This simple line 18W LED fluorescent lamp driving scheme introduced in this paper is smaller than a dollar coin, and the cost is about half of the current mainstream scheme in the market. It is born to meet this “harsh” requirement. The philosophy that simplicity is perfection is played incisively and vividly in this scheme.
2.1 introduction to the closed loop algorithm truec2 of the third generation non isolated LED constant current control technology
According to the characteristics of LED lighting load, the topology of non isolated constant current driving power supply is basically buck buck buck structure. The development of non isolated LED constant current control technology is divided into three generations. Strictly speaking, it is not a real closed-loop constant current from the initial fixed frequency mode to the later fixed off time mode. Until today, the closed-loop algorithm of the third generation non isolated control technology is used, which is called the control strategy of truec2. The du8618 chip of Shanghai duty cycle semiconductor company, based on truec2 driving technology, is insensitive to various peripheral components, which not only improves the reliability and accuracy, but also reduces the cost of the overall scheme.
The so-called closed loop is to really detect the output current value and take it as the standard to send PWM signal. The so-called open-loop does not use the detected output current value as a reference for sending PWM signal. In terms of circuit topology, there is no difference between them. They are both non isolated buck buck topology. However, there are two benchmarks inside the chip, one is the peak reference and the other is the average reference. The chip detects the inductance current signal and processes it with patented technology, as shown in Figure 1. The part of truec2 performs integral processing and peak value control. In this way, the average value of the inductance current, that is, the average value of the output current, is detected. According to the detected value, the chip controls the output duty cycle and realizes the closed-loop control. This control structure makes the line extremely simple. Due to the closed-loop control current, compared with the open-loop mode, the short circuit of the line is also constant current. This is very important in production application, which greatly reduces the risk of system failure. This makes this scheme not only have a qualitative leap in performance, but also greatly improve the reliability. The du8618 integrates 1A high-voltage MOSFET and makes the simplified circuit to the extreme. Through the design of low current and high voltage, the loss is reduced, so that the 1A MOSFET is integrated and encapsulated in a small volume sop8, which can meet the driving requirements of all non isolated LEDs within 18W.
Figure 1: working diagram of some modules of truec2 in the chip
2.2 the du8618 integrated switch simplifies the circuit to realize the full closed-loop constant current control of 18W LED fluorescent lamp
Du8618 is a chip based on truec2 technology, which is specially used for driving 18W led non isolated fluorescent lamps.
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 driving power supply of full closed-loop non isolated step-down constant current 18W LED fluorescent lamp
Figure 3: dimensions of driving power supply for du8618 output 18W non isolated LED fluorescent lamp
Figure 4: PCB layout
The schematic diagram in Figure 2 shows that only 14 elements are required to achieve 18W output power. The visual display of Figure 3 and Figure 4 shows that the scheme is less than one dollar coin. The advanced control mode described above is the fundamental reason for achieving higher power density. The practical significance is not just to improve the power density. Technological progress has also brought about simpler lines, lower costs and higher constant current accuracy. The efficiency of 95% is higher than most schemes in the market. The design idea of low current and high voltage is the key to achieve such high efficiency.
2.3 experimental results
When the input voltage and load led change, we get the following linear and load adjustment rate results:
Figure 5: system linear adjustment rate
The linear adjustment rate of Fig. 5 is close to 0 because the chip is closed-loop controlled cycle by cycle and responds immediately without causing the change of output current. While realizing such an ideal linear adjustment rate, it also saves the second generation control chip, because many peripheral components of linear compensation, the simple and perfect philosophy is reflected again in this design.
Figure 6: system load adjustment rate
The practical significance of high load regulation rate is that multiple sets of lamp loads can use one set of power supply. 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 plant, it has significant value for material preparation and inventory management in production. It is worth mentioning that, as shown in Figure 6, the system still realizes constant current in case of short circuit, which means: 1. Short circuit protection is realized in the safest way. 2. This is the true full load constant current.
It can be seen from figures 5 and 6 that due to the closed-loop control, the output current maintains a fixed value within the designed normal working range, and a single system can be considered as 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 consistent distribution of parameters, a large number of trial production data show that the constant current accuracy is less than ± 2%
Figure 7: switching waveform
Let’s take a look at the component list.
The total cost BOM cost is less than $3.5. This achievement is not special compared with some resistance capacitance step-down or linear high voltage schemes, but considering such linear adjustment rate, load adjustment rate and system efficiency, as well as the output constant current effect without any power frequency ripple, the author has no intention to blindly follow all sentient beings and hold high the gimmick of “low cost”, Just want to express the simple design philosophy, which is reflected incisively and vividly in cost accounting.
3 Trend Outlook
The author believes that with the optimization and popularization of insulating heat dissipation materials, the non isolated driving scheme is the general trend. Its higher efficiency not only reduces energy consumption, but also improves the utilization rate of LED beads, reduces costs and brings power to consumers. In fact, some low-cost isolation schemes do not meet the safety regulations of isolation, and the sword is at the wrong edge, which is not conducive to long-term development.
Fortunately, the LED industry is a typical short cycle industry, and technology rather than capital is the core competitiveness. Bold predictions:
-The scheme of isolating single-stage pf high ripple will sooner or later be replaced by the “one and a half” scheme (high PF, low ripple and compact volume).
-The linear high voltage LED scheme will gradually replace the resistance capacitance step-down scheme.
-Before a large number of mature “one and a half” schemes are put on the market, the non isolation scheme will gradually seize the share of the isolation scheme.
-The non isolation scheme is also developing in two directions: one is the high-end valley filling scheme with high efficiency, high PF and low ripple. One is the full closed-loop ultra-high cost performance scheme in this paper. Its goal is to make the market price of LED fluorescent lamps close to 30 yuan while maintaining high quality, which will lead to market explosion.
– Although the thyristor dimming scheme will remain with the stubborn thyristor on the wall, and a large number of abandoned manufacturers try to make a “fully compatible scheme”, it will withdraw from the historical stage with the rise of advanced LED dimming technology. The alternative will give people a very novel light experience.
Responsible editor; zl