With the continuous development of LED technology, promoting the advent of white LEDs, the lighting industry has begun the era of green lighting. As 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 have become the primary issues today. High-power LEDs should become the mainstay of the lighting industry, and safe and efficient drive research is the key to popularizing and applying high-power LEDs.
1 High-power LED operating characteristicsLED is a new type of semiconductor solid-state cold light source, which is a photoelectric device that can convert electrical energy into visible light. Generally speaking, the power of high-power LEDs is at least 1 W or more. At present, the more common ones are 1 W, 3 W, 5 W, 8 W and 10 W; LEDs called "green light sources" are moving towards high current (300 mA～1.4 A), high efficiency (60～120 lm/W), brightness adjustable direction development.
(1) Volt-ampere characteristics
High-power LED is a low-voltage, high-current drive device. When the LED voltage changes very small, the current changes greatly. When the forward voltage exceeds a certain threshold, that is, the so-called on-voltage, it can be approximated that IF is proportional to VF, as shown in Figure 1.
(2) Optical characteristics
According to the light-emitting principle of the LED, the light-emitting brightness of the LED basically changes in the positive direction with the current of the LED. Controlling the brightness of high-power LEDs is essentially controlling its output luminous flux.
(3) Temperature
The magnitude of the LED forward current also changes with temperature. Once the ambient temperature exceeds a certain value, the allowable forward current of the white LED will be greatly reduced. In this case, if a large current is still applied, it is easy to cause the white LED to age. Figure 2 is the relationship curve between the temperature of the LED and the forward current.
The stability of the light source system has a great relationship with the driving power supply, and 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 and stability of the entire light source system. The driving power required for high-power LEDs is a low-voltage DC. Therefore, the traditional power supplies used to drive light sources such as light bulbs (tungsten filaments), mouth lamps, energy-saving lamps, sodium lamps, etc. are not suitable for directly driving high-power LEDs. According to the above high-power LED characteristics, a small change in VF will cause a large IF change; too strong current will cause LED light attenuation, and too weak current will affect the luminous intensity of LED; when the temperature rises, the barrier potential of the LED decreases. The current will get bigger and bigger. Therefore, the constant voltage source drive cannot guarantee the consistency of LED brightness. And it affects the reliability, life and light decay of the LED, so the super bright LED is usually suitable for driving with a constant current source. In addition, to improve the efficiency of light emission, design an LED drive system with complete 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 the method of combining hardware circuit design and software program design, taking the single-chip microcomputer as the core, and adjusting the output current through negative feedback to achieve the purpose of stabilization, so as to complete the intelligent driving system with adjustable brightness and suitable for a variety of high-power LEDs. The performance of the system has been greatly improved and improved, effectively solving the problem of the stability and reliability of the light source output. The system principle block diagram is shown as in Fig. 3.
2.1 Controllable constant current source
Figure 4 is the constant current source circuit used in the system. This circuit belongs to the topological structure of current series negative feedback, which is composed of an integrated operational amplifier and a MOS tube. In order to realize the adjustable constant current source control, the adjustable voltage signal Vin output by D/A is introduced into the non-inverting input end of the operational amplifier, making it a controlled constant current source. Connect the sampling resistor R at the reverse input terminal. 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. It has high gain, low offset voltage influence, a wide supply voltage range of 3 to 30 V, and can be used as a voltage follower. The op amp cooperates with the 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 op amp to increase the drive current. When the input voltage of the non-inverting terminal 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 0～30 V power supply, and control the constant current source circuit to obtain a current output of 0～2.4 A, which can calculate the resistance of the sampling resistor. The value 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 that affects the accuracy of the output current value of the constant current source. At the same time, A/D conversion provides data by sampling the voltage value on R for closed-loop control of the microcontroller. Because the maximum output current of the original design is 2.4 A, the power of R should be large enough. For this reason, a constantan material with a relatively small temperature coefficient is used, with a resistance of 1 Ω and a power of 10 W. In addition, the MOSFET is a voltage-controlled device, and its gate required control current IG is almost 0 in steady state, 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 an N-channel enhancement type MOSFET tube RF830 with a drain current of 4.5 A and a power dissipation of 74 W.
2.2 Automatic control unit The design of the above-mentioned controllable constant current source has met the requirement 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) with functions such as A/D and D/A control. Among them, 4 buttons (S1～S4) control to 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. Set the current required current value of the LED by selecting button 1; when the LED light source is fixed and controlled to reach a constant current working condition, pass S2 can easily set the brightness of the LED. The circuit design principle of the system is shown as in Fig. 5.
The main function of this part is to provide the precise voltage signal required to adjust the output current according to a given current value. First, use the keyboard input method to set the given current value. According to the data written by the single-chip microcomputer through its built-in 12-bit D/A conversion, the output DC voltage is provided to the input voltage Vin of the constant current source to obtain a stable constant current output, and then the LED is output through 12-bit A/D sampling The current data is sent to the single-chip microcomputer, and the control voltage is calculated by the single-chip 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 feedback to the controllable constant current source circuit to achieve the The output current of the constant current source is accurately adjusted, and finally the digital tube displays the value of the set current and the output current respectively.
As the core of the control system, C8051F040 has built-in 12-bit A/D, D/A conversion, and built-in 2.4V reference voltage, which is more convenient for system circuit design. According to the reference voltage, the A/D output current corresponds to the voltage range of the D/A input one-to-one, and the current accuracy obtained by the 12-bit A/D conversion can reach 0.6 mA, which meets the design requirements.
3 system software designThe 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 software of the whole one-chip computer is shown as in Fig. 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, subtract the difference from the original D/A entry value and send it to D/A conversion; if the actual value is less than the set value, add the original D/A entry value to the difference The value is then sent for conversion. Cyclic comparison, after making the actual value consistent with the set value, the stable actual value is displayed through the digital tube.
The performance index of the system is mainly determined by two relationships: the relationship between the set value and the displayed value of the A/D sampling; the relationship between the internal measurement value and the actual measurement value. The latter is caused by the influence of temperature on the sampling resistance and load resistance and the amplification factor of the transistor 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 resulting error is added to the system program .
4 Data processing and result analysisData testing is an important indicator reflecting system performance. This test selects 1 W, 2 W, 10 W LEDs, and sequentially adds 9 V, 12 V, 15 V power supply voltage, and sets the output current value corresponding to the selected power LED (1 W-0.35 mA, 2 W) by pressing the button. -0.70 mA, 10 W-1 A), respectively detect the corresponding D/A conversion output voltage, the actual output current value detected by the current source itself, the current value measured by an external ammeter and the two data display values ​​of the digital tube. Secondly, the current is adjusted separately for the 2 W LED, with 10 mA steps increasing and decreasing, and observing the change of its luminous brightness. Related data are listed in Table 1 and Table 2.
It can be seen from the above test results that the system realizes a precise constant current source controlled by the single-chip microcomputer C8051F040 to ensure the stable operation of high-power LEDs. The output current of different power LEDs meets the requirement of error accuracy within ±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 performance of the constant current source power supply is degraded due to insufficient heat dissipation of the system, which causes the error to increase. The main reason for the error is the manufacturing error of the sampling resistor. At the same time, the sampling resistor heats up when the system is working, and the resistance change will also cause errors. But in general, the system has high stability and accuracy.
The system is controlled by a single-chip microcomputer, which effectively improves the current stability of the light source output. The realization of the intelligent control of the digital light source drive is of great significance to the development of high-power LED lighting. In terms of data testing and debugging, the measurement data is not very accurate due to the error of the instrument and the temperature rise of the circuit device due to the long working time. The software design minimizes the existence of the error, so as to make the error of the output current. The range is reduced to ±2 mA, which greatly improves the accuracy of the system.
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