difference in design from the switching power supply, so it is reasonable to borrow it for LED ; 2. High-power isolated lighting power supply. In this technology, all LED drivers are designed with switching power supplies. No company has launched AC drive devices dedicated to LEDs so far . The isolated power supply is the switching power supply. It can be basically asserted that the switching power supply is LED-AC drive, which is also one of the main application methods; 3. Non-isolated LED driver power supply. Non-isolated LED drivers have launched more targeted power supply technologies, which is worthy of recognition and is a direction for LED power supplies. Isolation safety mainly depends on the design of the creepage clearance with the shell. There is no requirement that the LED power supply must be isolated, and there is no limit on the output voltage value. The key is which isolation is safer. Non-isolation has the advantages of high driving efficiency, and the combined structure can also design high-quality, safe and efficient LED products; 4. Thyristor dimming compatible power supply. This technology has special applications and limited quantity, and is a transitional technology.

In the current mainstream AC power supply market, the focus is on the first three items. Among them, non-isolated technology has developed rapidly. The first two isolated LED driver designs are all transplanted switching power supply chips, which are essentially not out of the original mode of switching power supply. Although many experts and scholars from various companies are committed to the research of LED lighting power supply, the current technology still remains at the switching power supply stage. Therefore, the main research focus of LED lighting power supply should be on the secondary DC part, combining constant voltage switching power supply with distributed constant current technology.

Keywords：LED Reference address：LED constant current precise control

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