3W LED Bulb Trend Research: Non-isolated Bulbs Reverse Isolation

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 3×1W, with each bulb being 3.3V300mA. The load is formed by connecting three bulbs in series, and the output load is 10V/300mA. Since the grid voltage after rectification is quite different from the output load voltage, the most suitable method is to drive it through flyback isolation and buck. 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, uses fewer components, is small in size, has a lower cost, and is more efficient. 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 .

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

　　DU8613 integrated switch simplifies circuit 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 internally, 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 based on TRUEC2 technology to achieve a fully closed-loop high-precision constant current control method. It is a chip specifically used for driving 3WLED non-isolated bulbs. Figure 1 shows a step-down 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 by detecting different output current average values, thereby achieving closed-loop control.

　　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 linear and load regulation results:

　　Figure 4 System linear regulation rate

　　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

　　图5中的系统负载调整率为0.3%，它的实际意义是多套灯负载可以用一套电源。例如12串LED输出是36V左右，24串输出是72V左右，如果设计电流值相同，可以使用同一套电源，对于电源厂，在生产中对于备料、库存管理有显著价值。值得一提的是，如图6，此系统在短路的时候依然实现了恒流，这就意味着：1.短路保护通过最安全的方式实现。2.这是真正意义的全负载恒流。

　　Figure 6 Inductance Regulation

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

　　Average inductor current: ILav = ILpk - (1/2) × ILr

　　ILpk is the inductor peak current: ILpk=Vcs/Rcs

　　ILr is the inductor ripple current: ILr=(Vout/L)×Toff

　　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 procurement costs.

　　Figure 7 Efficiency curve

　　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 ±0.5%. 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.

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

　　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 3×1W isolation solution that is currently 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.