Recently, the price of phosphors has increased dramatically, resulting in an increase in the cost of fluorescent lamps; The manufacturing technology of LED light source has developed rapidly, which makes its cost drop rapidly; The post earthquake nuclear power problem in Japan has led to a rapid growth in the demand for LED lamps in the post earthquake reconstruction market. At present, governments of various countries continue to strengthen efforts to eliminate incandescent lamps. For example, Europe stipulates to ban 100W incandescent lamps at the end of 2009 and 75W incandescent lamps at the end of 2010, which undoubtedly brings great opportunities to the LED lighting market. With the progress of constant current control technology of LED lighting driving power supply and the continuous reduction of LED light source price, the distance between LED lighting and thousands of households is further shortened.

The progress of constant current control technology of LED lighting driving power supply is mainly reflected in the following aspects: 1) with the continuous simplification of chip peripheral control circuit, the main stage side constant current technology (PSR) has become mature; 2) The constant current accuracy of the chip is continuously improved, which provides a strong guarantee for reducing the deviation of production and improving the reliability of mass production of power supply; 3) The emergence and mature technology of single-stage APFC chip for LED lighting further reduce the volume and cost of power supply. In line with the development trend of LED lighting market, bpsemi launched bp31xx series driver chip of main stage side constant current control (PSR) in 2011. For example, bp3102, bp3105 and bp3108 suitable for PSR isolation scheme, as well as the driving chips bp3309 and bp3308 of bp33xx series controlled by single-stage APFC.

Innovative LED driving power chip design technology

At present, the driving power supply of LED lighting lamps has very high requirements for the parameters of transformer and inductor. In order to achieve accuracy, it needs to spend a lot of manpower and financial resources, and the cost of industrial production is difficult to be further reduced. Through the innovation and breakthrough in the design technology of LED driven constant current source chip, the new solution can be used in the practical application of LED lamp driving power supply. The output current is not sensitive to the inductance of transformer and inductance and the VF of LED. The key of innovative technology is to optimize the chip algorithm language. Figure 1 shows a new principle of constant current control at the main stage side of bp31xx series. It can be found from the figure that the chip controls the peak value of output current and feeds back through the auxiliary winding to fix the proportion of demagnetization time and chip switching cycle, so that the output current can be independent of the deviation of peripheral inductance and output voltage, The deviation of VF value and inductance of LED is the biggest problem faced by mass production. Generally speaking, there is a deviation of about 5 ~ 10% between the inductance of mass-produced transformer and the VF of LED. The batch accuracy of bp31xx series LED lighting constant current drive power supply can reach less than 3%, which can allow large tolerance of transformer and inductance parameters, which reduces the cost of large-scale industrial production and promotes the continuous and rapid production of the assembly line.

Analysis of design technology of LED driving power chip

Fig. 1 working principle of bp31xx series main stage side constant current control technology.

The application circuit of bp31xx is very simplified, and the surrounding parts are few, which is in line with the concept of low-cost and industrial production. Fig. 2 is a typical application circuit. After the main stage side constant current technology is adopted, the secondary feedback circuit will no longer become an obstacle to the power supply volume and cost, which brings great convenience to those occasions with extremely strict requirements for power supply volume, such as bulb lamp, GU10 spotlight, etc. The traditional optocoupler feedback structure also has the problems of optocoupler aging and difficult safety test. For the application ambient temperature of 60-90 ℃ inside the bulb lamp, the reliability problem has become the focus of manufacturers and customers. The maturity of the main stage side constant current technology, especially the innovative drive power chip design technology, makes the application circuit insensitive to some difficult quality control parameters of transformer and inductance. For LED lighting power supply and lamp manufacturers, it is not only convenient to improve product performance, but also reduce cost, which is a win-win choice.

Analysis of design technology of LED driving power chip

Figure 2 typical application circuit of bp3105

Although there are fewer parts and devices around the circuit, the power circuit designer needs to select high-quality limited devices, such as high withstand voltage of MOS, long-life products with high sealing, high hydration resistance, high overload resistance and 125 ℃ 10000 hours, so as to adapt to long-term operation in high temperature environment.

Single stage APFC chip for LED lighting

Another hot trend of LED constant current drive power supply is the single-stage APFC control chip with automatic high power factor compensation and low harmonic. The American energy star standard has specific requirements for PF value for solid-state lamps above 5W. Therefore, the demand of LED lighting manufacturers for single-stage APFC chip has greatly increased this year. Because the existing single-stage PFC chip is not specially designed for LED lighting, there are some common defects, such as long start-up time, the need for optocoupler and external reference feedback, poor mass production current accuracy of primary side feedback, etc., so that the current APFC constant current scheme represented by 6562 and 7527 continues to be widely used, In this way, it is difficult to further reduce the volume and cost of LED lighting driving power supply. Therefore, the new single-stage APFC scheme for LED lighting is also prayed by the LED lighting industry.

Bp3309 is a new generation of single-stage APFC chip dedicated to LED lighting. Figure 3 shows the typical application circuit of bp3309. The chip has constant current control (PSR) on the main stage side, without secondary optocoupler, TL431 and other feedback circuits; The chip adopts patented source level driving technology, and the start-up time is very short. Under the condition of 85vac and 1m ohm start-up resistance, the start-up time is less than 200ms, which improves the system efficiency; Soft start and no overshoot technology necessary for LED lighting lamps shall be adopted; The chip has complete protection functions, including short-circuit protection, open circuit protection, overvoltage protection, undervoltage protection, over temperature protection and sampling resistance short-circuit protection. When the fault is removed, the chip can automatically return to normal operation. In addition, bp3309 chip solves the problems of deviation of linear adjustment rate, load adjustment rate and batch constant current accuracy (as shown in Table 1). This led power drive scheme can meet the requirements of mass production. Compared with the traditional side feedback scheme, the advanced PSR single-stage APFC scheme not only saves the space and cost of power supply, but also improves the reliability.

Analysis of design technology of LED driving power chip

Figure 3 typical application diagram of bp3309 of single-stage APFC.

Analysis of design technology of LED driving power chip

Table 1 linear adjustment rate and load adjustment rate of bp3309

Summary of this paper

With the continuous development of LED lighting industry and the introduction of national energy standards, on the premise of ensuring transformation efficiency, the development of LED lighting power supply will continue to develop in the direction of smaller volume, fewer components, higher precision and more conducive to production. Therefore, the new power chip constant current control technology and single-stage APFC technology are further improved and mature, It plays a very significant role in improving the performance, reducing the cost and reducing the volume of LED lighting power supply.

Source; Electronic Engineering Network

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