Design of high-power LED intelligent lighting control system

With the continuous development of LED technology, white light LED has been introduced, and the lighting industry has entered the era of green lighting. Since LED has low energy consumption, low heat radiation and high luminous efficiency, it is a new type of lighting device that is energy-saving, environmentally friendly, economical and safe. Therefore, accelerating technical research and improving its luminous efficiency has become the primary issue today. High-power LED is to become the main body of the lighting industry, and safe and efficient driving research is the key to promoting the application of high-power LED.

1 High-power LED working characteristics

LED is a new type of semiconductor solid-state cold light source. It is a photoelectric device that can convert electrical energy into visible light. Generally speaking, the power of high-power LED is at least 1 W. Currently, the more common ones are 1 W, 3 W, 5 W, 8 W and 10 W. LED, known as the "green light source", is developing towards high current (300 mA ~ 1.4 A), high efficiency (60 ~ 120 lm / W), and adjustable brightness.

(1) Volt-ampere characteristics

High-power LED is a low-voltage, high-current driving device. When the LED voltage changes very little, the current changes greatly. When the forward voltage exceeds a certain threshold, commonly known as the on-state voltage, it can be approximately considered that IF is proportional to VF, as shown in Figure 1.


(2) Light characteristics

According to the light-emitting principle of LEDs, the brightness of LEDs basically changes with the forward current of LEDs. Controlling the brightness of high-power LEDs is essentially controlling their output luminous flux.

(3) Temperature

The magnitude of LED forward current also changes with temperature. Once the ambient temperature exceeds a certain value, the allowable forward current of white light LEDs will be greatly reduced. In this case, if a large current is still applied, it is easy to cause white light LEDs to age. Figure 2 is a curve showing the relationship between LED temperature and forward current.

2 System design

The stability of the light source system is closely related to the driving power supply. Many factors such as transient current or voltage spikes can easily damage it. The performance of the driving power supply directly affects the working life, stability and other performance of the entire light source system. The driving power supply required for high-power LEDs is a low-voltage DC power supply. Therefore, the power supply traditionally used to drive light bulbs (tungsten filaments), light bulbs, energy-saving lamps, sodium lamps and other light sources is not suitable for directly driving high-power LEDs. According to the above characteristics of high-power LEDs, a small change in VF will cause a large change in IF; too strong current will cause LED light attenuation, and too weak current will affect the luminous intensity of the LED; when the temperature rises, the barrier potential of the LED decreases, and the current will become larger and larger. Therefore, the use of a constant voltage source drive cannot guarantee the consistency of the LED brightness. It also affects the reliability, life and light decay of the LED, so ultra-bright LEDs are usually suitable for constant current source drive. In addition, to improve the efficiency of luminescence, design an LED drive system with perfect and reliable protection functions, and an intelligent LED drive with automatic control and detection has become a necessary way for technological development. This paper adopts a combination of hardware circuit design and software program design, with a single-chip microcomputer as the core, and adjusts the output current through negative feedback to achieve stability, thereby completing a brightness-adjustable intelligent drive system suitable for a variety of high-power LEDs, which greatly improves and improves the performance of the system and effectively solves the problem of light source output stability and reliability. The system principle block diagram is shown in Figure 3.


2.1 Controllable constant current source

Figure 4 is the constant current source circuit used in the system. The circuit belongs to the topology of current series negative feedback, which is composed of an integrated operational amplifier and a MOS tube. In order to realize the control of adjustable constant current source, the adjustable voltage signal Vin output by D/A is introduced to the in-phase input of the operational amplifier to make it a controlled constant current source. The sampling resistor R is connected to the reverse input. The output current of the constant current source directly depends on the ratio of the output voltage of D/A and the sampling resistor R1, which is expressed by the formula: Is=Vin/R.


The integrated operational amplifier LM358 includes two independent, high-gain, internal frequency-compensated operational amplifiers, which have high gain, small offset voltage effect, a wide power supply voltage range of 3 to 30 V, and can be used as a voltage follower. The operational amplifier cooperates with MOSFET RF830 to follow the input voltage Vin through feedback, and the base of the power MOSFET is connected to the output stage of the operational amplifier to increase the driving current. When the in-phase input voltage of LM358 is constant, due to the existence of negative feedback, the output voltage of LM358 is guaranteed to be constant, so that the current flowing through the LED load is a constant current. This design is to adjust the input voltage Vin of the current source from 0 to 2.4 V under the condition of 0 to 30 V power supply, control the constant current source circuit to obtain a current output of 0 to 2.4 A, and thus calculate that the resistance of the sampling resistor should be 1 Ω to ensure the required constant current value.

The selection of the sampling resistor will directly affect the stability of the constant current source. When the output current reaches a certain level, R will inevitably heat up and cause its own resistance to change, which is a key factor affecting the accuracy of the output current value of the constant current source. At the same time, the A/D conversion provides data for the microcontroller to perform closed-loop control by sampling the voltage value on R. Since the maximum output current of this design is 2.4 A, the power of R should be large enough. For this reason, a resistor with a resistance of 1 Ω and a power of 10 W made of constantan material with a relatively small temperature coefficient is used. In addition, MOSFET is a voltage-controlled device. In steady state, the control current IG required for its gate is almost 0, which will not affect the accuracy of the output current UD, thereby ensuring the output current accuracy of the constant current source. For the MOSFET tube in the circuit, a high-power tube should also be selected to meet the current requirements. This system uses the N-channel enhancement MOSFET tube RF830 with a drain current of 4.5 A and a dissipation power of 74 W.

2.2 Automatic control unit

The design of the above controllable constant current source has met the requirements of stable output of the power supply, but the stability of the power supply is only a necessary condition for the stability of the light source. Because when the power supply is stable, the output current of the light source will still fluctuate during long-term operation. The automatic control module in the system is mainly composed of a keyboard, LED digital display, and a single-chip microcomputer (C8051F040) system with A/D and D/A control functions. Among them, 4 buttons (S1~S4) control and realize 2 functions, 2 selection buttons, and 2 plus and minus buttons. When the LED light source changes, the electrical parameters of the LED change accordingly, and the required constant current value also changes. The required current value of the current LED is set by selecting button 1; when the LED light source is fixed and controlled to achieve constant current working conditions, the LED brightness can be conveniently set through S2. The principle of system circuit design is shown in Figure 5.

The main function of this part is to provide the precise voltage signal required to adjust the output current according to the given current value. First, the given current value is set by keyboard input. According to the data written by the single-chip microcomputer, the built-in 12-bit D/A converter outputs a DC voltage to provide it to the input voltage Vin of the constant current source to obtain a stable constant current output. Then, the current data of the LED output is sent to the single-chip microcomputer through 12-bit A/D sampling. The control voltage is calculated by the single-chip microcomputer processing. According to the comparison between the actual current and the set current, new data is written to the single-chip microcomputer to update the output current, and then fed back to the controllable constant current source circuit to achieve precise adjustment of the constant current source output current. Finally, the digital tube displays the set current and output current values ​​respectively.

C8051F040 is the core of the control system. It has built-in 12-bit A/D, D/A conversion, and built-in 2.4V reference voltage, which is more convenient for the design of the system circuit. According to the reference voltage, the A/D output current corresponds to the voltage range of the D/A input. The current accuracy obtained by 12-bit A/D conversion can reach 0.6 mA, which meets the design requirements.

3 System software design

The design of the software program mainly includes the initialization management module, the key management module, the data processing module and the display module. All modules are written in the single-chip C51 language. According to the hardware circuit, the main program flow of the entire single-chip software part is shown in Figure 6.

In the closed-loop comparison operation, the standard value is approached by comparing the difference between the actual value and the set value. If the actual value is greater than the set value, the original D/A input value is subtracted from the difference and sent to D/A conversion; if the actual value is less than the set value, the original D/A input value is added to the difference and sent to conversion. After the cyclic comparison, the actual value and the set value are consistent, and the stable actual value is displayed through the digital tube.

The performance indicators of the system are mainly determined by two major relationships: the relationship between the set value and the A/D sampling display value; the relationship between the internal measurement value and the actual measurement value. The latter is caused by the influence of temperature on the sampling resistor, the load resistor and the transistor amplification factor and the error of the measuring instrument. In order to reduce this error, a resistor with a low temperature coefficient must be selected as the sampling resistor; and the error of the A/D and D/A conversion process can be obtained through multiple experiments to obtain a certain proportional relationship, and the obtained error is added to the system program.

4 Data processing and result analysis

Data testing is an important indicator reflecting system performance. This test uses 1 W, 2 W, and 10 W LEDs, and adds 9 V, 12 V, and 15 V power supply voltages in turn. The output current value corresponding to the selected power LED is set by pressing the button (1 W-0.35 mA, 2 W-0.70 mA, 10 W-1 A), and the corresponding D/A conversion output voltage, the actual output current value detected by the current source itself, the current value measured by the external ammeter, and the two data display values ​​of the digital tube are detected respectively. Secondly, the current of the 2 W LED is adjusted separately, increasing and decreasing in steps of 10 mA, and the change of its luminous brightness is observed. The relevant data are listed in Tables 1 and 2.


From the above test results, it can be seen that the system realizes the precise constant current source controlled by the single-chip microcomputer C8051F040, ensuring the stable operation of the high-power LED, and the output current of different power LEDs meets the requirement of error accuracy within the range of ±3 mA. In addition, when adjusting the brightness, it can be seen that when the current value is small, the output current is closer to the given current; when the current value is large, the constant current source power supply performance decreases due to the poor heat dissipation performance of the system, causing the error to increase. The main reason for the error is the sampling resistor manufacturing error. At the same time, the sampling resistor heats up when the system is working, and the change in resistance value will also cause errors. But in general, the system has high stability and accuracy.

Conclusion

This system effectively improves the current stability of the light source output through single-chip control. It realizes the intelligent control of digital light source drive, which is of great significance to the development of high-power LED lighting. In terms of data testing and debugging, due to the error of the instrument and the error caused by the temperature rise of the circuit components due to long working time, the measurement data is not very accurate. Through software design, the error is reduced as much as possible, so that the error range of the output current is reduced to ±2 mA, which greatly improves the accuracy of the system.