Small size non-isolated constant current 18WLED fluorescent lamp driving scheme

The minimalist line 18W LED fluorescent lamp driving scheme described in this paper is less than one dollar coin. The cost is about half of the current mainstream market solution, which is in response to this “demanding” requirement. The simple, perfect philosophy is played in this program.

2.1 Introduction to the third-generation non-isolated LED constant current control technology closed-loop algorithm TRUEC2

For the characteristics of LED lighting load, the topology of the current non-isolated constant current driving power supply is basically a BUCK buck structure. The development of non-isolated LED constant current control technology is divided into three generations. From the initial fixed frequency mode to the later fixed off-time mode, strictly speaking, it is not a true closed-loop constant current. It has been developed to today, and the third is used. A closed-loop algorithm for non-isolated control technology, called the control strategy of TRUEC2. Shanghai Duty Cycle Semiconductor's DU8618 chip, based on TRUEC2 drive technology, is insensitive to a variety of peripheral components, which improves reliability and accuracy while reducing overall system cost.

The so-called closed loop, that is, the true detection of the output current value, is used as a standard to issue the PWM signal. The so-called open loop does not use the detected output current value as a reference for issuing a PWM signal. From the circuit topology, there is no difference between the two, they are non-isolated BUCK buck topology. But there are two benchmarks inside the chip, one is the peak reference and the other is the average reference. The chip detects the inductor current signal and performs patented processing. As shown in Figure 1, the TRUEC2 part performs integral processing and peak control. Thus, the average value of the inductor current, that is, the average value of the output current, is detected. The chip controls the output duty cycle for the detected value and implements closed-loop control. This control structure makes the circuit extremely simple. Due to the closed-loop control current, the line is also short-circuited compared to the open-loop mode. This is very important in production applications, which is equivalent to greatly reducing the risk of system failure. This makes the solution not only a qualitative leap in performance, but also greatly improved in reliability. The DU8618 integrates a 1A high-voltage MOSFET, which simplifies the circuit. The low-current, high-voltage design reduces losses. The 1A MOSFET is packaged in a small-volume SOP8, which is suitable for all non-isolated LED drivers within 18W. demand.

Figure 1: Schematic diagram of the internal TRUEC2 module of the chip

2.2 DU8618 integrated switch simplifies the line to achieve full-closed 18W LED fluorescent lamp constant current control

The DU8618 is based on TRUEC2 technology and is specifically designed for 18W LED non-isolated fluorescent lamp driven chips.

The basic electrical parameters are as follows:

Input voltage range: 180~265VAC/50Hz Typical efficiency: >95%

Output voltage range: 60~180VDC Output current: 90mA

Nominal output power: 18W

Figure 2: Schematic diagram of full-closed non-isolated buck constant current 18W LED fluorescent lamp driving power supply

Figure 3: DU8618 output 18W non-isolated LED fluorescent lamp drive power supply size indication

Figure 4: PCB layout

The schematic of Figure 2 shows that to achieve an output power of 18W, all the components required for the solution are only 14. The visual display of Figures 3 and 4 shows that the solution is smaller than a single coin. The advanced control methods described above are the root cause of achieving higher power densities. The practical significance is not just to increase the power density. Technological advances also include: simpler lines, lower costs, and higher constant current accuracy. 95% efficiency is higher than most of the market, and the design of small current and high voltage is the key to achieving such high efficiency.

2.3 Experimental results

For the input voltage and load LED changes, we tested the following linear, load regulation results:

Figure 5: System linear adjustment rate

The linear adjustment rate of Figure 5 is close to 0. This is because the chip is controlled cycle-by-cycle and responds immediately, without causing a change in output current. In order to achieve such an ideal linear adjustment rate, the second generation of control chips is also eliminated. Because of the many peripheral components of linear compensation, the simple and perfect philosophy is once again embodied in this design.

Figure 6: System load adjustment rate

The practical significance of high load regulation is that a set of power supplies can be used for multiple sets of lamp loads. For example, the 12-string output is about 36V, and the 24-string output is about 72V. If the design current value is the same, the same set of power supply can be used. For the power supply plant, it has significant value for the preparation and inventory management in production. It is worth mentioning that, as shown in Figure 6, the system still achieves constant current during short circuit, which means: 1. Short circuit protection is achieved in the safest way. 2. This is the true full load constant current.

As can be seen from Figures 5 and 6, due to the closed-loop control, the output current maintains a constant value within the normal operating range of the design. A single system can be considered as a constant output current, that is, the linear adjustment rate is close to 0, and the load regulation rate is ±0.5. %. When mass production, due to the consistent distribution of parameters, a large number of trial production data shows that the constant current accuracy is less than ± 2%.

Figure 7: Switching Waveform

Let's take a look at the list of components.

The total cost BOM cost is less than $3.5. This score is not as special as some RC or linear high pressure solutions, but considering such linear adjustment rate, load regulation and system efficiency, and no power frequency pattern. The wave's output constant current effect, the author does not intend to blindly follow the sentient beings, hold high the "low cost" gimmick, just want to express a simple design philosophy in the cost accounting.

3 Trends

The author believes that with the optimization and popularization of insulation and heat dissipation materials, the non-isolated drive scheme is the trend of the times. The higher efficiency of its practical significance, in addition to reducing energy consumption, has increased the use of LED lamp beads, reduced costs, and brought power to consumers. Some low-cost isolation schemes have in fact not completely achieved the isolation of safety regulations, and the sword is slanting, which is not conducive to long-term development.

Fortunately, the LED industry is currently a typical short-cycle industry, and technology rather than capital is the core competitiveness. Bold predictions:

- The solution to isolate single-stage PF high ripple is replaced by the "first-and-a-half" solution (high PF, low ripple, compact size) sooner or later.

- The linear high voltage LED solution will gradually replace the RC solution.

- Before the maturity of the “first-and-a-half-year” plan, the non-isolated solution will gradually take the share of the isolation plan.

- Non-isolated solutions are also moving in two directions: one is a high-end valley-filled high-efficiency, high-PF, low-ripple solution. One is the full-closed ultra-high cost-effective solution of this paper. The goal is to make the LED fluorescent lamp close to 30 yuan in the case of maintaining high quality, which will cause the market to explode.

- SCR dimming solution, although the stubborn wall-mounted thyristors will remain, and there are a large number of abandoned manufacturers trying to do "all compatible solutions", but with the rise of advanced LED dimming technology Will withdraw from the historical stage. The alternative solution will give people a very new light experience.

Chongxiaotian, humble land. As an enthusiastic pursuer of energy conservation and environmental protection, the author, as a person in the LED industry, is convinced that advanced technology is the only driving force for a simple, perfect and bright world in the future. Creativity is always the driving force behind human survival and evolution. The so-called "commercial behavior" of blind low price and low quality vicious competition can only seek short-term self-interest and self-destruct the Great Wall, so that consumers lose confidence in LED products. The Guangdong Provincial Bureau of Quality and Technical Supervision notified the provincial-level special supervision and spot check results of LED lamp product quality in August last year. There were 23 batches of LED lamp products produced by 21 companies in 7 cities and cities, and the product failure rate was 73.9%. Looking at the data, do you dare to use LED products? Can you still believe that it is energy efficient? This is the result of ignorance. A conscientious insider, only the tireless pursuit of advanced technology, is the only way to benefit mankind.