LED bulb power supply analysis: non-isolation reverses isolation

The article first analyzes the current mainstream 3W LED bulb solution with the largest market share, 3MW isolation. The advantages and disadvantages of this solution are proposed. Based on these advantages and disadvantages, the market development direction of the future solution is proposed. The selection of China's LED standards will be based on the most optimized solution on the market. In line with market development, this article uses the DU8613 chip of Shanghai Duty Cycle Semiconductor Company to provide a 3W bulb LED constant current control drive solution based on the non-isolated mUOK topology, and provides experimental data and related waveforms, and looks forward to the development space of non-isolated drive.

　　With the optimization and popularization of insulating heat dissipation materials, non-isolated drive solutions are the general trend. The practical significance of higher efficiency is not only to reduce energy consumption, but also to increase the utilization rate of LED lamp beads, reduce costs, and bring motivation to consumers. At present, China's LED standards have not yet been established. Due to fierce market competition, the current 3W isolated drive solution in the market can achieve extremely low costs, but there are some problems.

　　Introduction to the current mainstream 3W driver power supply in the market

　　3WLED bulbs are one of the markets with the largest consumer demand. The current mainstream solution is 3x1W, with each lamp bead 3.3V/300mA. The load is formed by connecting three lamp beads in series, and the output load is 10V/300mA. Since there is a large difference between the grid voltage after rectification and the output load voltage, the most suitable method is to drive it through flyback isolation and step-down. However, the isolation circuit has a complex structure, many components, high cost, low efficiency, and low reliability, and is not suitable for mass production. Compared with the isolation circuit, the non-isolated circuit has a simple circuit, fewer components, small size, low cost, and high efficiency. With the market's requirements for cost and efficiency, non-isolated circuits have begun to take the stage in the field of low-power LED drivers .

　　DU8613 integrated switch simplifies circuits to achieve full closed-loop 3WLED bulb constant current control

　　DU8613 is a step-down constant current controller in continuous current working mode with 3% system constant current accuracy. It integrates 500V high-voltage MOSFET, simplifies the peripheral circuit to a certain extent, and sets protection functions such as sampling resistor open circuit, short circuit protection, output overcurrent, short circuit protection, and over-temperature protection.

　　DU8613 is a chip based on TRUEC2 technology that implements a fully closed-loop, high-precision constant current control method. It is specifically used for driving 3WLED non-isolated bulbs. Figure 1 shows a buck drive solution implemented using this chip. As shown in Figure 1, the inductor current signal detected at the CS end of the chip is processed by patented technology, such as the TRUEC2 closed-loop constant current control module in Figure 2, to obtain the average value of the inductor current (i.e., output current). The chip controls the duty cycle of the switch tube through the different output current average values ​​detected, thereby achieving closed-loop control.

Figure 1 Fully closed-loop non-isolated buck constant current LED driver power supply reference schematic

Figure 2 DU8613 internal function diagram

Figure 3 DU8613 bulb application actual picture and PCB wiring diagram

　　Experimental results:

　　We select a typical LED bulb application to verify the IC function. The basic electrical parameters are as follows:

　　Input voltage range: 180?265VAC/50Hz Typical efficiency: >88%

　　Output voltage range: 3?52VDC Output current: 60mA Nominal output power: 3W

　　When the input voltage and load LED change, we tested and obtained the following linearity and load regulation results.

Figure 4 Linear Regulation

　　The linear regulation rate in Figure 4 is close to 0, because the chip performs closed-loop control cycle by cycle, responds immediately, does not cause output current changes, and effectively suppresses disturbances caused by grid voltage fluctuations. While achieving such an ideal linear regulation rate, many peripheral components for linear compensation of the second-generation control chip are eliminated. The philosophy of simplicity is perfect is once again reflected in this design.

Figure 5 System load regulation

　　The system load adjustment rate in Figure 5 is 0.3%. Its practical meaning is that multiple sets of lamp loads can use one set of power supply. For example, the output of 12 strings of LEDs is about 36V, and the output of 24 strings is about 72V. If the design current value is the same, the same set of power supply can be used. For power supply plants, it has significant value for material preparation and inventory management in production. It is worth mentioning that, as shown in Figure 6, this system still achieves constant current when short-circuited, which means: 1. Short-circuit protection is achieved in the safest way. 2. This is a true full-load constant current.

Figure 6 Inductance Regulation

　　Traditional chips on the market use an open-loop control technology, a fixed Toff control technology, and the current derivation formula is as follows:

　　If traditional chips are used, changes in inductance will cause changes in output current, but the consistency of inductance is difficult to control during mass production. Figure 6 shows that the use of the DU8613 chip can still keep the output current constant under different power inductors, thus reducing the requirements for inductance consistency, which is conducive to mass purchase of inductors, thereby reducing purchase costs.

Figure 7 Efficiency curve

Figure 8: Efficiency curve of a company's 3W isolated LED driver solution

　　As can be seen from Figures 4, 5, and 6, due to closed-loop control, within the normal working range of the design, the output current maintains a constant value, and the single system can be considered to have a constant output current, that is, the linear adjustment rate is close to 0, and the load adjustment rate is ±0.3%. During mass production, due to the distribution of parameter consistency, a large amount of trial production data shows that the constant current accuracy is less than ±2%. Figure 7 shows that when the input is properly adjusted and the output nominal power is 3W, the system efficiency can be above 88%, and even within the full voltage input range, the efficiency can reach above 86%. Figure 8 is the efficiency curve of an isolated 3W solution on the market. The comparison shows that for low-power LED bulbs, non-isolation is a huge improvement compared to isolation from the perspective of energy saving. If converted to the global power consumption of bulbs, the effect of energy saving and emission reduction is quite considerable.

　　Experiments show that DU8613 can achieve high-precision constant current control using full closed-loop constant current control. It is used to drive 3W bulbs, and the total BOM cost is less than ￥1.5. This result is comparable to some RC buck or constant current diode solutions, and its linear adjustment rate, load adjustment rate, system efficiency, and output constant current effect without any power frequency ripple give the chip an advantage. In addition, the circuit board has fewer components and is easy to weld, which is conducive to mass production. Due to the weak dependence of the chip's full current feedback on external conditions, it is convenient to purchase raw materials and can reduce costs.

　　Trend Outlook

　　In response to market demands, reducing costs, improving efficiency and reliability have become the goals pursued by every enterprise.

　　The circuit using isolated flyback converter is relatively complex, and the transformer has large losses, which leads to low efficiency. LED itself is an emerging alternative light source that is energy-saving and environmentally friendly. The 3x1W isolation solution that is widely used in the market is based on the mainstream lamp beads in the market, but the efficiency of about 70% is disappointing. If the driving power supply does not have a high enough conversion efficiency, how can it be energy-saving and environmentally friendly?

　　Non-isolated circuits have the advantages of simple structure, fewer components, low cost, and high efficiency. In view of the above advantages, non-isolated circuits will gradually replace isolated circuits in low-power LED driver power supplies and become the mainstream of such driver power supplies. The driving solution based on DU8613 of Shanghai Duty Cycle Company has the advantages of high constant current accuracy and high efficiency while ensuring that the cost is low enough. The author believes that the characteristics of non-isolation and low current are in line with the needs of the market and will inevitably gradually replace the isolation solution.